general treatment planning Flashcards
1
Q
what is impact of dose inaccuracy (ex. treatment is 10% off of prescription?)
A
- 1% accuracy improvement yields 2 % increase in cure rate for early stage tumours
- 7% difference in dose is shown to manifest in patient’s response to radiation therapy
2
Q
what are min equilibrium depth and radii for TCPE for photons of 100 kEv, 4.5 MeV, and 10 MeV?
A
100 keV- 0.15 mm depth and 0 mm radius
4.5 MeV- 4.5 mm depth and 1.5 mm radius
10 MeV- 5 cm depth and 2 cm radius
3
Q
Define Fano’s theorem
A
when an infinite medium of varying density but constant atomic composition is exposed to a uniform photon fluence (i.e., CPEconditions), differential in energy and direction, then the fluence of charged
particles launched by the photons is also constant and not affected by density
variations.
4
Q
O’Connor scaling theorem
A
for dose in 2 media of equal Z but different density, the ratio of 2nd scattered radiation fluence to primary fluence is constant in the 2 media if all gemoteric distances including field sizes are scaled inversely with density
However, the primary penumbra width is inversely proportional to tissue density
whereas the density has the opposite effect on the scatter penumbra, since the scatter dose decreases with the decrease of density. Therefore, the inverse proportionality of penumbra width with density does not hold for the total dose
5
Q
define primary vs scattered dose
A
- 1st time photons interact with medium = primary
- scattered = photons which have previously interacted at least once in the medium
6
Q
categorize different inhomogeneity correction algorithms according to level of anatomy sampled (1D or 3D) and inclusion/exclusion (TERMA) of electron transport
A
TERMA 1 D includes linear attenuation coefficient correction, ratio of TAR, equivalent path length, effective SSD, isodose shift, Batho power law
TERMA 3D includes ETAR
Electron transport 1D includes pencil beam convolution
Electron transport 3D inclused superposition/convolution, monte carlo
7
Q
define effective attenuation coefficient for inhomogeneity correction
A
point correction = exp ((mu’)(d-d’)
u’ is attenuation coefficient
d is physical depth
d’ is radiological depth = sum of thickness of various layers with different densities * their density
8
Q
why does RTMR yield better results than RTAR for inhomogeneity correction?
A
(i) the TMR values include no inherent
backscatter; (ii) the TAR value includes inherent backscatter, (iii) in lung of
density 0.3 g/cm3, backscatter is reduced and (i) is more appropriate.
9
Q
What do different algorithms use forinhomogeneity corrections?
A
- Phillips pinnacle = adaptive collapsed cone convolution/superposition
- Nucletron = ETAR
- Elekta PrecisePlan = TAR and 3D SAR integration
- Eclipse AAA = density scaling
- Eclipse Acuros = boltzmann equation
10
Q
For what sites should you use lower energy photon beams due to inhomogeneities?
A
- larynx (air cavity)
- chest wall (breast and lung)
- lung
11
Q
What is the issue with using eclipse to “shift” the DVH for a VMAT plan (make it cooler or hotter)
A
Have to make sure plan is verified- since didn’t run optimizer won’t know that machine can mechanically finish the plan
12
Q
Draw POP PDD for a plan with water on edges and bone in center
A
should be POP PDD but with drop in bone, peaks around interface due to backscatter. Total PDD is smaller than water alone because the bone attenuates more than water
13
Q
How do you derive the PDD conversion to TMR equation
A
write PDD and TMR in terms of Dmax1 and Dmax2, then write Dmax1 as function of Dmax2 using IS and PSFs
14
Q
most basic constraint in TP?
A
beam weight must be non-negative
15
Q
how to deal with multiple isocenters?
A
- can make multiple plans and combine later; but target has to be covered in each individual plan as patient position can change between plans
- make a plan to optimize all isocenters at once- break them into separate plans after
16
Q
where are mantle and Y-shape plans used?
A
pediatric lymphoma
17
Q
what is HyperArc?
A
optimizes couch kicks automatically
18
Q
Issues with extended SSD
A
- slower dose rate
- no laser aids
- clearance - cone beam
- more penumbra
- integrity of couch movements may not be as good as at iso
19
Q
Considerations for CNS treatment with VMAT
A
- VMAT hence no couch kicks
- no junction
- optimizer feathers the dose in there
- use angled VMAT
20
Q
advantages of IMRT over VMAT
A
-more control over gantry angles
21
Q
if data is missing from the CT (i.e. large patient), how can one avoid entering through that sector?
A
draw in an avoidance structure and avoid entering through it using avoidance arc in eclipse
22
Q
what is luminal tumor
A
have to include entire lumen in tumor
23
Q
anatomical vs geometrical expansion
A
anatomical- go around the anatomy manually
geometrical- add same margin automatically all around
24
Q
what isodose line defines the field?
A
50 %
25
why not abbutt MLC leaves in field?
get highest leakage there
26
what happens to dose distribution for single beam as field size increases?
- PDD gets bigger
- surface dose gets bigger
- max dose relative to prescription is smaller because PDD is bigger
27
dmax, PDD10, and PDD20 for 6x, 10x, 18 x
6x : 1.5, 66%, 38%
10x: 2.3, 73%, 46%
18 x: 3.1, 78%, 52%
28
F factor to correct for SSD?
new PDD = old PDD * (( new SSD + dmax)/(new SSD +d ))^2/((old SSD + dmax)/(old SSD +d))^2
29
what field size do we use for Sp if we doing TMR calc? PDD?
TMR- FS at calc pt
| PDD- surface field size projected to dmax (norm point)
30
Is Sc the same for 30x40 field and 40x30 field?
no because of different positions of upper and lower collimators
31
AAPM report for MU beam calculations
TG71
32
isodose lines for POP
50% defines the field
100% is hourglass and goes through norm point
hot islands on either side, 105 %
islands get hotter as patient thickness increases or energy decreases
-hot points tend to be lateral (due to horns?)
33
How fast do PDD and TMR change?
TMR about 3 %/cm
| PDD about 4 %/cm
34
draw out hot spot vs patient thickness graphh for 6x, 10x, 18x single field
start at patient thickness 10 cm, ~105% for 18 x, 110% for 10x 120% for 6x, go to patient thickness 40 cm, 300 (6x), 250 (10x), and 210 (18x)
-exponential graphy
35
POP and 4-field box hot spots vs patient thickness for 6x,10x,18x
start at patient thickness 20 cm, hotspots around 108,105,103%, then at 30 cm 130,118,112%,
got to patient thickness 40 cm with POP hot spots of 160,140,130 for 6x,10x,18x
-4 field box hot spots are half those of POP
36
draw isodose lines for 4-field box
- clover leaf shape in middle (100%)
- draw 95% and 60% as boxes around that
- 50% isodose is a cross that defines and connects the 4 fields (make sure to connect them)
- may have hot spot islands at edges of each field
37
how to determine direction of gradient?
Draw out vectors from isocenter to source of each beam. Add up the vectors (considering beam weights)- the vector sum defines the gradient of the beam arragement
38
penumbra width at dmax and 10 cm depth for 6x, 18x
also lung
also field size
-penumbra at 10 cm are about twice that at dmax
-at dmax, 6x penumbra are 3 mm (20/80), 7 mm (90/10), 15 mm (95/5)
-10x penumbra slightly bigger than 6x; 18 x are double size of 6x
lung penumbra 2-3 times that in tissue
-larger FS = larger penumbra
39
why do penumbra get larger with depth?
scatter diverges out like a cone
40
definition of penumbra
-the region at the edge of a radiation beam, over which the rate of dose changes rapidly as a function of distance from the central axis.
41
transmission penumbra
variation in dose at edges of beam caused by collimator (different thickness of collimator attenuate diffrently)
42
geometric penumbra
- due to size of source
| - due to geometry of setup
43
scatter penumbra
created under collimator jaws into the region of penumbral tail
44
sources of beam positioning uncertainty that go into PTV margin
- image fusion
- target delineation uncertainty
- planning slice thickness (but if too thin, get too much noise)
- coincidence of radiation and imaging isocenters and mechanical isocenter
- couch shift accuracy and tolerance
- physiological effects
- patient motion
- accuracy of surroagte
- anatomical changes between plan and treatment delivery
45
difference in target volume definitions ICRU 50 vs 62
ICRU 50- GTV, CTV, PTV, treated volume, irradiated volume
ICRU 62 - GTV, CTV, ITV, PTV, treated volume, irradiated volume
-62 introduced setup margin, internal margin
46
Define D5%
minimum dose received by the hottest 5 % of the
volume
-important for serial organs
47
Define V5%
volume that receives at least 5 % of the prescription dose
| -important for parralell organs
48
explain cumulative vs differential DVH
where cumulative is flat, differential is 0
differential has peak where cumulative changes
peak of target differential DVH is ideally located around Rx dose. Narrower peak = more homogeneous dose
-differential DVH is summed volume of elements that receive dose within given interval vs binned dose intervals
any point on cumulative DVH gives volume that receives dose greater than or equal to particular dose value
49
limitatios of DVH
- no spatial info
- assumes all parts of organ are equally important- organ function is uniformly distributed
- doesn't consider changes in organ over time
- doesn't tell you anything outside of organ
- in lung, irradiation of heart in addition to lung increases risk of radiation
50
difference between conventional RT and 3DCRT
Conventional RT – uniform intensity across beams involving square or rectangular fields
3D-CRT – uniform intensity across beams but irregular field shapes conformed to target
51
is there intensity modulation across the field for a single VMAT arc?
no?
| intensity modulation happens when the arc modulations are added together?
52
what are ways the VMAT vs conventional RT DVH might be different for an OAR?
3DCRT may have more dose to certain organs in the beam directions (select locations)
- VMAT- more dose wash- all OARs receive a little bit of dose
- OARs outside of the path of the 3DCRT beams will do better with 3DCRT but worse with VMAT. OARs in the path of the 3DCRT beamcs will do worse with 3DCRT but better with VMAT
- VMAT target DVH less homogeneous
- VMAT- more of the whole body gets a small amount of dose
53
aims for coverage
- 100% of the target getting 95% of the Rx dose, and a maximum dose of 107%
- 95% of the volume should get 100% of the Rx dose
54
does dose conformality affect the PTV DVH?
NO!!! Cannot see this in PTV DVH
55
how would a multi-prescription PTV DVH look like?
"steps" to show what volumne gets high dose vs low dose
| -steps not totally squared off due to high dose spill into low dose PTV
56
What does Sc account for?
- not for in-phantom scatter
- scatter in jaws, collimator, head, FF, monitor chamber, air
- defined at FS at isocenter
57
How is Sc measured?
mini phantom 4x4 cm2 cross section
scatter for FS larger than this is the same, so we isolate the effect of Sc
-depth in phantom is 10cm to remove electron contamination
-for small fields, use high density or Z mini phantom and correct for pertubation effects
58
How is Sp measured?
vary effective field size at phantom while keeping all other machine settings constant (block using something other than collimator)
OR
measure ScSp and Sc and then calculate Sp
59
equation for equivalent field size if there is additional shielding
d= square root (equivalent square field size ^2 x fraction unshielded)
60
Field size for Sc, Sp
Sc- actual jaw size (not equivalent square) at isocenter
| Sp- equivalent square at depth of measurement for TMR, at dmax for PDD
61
collimator exchange effect
Sc differes for rectangular fields of opposite directions (5X10 vs 10x5) due to positiion of X and Y jaws
62
Sc and Sp ranges for FS between 5x5 to 40x40 cm2
+/- 5 %
63
how to measure TAR?
Dq/Dq'
Dq is dose at point Q on CAX in phantom
Dq' is dose in a small mass of water (full build-up) in air at same point
64
what is peak scatter factor
TAR when depth is dmax
| PSF usually defined at surface even though TAR is defined at depth
65
define TPR and TMOR
tisuee phantom ratio = Dp/Dto where Dto is reference depth
| tissue maximum ratio is TPR where reference depth is dmax
66
what is backscatter factor
PSF for low-energy photons where dmax is at the surface
| -in TG61 for ortho, BSF is used to convert from dose in air to dose on surface of phantom
67
PDD, TAR, TPR, TMR dependence on SSD
only PDD increases with SSD
| with others, numerator and denominator are same distance from source
68
mAYNORF f-FACTOR
PDD1/PDD2 = ((f1+zmax)/(f1+z))^2 divided by ((f2+zmax)/(f2+z))^2
69
SSD is incorrectly set at unit- how does this affect doze at zmax?
- wants to compare doses at same depth, same jaw setting at dmax, but different SSDs, but normalized to dmax for first SSD
- just use ISL (SSD1+zmax)^2/(SSD2+zmax)^2
70
why does PDD fall of more quickly than TMR?
inverse square
71
TMR for 6 and 18 MV beams, 10x10 cm2 field size, at 5 cm, 10 cm, and 20 cm depth
6 MV: 0.92, 0.78, 0.52
| 18 MV: 0.99, 0.8, 0.71
72
what accounts for scattr changes with depth vs scatter changes with field size?
output factors- field size
| TPR/PDD/TMR- depth
73
scatter air ratio
gives scatter contribution to dose at point Q in a phantom per unit dose of small mass of water at same pt in air
SAR(z,A,hv) = TAR(z,A, hv) - TAR(z,0,hv)
74
equivalent field size
4* area/perimeter for rectangular
| square root of pi times radius for circular
75
calrkson's integration method
find dose function at a point in an irregular field
-divide field into N sectors of beam originicating at point of interest (circular fields of radius r, r is distance from point to edge of field)
Nth sector contributes 1/N of the full field
76
equatioon for MU denominator trick
think of denominator as converting reference dose rate conditions to prescription conditions
77
how to handle beam weighting in MU equation
multiply numerator by beam weighting
78
ISL in isocentric versus fixed SSD cases
-isocentric- ISL corrects for distance from point of interest to source vs calibration point to source
fixed SSD- ISL corrects for difference in distance of dmax for scenario of interest vs calibration dmax
79
what is RTAR
ratio of TAR to correct for onhomogeneity
-calculates the primary beam contribution accurately, but does not calculate the scatter contribution accurately because the lateral size, shape and location of the inhomogeneity are not taken into account. RTAR assumes that the heterogeneity is infinite in lateral dimensions
CF= TAR(effective radiological depth, FS)/TAR(physical depth, field size)
80
what if CT not available and all you have are 2D radiographs? How do you plan?
- prescribe to deepest extend of the lesion
| - choose apertures according to BEV radiographs
81
requested PTV coverage for palliative vs curative
90% vs 95%
82
common fractionations for palliative treatment
8/1 - bone mets, cord compression
20/5- bone mets, cord compression, brain
30/10, bone mets, cord compression, brain
37.5/15- brain, cord compression
83
palliative brain 3DCRT treatment
lateral POPs- prescribe to mid-separation at widest separation
84
palliative bone met, spine, cord compression treatments
POP or single beam
| sometimes peace sign for spine
85
what happens when you move norm point towards higher dose region
plan gets colder
86
what happens if you increase weighing on a beam?
the side of that beam gets hotter
87
typical skin cancer prescription
50/20
| add or subctract 5/5 (ex. 45/15)
88
HU of water
0
89
HU of air
-1000
90
HU of dense bone
1000
91
HU of fat
-20 to 100
92
HU of muscle
+44 to +60
93
HU of lung
-300
94
HU of blood
+40 to +60
95
what % dose error was found to manifest in patient response
7 %
96
AAPM report on accuracy of dose modelling
TG85
97
min equilibrium depth and radius for 10 MV beam
5 cm depth and 2 cm radius
98
who introduced concept of ITV, IM, SM?
ICRU 62
99
what happens to CI and treated volume as number of beam directions increases?
conformity index decreases (improves) but treated volume also increases
100
ICRU 50 vs ICRU 62 dose reporting
50: reference point dose, min and max dose to PTV
62: above plus dose available in planes/volumes, OAR, PRV, treated volume, irradiated volume, GTV, CTV, PTV
101
requirements for reference point
clinically relevant
no steep dose gradient
dose can be accurately determined
102
TNM staging system
```
T = extend of tumor
N= lymph node involvement
M= metastasis
```
103
definition of hot spot
region outside PTV where dose exceeds Rx, min diameter 15 mm
104
RBE
Dref/Dtest to achieve same biological endpoint
105
RBE for ortho beams, electron beams, photon beams
1.18 for ortho
| 1 for electron beams 1-50 MeV and photon beams 2 MeV
106
therapeutic ratio
TCP/NTCP
107
what does internal margin account for?
variation in size, shape and position of the CTV in relation the anatomical reference points
108
serial vs parrallel organ
-dose above tolerance even to small vo,ume can impair function (spinal cord)
parrallel- main parameter for function impairement is volume of organ that receives dose (lung)
109
internal and external reference points
-used to align patient
-internal= anatomical landmarks
external = tatoos, marks on immobilization device
110
PTVeval
used to evaluate DVH if PTV goes outside body structure
111
examples of systematic error
weight loss, tumor swelling or shrinking, technical errors
112
examples of random error
physiological processes, patient movement
113
density of air
0.001 g/cm3
114
density of fat/muscle
0.9-1.1 g/cm3
115
density of bone
1.1-1.8 g/cm3
116
density of metallic implants
3.8 g/cm3
117
CT sim rotation speed
1 rotation/s
118
CT Sim slice thickness
2. 5 mm
| 1. 25 mm for brain, stereo, H/N
119
2 categories of image fusion
also remember mutual information!
geometic-based: based on contour and surface matching
intensity-based: uses image intensity info
120
how DRRs generated?
digitally reconstructed radiograph
| -generated from 3DCT using BEV and ray tracing
121
what does obliquity do to skin dose?
increase it
122
methods to correct contour irregularities
- effective SSD (essentially corrects PDD for IS), ((f+zmax)/(f+h+zmax))^2, where PDD is evaluated at actual deoth z but with original SSD f
- ratio of TAR or TMR, T(d,r)/(T(d+h,r) where PDD is evaluated at d+h using original SSD
isodose shift method - shift entire dose distributions. For missing tissue, dshift away from source
123
batho power low method
-assumes slab
-considers position of inhomogeneity
-only applies to points within or downstream of inhomogeneity
CF = ((TPR(d3)^(p3-p2))/TPR(d2+d3)^(1-p2)))
-primary only
124
RTAR for inhomogeneities
Does not take into account the position of the inhomogeneity relative to the point of interest, its lateral size, or its shape. Therefore does not account for change in scatter dose; only correct primary contribution. Based on O’Connor scaling theorem.
CF= T(deff)/T(d)
-primary only
125
equivalent TAR method for inhomogeneities
ratio of TAR except the field size for effective depth is field size at the point of interest multiplied by the weighted density of the irradiated volume (found by averaging electron densities over all pixels, weighted by their relative contribution to the dose at the point of calculation – which can be obtained from Compton scatter cross sections
-primary and scatter
126
isodose shift for inhomogeneities
isodoses beyond inhomogeneity are moved by n* thickness of inhomogeneity
127
O'connor scaling theorem
if medium is half as dence, particle will travel twice as far
128
trick for remembering equation to convert from TMR to PDD (ignoring the ratio of PSF)
want PDD-
| ((SSD+ dmax))/(SSD + d))^2
129
what happens in bone for low vs high energies?
kV energy: higher dose due to high Z. Dose lower downstream due to attenuation
MV: higher electron density. dose downstream fro bone is lower due to more attenuation. Can be increased dose at interface due to pair production at high energies. For 1-6 MV, dose is lower at interface due to scatter build-up
-may be dose enhancement upstream due to backscatter (especially at high MV)
-Kv effects are more pronounced
130
acceptable transmission for a block
5%, achieved with 4.3 HVLs
131
why are blocks made with divergent shape?
to match divergence of beam, otherwise some of beam would go through part of the block leading to larger penumbra
132
difference between bolus and compensator
compensator maintains skin sparing due to large air gap
133
isodose curves with wedges
isodose curves tilt closer to surface at thick edge of wedge
134
how do wedges perturb beam quality?
mostly harden the beam
can soften beam due to PP interactions (occurs for > 15 MV)
-with depth, wedge becomes less effective at tilting isodose lines due to beam hardening
135
how far should wedges be from skin to prevent electron contamination from reaching patient skin?
15 cm
136
what is wedge angle?
angle between isodose line and line perpendicular to CAX
137
hinge angle
angle between central axes of wedged beam
| wedge angle = 90- hinge angle/2
138
pros of POP over single beam
more uniform dose in PTV, better covergage, lower hot spot
139
benefit of 3dcrt compared to vmat
no dose wash
| 3dcrt is less conformal- small geometric misses won't has as large an impact on coverage
140
what happens with increasing SSD?
slower fall-off (i.e. PDD is larger)
decreased dose rate
larger field size at particular depth
141
optimization parameters
```
beam quality/energy
angle/entry point
field size
number of beams
modulators
location of isocenter and reference point
SSD vs SAD setup
```
142
where might it be tough to achieve dose within 95-107% in PTV and no hot spots outside PTV?
breast treated with oblique tangents with wide separation where you only have access to low MV photons
(PERIPHERAL HOT SPOTS)
143
conventional simulation
fluoroscopic and radiographic imaging to identify and define treatment fields. 2D images.
144
why are leaf ends in TB rounded?
keeps leaf transmission ~ constant regardless of position in field
145
penumbra of MLCs vs jaws or blocks
MLC penumbra larger
146
MLC inter-lead and midleaf transmission
midleaf 2 %
interleaf <3 %
between closed leaves larger (thus close leaves under jaws)
147
how to oerient collmator?
nt at 0 degrees (or lines of dose leakage are consistent and not blurred across patient)
want direction of motion of leaves to be parrallel with smaller target dimension (smaller travel range)
148
what does leaf motion calculator do?
converts optimized fluence distribution to apertures
| -considers leaf edge shapes, motion limitations, transmission
149
FIF subfields- when to expose or cover norm point
when adding in subfields to block hot spot regions by removing weight from main fields, expose norm point
when adding it fields to add heat, cover norm point
-can merge subfields for convenience if same collimator angle etc.
150
penumbra spoliers
-for junctioning
creating a larger penumbra region so that overlap region is larger and effect of differences in field position are minimized
151
junctioning methods
- penumbra spoilers
- feathering (location of junction changes)
- beam blocking (if all beams blocked, probably no reason to feather since patient doesn't move in between)
- divergence matching (colli and couch)
152
is junctioning important in VMAT?
not as much as 3DCRT because VMAT has low dose gradients so is less sensitive to junctioning
153
what hapens when using half beam block?
tilting of isodose curves toward surface at blocked edge due to less scatter from blocked part of field into open part of field
154
where is junction for spinal fields chosen for cranial-spinal irradiation
-gap at skin surface is chosen so junction occurs at the cord
155
when is prone position difficult?
anasthesized patients (kids)
156
spine treated prone- how can we reduce dose to ant OARs?
use electrons
6-45 Gy dose,
-ex: 36 Gy in 20 fx
157
effect of beam energy on penumbra
kV energy photons have very lateral scattering resulting in large penumbra (but these are made sharp with collimation)
MV beams penumbra have longer ranges as energy increases
158
what happens to coverage for norm point at thin vs wide separation
thin separation- less coverage and less hot spots
| wide separation- better coverage but more hot spots
159
what occurs at low density inhomogeneity?
lower dose within due to lower cross-section, faster dose after due to less attenuation
at interface, lower dose due to reduced backscatter (fron of inhomogeneity) and (end of inhomogeneity)
160
size of horns vs beam energy
more apparent horns for lower energy
161
define quality of coverage
minimum volume isodose which covers the target / Rx dose
162
what isodose line in 4 field box plan has clover shape vs square shape vs cross shape?
99, 90, 50
163
POP hot spot values relative to mid separation dose for (10,20,30,40) cm patient separation for 6 M, 10MV, 18MV
6 MV: 100, 108, 130, 160
10 MV: 100, 105, 118, 140
18 MV:100, 103, 112, 130
164
what happens if you set patient up as SAD and patient was supposed to be SSD?
dose rate will be higher
FS will be larger
PDD will be smaller at shorter SSD
165
pro of using SSD set-up
allows for larger field size (but will have lower dose rate)
166
when do you include skin in evaluation structure?
when you are treating the skin
167
interface effects
bone
-enhanced upstram of bone due to increased backscatter. Enhanced downstream of bone due to 2nd range electrons. Then drops due to more attenuaion in bone
Lung
- dose low after lung due to build-up of scatter taking more distance
- higher dose since less attenuation in lung
-effects are worse for small field sizes and higher energy
168
kVp effect on PDD
| FS effect on kV PDD
falls off less steeply
| larger FS doesn't fall off as quickly
169
differences between kV, MV, and MeV isodose profiles
kV- large latersal penumbra, but electrons have less depth dose
- electron beams more uniform than kV for larger fields
- MV has sharper penumbra and penetrates deeper
170
why might you hesitate to use electrons near the eye?
electron beams have larger lateral penumbra
171
DVH for EBRT vs brachy
EBRT has sharp edge- brachy slope is shallower (more hetereogeneity)
172
• Describe the construction of an internal lip shield for electron therapy.
o Energy of beam determines thickness required. Range [cm] of electron in water ~ energy [MeV] / 2. Pb density is 11.34 g/cc. Can divide by this (or divide by 10 as rough approx.) to get Pb thickness required.
o Cover with wax to absorb low energy scatter off lead.
o Also want to cover with something because lead is toxic and this will go in someone’s mouth.
173
you find out MLC positions were not dowloaded for a 1/7 fields. What do you do?
calculate dose given only 6 fields delivered properly
do plan sum with this fraction and the rest of them to assess impact
discuss with RO. possibility of adjusting plan to make up for it
174
what are early responding OARs?
-skin, colon, testis
175
what are late responding OARs?
spinal cord
kidney
lung
bladder
176
when should imaging dose be added to the treatment plan?
when it exceeds 5 % of prescription, per TG180
177
why are patients biased on CTSim couch?
to move user origin closer to isocenter so less shift is required on bed, and patient can still get CBCT
178
what do tattoos help insure?
patient has no yaw or roll
| standardized to be in particular location depending on treatment site
179
where is user origin
interesection of lines through BBs and parrallel to coordinate axes
180
• A patient has lost weight during the course of their head and neck treatment and their mask no longer fits. The oncologist has ordered a new mask and CT scan for the patient and would like to continue treating with the current plan until a new plan is ready. He has contacted you to determine the dosimetric implications of this. Describe what you would do in this scenario to help the oncologist make his decision.
o Discuss potential for intra-fraction motion given that mask is now too loose.
o Can deform planning CT to match daily treatment CBCT (use extended range CBCT ideally). Then recalc current plan on deformed CT to assess dosimetric implications. Discuss with RO, noting that deformed contours may not be accurate (don’t just blindly look at DVHs). Tell RO that they must assess accuracy of deformed contours, image registration and dose distribution.
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• Patient abdomen body contour consistently smaller than it was at CT sim. RO asks for help deciding how to proceed. What do you do?
-assess deformed CBCT- run plan to assess impact and discuss with RO
-o Assess whether it is weight loss or gas.
Can argue that gas won’t be a problem since amount of tissue traversed by beam is unchanged. This is valid as long as the gas is not directly adjacent to target.
o Assess patient setup (have they been setup wrong, with a rotation)
o Assess beam entry points. Contour change that does not occur at beam entry point is generally less of an issue.
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• The dosimetrist calls you with a patient who has bilateral hip replacement as seen on a CT scan. The physician wants to treat the pelvis with a standard 4-field beam arrangement. What advice would you give the dosimetrist in the case?
o Consider acquiring planning CT with higher energy beam, if available (e.g., MV CBCT using EPID, tomotherapy)
o Regions of artefact (e.g., dark streaks) that are outside of prostheses resulting in non-representative HU values should be contoured and set to HU=0 (assuming water equivalent region is more appropriate than streaky artefacts).
o Instead of ant/post/right/left beams, should use oblique beams that avoid entering or exiting through prostheses since this may result in strong attenuation and may not be properly modelled in TPS.
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• Create a plan QA checklist for standard 2 field tangent-breast technique.
Check that there is 2 cm flash.
Isocentre should be near chest wall-lung interface to minimize divergence into lung.
Check documentation to see if it is a breath hold patient (for left sided treatments). If so, this should be indicated appropriately (for example in the plan name, consistent across all documents, etc.)
Arms should be above head using breast board immobilization equipment (unless particular reason why patient can’t do this)
In evaluating treatment plan, look for intensity modulation using FiF technique or wedges.
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• A large patient, being simulated for a right breast treatment, undergoes a CT scan and her anatomy does not fit within the standard 50 cm field-of-view. The patient has missing anatomy on both the right and left sides. The therapists call you for advice; describe what options you have at the CT simulator in this case.
o Breast typically treated using partial arcs or tangents so beam enter/exit on one side. So it is more desirable to have more missing anatomy on the not treated side than a smaller amount of missing anatomy on both sides (which will have a larger clinical impact in terms of calculating dose accurately). Therefore, shift patient so that right breast (being treated) is closer to centre of CT bore [this means user origin closer to right breast I think].
o Otherwise could manually add in extrapolated contours set to HU=0 where there is missing tissue. However, this is an approximation since don’t actually know where body contour is and won’t be able to account for heterogeneities.
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• List pros and cons of MR sim (see III.73)
o Pros: potentially more accurate target contouring due to improved soft tissue contrast, treatment planning for MV photons not strongly affected by small changes in HU (so requirements for synthetic CT are not particularly onerous), only one sim appointment instead of two in cases where MRI would be requested by the RO anyway.
o Cons: synthetic CT generation may run into issues with tissues that have similar MR signals but different HU values and therefore different x-ray attenuation properties. Unlike x-ray CT, MRI is prone to geometric distortion due to e.g., susceptibility artefacts
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• What is the most common PET radiotracer? What is its half life? Why do you use PET in radiation oncology?
```
o Fluorodeoxyglucose (FDG); half-life = 110 minutes
o FDG is a glucose analog, which has more uptake in metabolically active cells such as cancer cells. This is helpful for localizing the target, for identifying lymph node involvement and for finding distant metastases.
```
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what does dose verification accomplish?
1) data transfered from TPS to machine properly
2) the machine can deliver the plan
3) ensure the TPS is calculating the dose properly
188
different ways of doing dose verification
1) portal dosimetry
2) 2d or 3d detector arrays
3) film and ion chamber in solid water phantom
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• A physician wishes to use in 18 MeV electron beam and an electron cut out of 4 x 4 cm2 to treat a 3 cm diameter area extending from the surface to a depth of 6 cm. The dose is prescribed to the 80% isodose line. Are there any issues with this treatment?
o Using {2,3,4,5} rule, {R100, R90, R50, Rp} = {3.6, 5.4, 7.2, 9} cm
Also R80 [cm] ~ E [MeV] / 3 = 6 cm so choice of energy is appropriate at first glance
o ISSUES WITH SMALL ELECTRON FIELDS: Rule of thumb states that if FS < Rp, the lateral scatter equilibrium does not exist on CAX – dmax, R90 (therapeutic range) and R80 decrease from values expected with larger FS according to {2,3,4,5} rule, dose fall off is less steep (Rp unaffected because is determined by max beam energy), surface dose increases relative to dmax, flatness of the beam profile compromised
So target may not actually be covered by desired dose level.
Flatness compromised: lateral constriction of high value isodose curves especially with deeper depths, small FS and higher energies means that lateral edges of target may not be well covered.
o Should measure actual output factor for this cutout due to issues described above.
o At dmax, Generally the 20%–80% width is expected to be 10 mm to 12 mm for electron beams below 10 MeV, and 8 mm to 10 mm for electron beams between 10 MeV and 20 MeV (smaller penumbra for higher energy because higher energy electrons more forward directed). So the choice of field size is appropriate, given size of penumbra.
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• How is the commissioning process different for a soft/dynamic wedge compared to a hard/physical wedge?
- entire jaw sequence must be considered; jaw motion must be QAd
- hard wedge- must check interlock functionality
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• What if the DVHs for a critical structure from two competing plans cross? Which would you select and why?.
depends on if it is serial or parrallel structure- want either minimal dmax or minimal volume exposed
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tolerance dose for pacemakers
TG-203
- 2 Gy, preferable < 0.5 Gy
- acceptable distance from field edge is 10 cm
- if it has to be in field, monitor
- avoid E of 10 MV or more as don't want to expose pacemaker to neutrons
o To estimate CIED dose:
If > 10 cm from field edge – dose probably < 2 Gy
If < 10 cm but > 3 cm – need to measure dose to know (use in vivo dosimetry e.g., OSLD)
Within 3 cm – can rely on TPS
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dose from MV image, kV image
1-2 cGy MV
| 0.1 cGy kV
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for static beams, pros and cons of more/less beams
-more beams can be splay of lower isodose lines due to overlap, but also yields lower entrance and exit doses
195
For 4DCT, in on what phase images does the physician contour the target?
0, 50, MIP
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draw isodose lines around a target
remember only 95% of volume gets the coverage- so target should always be surrounded by 95% isodose line or 86% (stereo)
DONT EVEN MAKE A POINT ON DVH 0R DIAGRAM USING 100 % VOLUME GETS 100 % DOSE
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internal margin and set-up margin
IM- variation in size, shape and position of CTV in relation to anatomical reference point
SM- inaccuracies and lack of reproducibility in patient-beam positioning (technical factors that can in theory be reduced by more accurate setup and immobilization, improved mechanical stability of the machine (elimination of sag))
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is heart parrallel or serial?
coronary arteries are serial aspect, and myocardium is the parallel aspect
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universal wedges
can be used with any field size
| part of wedge only contributes to attenuation of beam, not to isodose tilting
200
optimal relationship between hinge angle and wedge angle
wedge angle = 90 - hinge angle/2
201
POP max dose with regards to patient separation and energy
higher for larger separation and smaller energy
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millenium MLCs
5 mm x 40 pairs plus 10mm x 20 pairs, max FS 40x40
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HD MLCs
2.5 mm x 32 pairs plus 5 mm x 28 pairs, FS 40X22
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how to avoid 3-field overlap when using opposing fields?
make ratio of field sizes = ratio of SSDs
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angles to rotate colli or couch if divergent matching
always arcatan (1/2 L / SSD or SAD)
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equation for gap for junction at depth z
gap = (1/2 L1 *z/SSD) + (1/2 L2 *z/SSD), L1 and L2 are the field sizes
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important point about half beam block and isocentere location
isocentre must be at location where you want to block the beam
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for junctioning with opposiung fields in spine, what is equation for region of 3 beam overlap?
3 beam overlap = (1/2 L1 *z/SSD) - (1/2 L2 *z/SSD), L1 and L2 are the field sizes, z is depth of junction. Thus, if SSD and FS of beams or ratio is same, the overlap will be 0
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As FS increases from 5x5 to 40x40, how much does dmax decrease by?
~ 1/3
210
effect of inverse square law near isocentre
~ 2%/cm
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surface dose of electron beams rule of thumb
76 + E
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Output factor of fields vs 10x10 field
Halve the field- drop by 10%
double the field - increase by 10%
quadruple field- increase by 20%
and so on
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Varian Millenium MLC specs
- tongue or groove width = 0.078 cm
- mid-leaf transmission = 1.1 % (epsilon) - throught a solid leaf
- interleaf transmission = 2.6 % (lambda) m-i.e. tongue and groove are touch
- tongue or groove transmission = 19 % between leaf ends
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differences and pros cons for AAA, Acuros
AAA- analytical anasotropic algorithm
-anisotropic analytical algorithm
-convolution-superposition
-divides the calculations in depth and lateral
-separate convolutions for the primary source, secondary source, wedge, and electron contamination. Uses separate Monte Carlo derived modeling for primary photons, scattered extra-focal photons, and electrons scattered.
-The lateral dose deposition characteristics are modeled with six exponential curves.
-The functional shapes of the fundamental physical expressions in the AAA enable analytical convolution, which significantly reduces the computational time.
-The AAA accounts for tissue heterogeneity anisotropically in the entire three-dimensional
neighborhood of an interaction site, by using photon scatter kernels in multiple lateral directions
(Scatter Kernels).
- A polyenergetic scatter kernel is constructed as a weighted sum of the monoenergetic
scatter kernels. During the 3D dose calculation these kernels are scaled according to the
densities of the actual patient tissues determined from the CT images.
- patient body volume is divided into a matrix of 3D calculation voxels. The geometry of the calculation voxel grid is divergent, aligning the coordinate system with the beam fanlines.
-convolutions are performed for all finite-sized beamlets that make up the broad beam: each beamlet is assumed to have uniform photon fluence
-The final dose distribution is obtained by the superposition of the dose calculated with photon and electron convolutions.
-For 4 MV to 6 MV energies and field sizes larger than or equal to 5×5 cm2, AAA tends to
underestimate the dose in lung and overestimate the dose in water-equivalent tissue after the
lung.
-For 10 MV to 20 MV energy modes and field sizes smaller than or equal to 5×5 cm2, AAA tends
to overestimate the dose in lung.
Acuros- solves linear boltzmann equation using numerical methods
- linear BTE assumes radiation only interact with the matter they are passing through and not with each other; is valid for conditions without external magnetic fields
- uses cartesian system and covers entire calc volume
- similar dose accuracy for homogeneous tissue, acuros more accurate for inhomogeneous tissue
- errors are systematic and result from discretization of variables in space, energy, angle
- in eclipse, uses same source model as AAA
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AAA overview
-MC is used to derive kernals that describe distribution of secondary particles and energy transport at the interaction point
-uses separate convolution models for primary photons, scattered photons, scattered electrons
-convolutions are applied to many small beamlets into which the beam is divided
-superposition of the dose calculated by photon and electron convolutions for each beamlet allos us to obtain the final dose distribution
-corrects for tissue heterogeneity anisitropically surounding point irradiation
-employs density scaling
'
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Acuros overview
- applies deterministic solution of linear boltzmann equation, uses numerical methods
- takes effect of heterogeneity directly into account
- reports dose to water and dose to medium
1) machine sources are modelled (same as AAA) and ray tracing is used to create primary fluence of photons and electrons in the patient
2) acuros discretizes in space, angle, and energy to iteratively solve the LBTE for electron and photon fluence in the patient
3) dose in all voxels is calculated using energy-dependent fluence to dose response function to loval energy dependent electron fluence in that voxel
- since it calculates dose with the actual material, requires not only density but also chemical composition of the materials
- acuros XB includes 5 biologic materials: bone, lung, cartilage, adipose tissue, muscle
- has 2 reporting options: dose to water and dose to medium, which bases the energy dependent response function on either water or the voxel material
- Dose to water is what a small mass of water in the mterial would have (i.e. if an ion chamber were there, and didn't perturb the electron fluence)
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MC overview
- stochastic solution to linear boltzmann equation
- random samplings of particles in media
- requires interaction probability distributions and random number generator
- select 2 random numbers, R1, R2
- distance photon goes before interacting X = -ln (R1)
- R2 determines if its compton vs pair production vs photoelectric (from probability distribution function)
Simple MC simulation:
1) sample particle, energy, direction, starting position
2) sample distance to interaction
3) sample type of interaction
4) sample direction, enrgy, of new particles
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why use a long cone in ortho?
if treating uneven surface like the nose, effect of IS on different contours is less signiciant at long SSD
-also easier to get more uniform, larger field when needed
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condensed history steps in MC
-individual scattering events are grouped via multipke scattering theories
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Fano theorem
Under conditions of equilibrium in an infinite
medium, the particle fluence will not be altered
by density variations from point to point”
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what lesions can be treated with ortho
skin, mouth, rectal carcinoma
| -usually no more than 2-3 cm deep
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difference between ortho vs MV calibration
electron range so small in ortho that dose is all from photons (not from electron stopping power like it is with MV)
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Dose fall off for brachy (400 kV) is faster than ortho (300 kV) – why is this?
In brachy we are on the steep decline of the IS curve- thus fall off is fast. In ortho we are on the flatter part- thus fall off Is slow (ortho PDD starts on the flatter part of the curve).
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why do long ortho cones have closed ends?
because the electron contamination becomes problematic throughout the cone so the bottom is closed
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how to get ortho calibration factor in air Nk?
-electrons in air have range- but calibrate ion chamber in free air standard to account for the electrons in air (Nk). Box has to be large enough such that electrons created in the wall are not collected. For tissue, electrons have small range so use ratio of uen/p.
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In ortho, is it sufficient to estimate output factors for different applicators using the ratio of
the backscatter factors corresponding to the respective field sizes
no because scatter from inside cone may be significant
The output factor
for each individual applicator must be measured at each beam quality. Can do this using ion chamber and BW scatter factors from TG-61 for each sized cone (preferred, BW is needed since we look at dose to surface) or by using solid water with PP chamber- this is essentially measuring your own BW factor. Remember when measuring output factors for long cones that they have virtual sources and thus different IS! Have to correct for IS.
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How does MU count work in ortho?
-ortho uses primary MU chamber and secondary timer. Physicist tells it what the dose for a number of counts and time is.
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differences in backscatter that the ortho vs MV ion chamber sees
-for MV linacs, the ion chamber in the linac will see different backscatter depending on the FS. It does not correct for this (assumes 100 MU is 100 cGy at 10x10); however, the impact is very small. Photon backscatter from the field-defining collimators into the monitor can also
have some effect on the output because monitor chambers are calibrated to count an
MU for a known amount of charge collection. Several investigators (Sharpe et al.
1995; Yu et al. 1996; Liu et al. 1997a; Lam et al. 1998) have shown that the monitor
backscatter signal can vary from 1% to 3% for the largest to the smallest fields.
The effect of the monitor backscatter is to increase the output per monitor unit with
increasing field size. For ortho, the impact of using different cones is large (backscatter is very dependent, ion chamber sees very different amount of backscatter). Have to commission the # of counts for 100 MU for each cone at commissioning so that we account for this
-machine saves this factor as relative to cone C or G- so if we change the output per calibration of cone C, the relative factor will take care of it for all the cones
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uncertainties for ortho dosimetry
5%
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calibation for ortho in air at surface
D = MNkBwPstem * ratio of uen/p to water from air
- uen/p depends on HVL
- Bw depends on field diameter, SSD, and HVL
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how does TMR relate to TAR?
TMR = TAR/PSF
```
have 3 beams with dose D at depth, dose dmax at dmax depth, and dose Q in air, all have same SAD
write TAR = D/Q
PSF= dmax/Q
and TMR = D/dmax
rearrange
```
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why do cobalt sources have larger penumbra?
cobalt source is a cylinder 1-2 cm diameter vs 3 mm diameter pencil electron beam in MV linac that interacts with target
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dose rate for fresh Co-60 machine
240 cGy/min at SAD of 80 cm
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How to calibrate Co-60 machine?
measure time it takes for source to deposit dose (source is always on)
235
advantages of treating rectal patient prone
pushes bowel out of way
| however less stable
236
formula for calculating geometric penumbra
P = source size * (SSD+depth-SCD)/SCD)
237
formula for calculating geometric penumbra
P = source size * (SSD+depth-SCD)/SCD
238
at what separation should you increase from 6 MV o 10 MV?
23 cm
239
how much does dose increase behind lung for a 10 MV beam?
2%/cm
240
why use electrons to treat IMN nodes?
-can reduce dose to lungs by using a less wide tangent field for the breasts
241
treated volume vs irradiated volume
treated volume is encompassed by prscription isodose line
| irradiated volume is encompassed by 50% isodose line
242
where is GTV with regards to PTV on DVH?
GTV always has to get more dose than PTV- to right of PTV
243
describe collapsed cone convolution
A method for photon beam dose calculations is described. The primary photon beam is raytraced through the patient, and the distribution of total radiant energy released into the patient is calculated. Polyenergetic energy deposition kernels are calculated from the spectrum of the beam, using a database of monoenergetic kernels. It is shown that the polyenergetic kernels can be analytically described with high precision by (A exp( -ar) + B exp( -br)/r2, where A, a, B, and b depend on the angle with respect to the impinging photons and the accelerating potential, and r is the radial distance. Numerical values of A, a, B, and b are derived and used to convolve energy deposition kernels with the total energy released per unit mass (TERMA) to yield dose distributions. The convolution is facilitated by the introduction of the collapsed cone approximation. In this approximation, all energy released into coaxial cones of equal solid angle, from volume elements on the cone axis, is rectilinearly transported, attenuated, and deposited in elements on the axis. Scaling of the kernels is implicitly done during the convolution procedure to fully account for inhomogeneities present in the irradiated volume. The number of computational operations needed to compute the dose with the method is proportional to the number of calculation points.
think similar to comvolution superposition, just uses cone approximation to reduce amount of calculation
-computes dose to less points (number of voxels times number of cones Nx M instead of NxNxN points)
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what is wedged pair used to do?
reduce dose where the 2 beams overlap the most, at the shallowest depth
245
field borders for tangent breasts
superior- 1 cm above breast tissue
inferior- 2 cm below inframmary line
medial- midsternum
lateral- midaxillary line
246
why does surface dose increase with FS?
more electron contamination of the beam due to scatter interactions with the collimator and air
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what happens to surface dose and dmax when using oblique photon beam?
surface dose increases
-dmax depth decreases from perpendicular surface. It is mesured perpendicular to the skin and when beam i oblique, it travels a longer path length to reach a particular perpendicular depth
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how to increase surface dose without decreasing penetration at depth?
use a beam spoiler to increase electron contamination without sigificantly attenuating the beam
249
impact of SSD on depth of dmax
increasesdepth of dmax increases with SSD due to less contaminant electrons
250
why use rounded MLC leaf ends?
attenution occurs in rounded ends along chords of the circle
- penumbra is same width regardless of position of leaf in beam
- constant beam transmission through lead ends, regardless of position of leaf in beam
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how to lower dose variation in TBI?
• Higher energy lower dose variation (excluding effects of buildup region which is somewhat mitigated by exit dose from opposing beam anyway; can also be mitigated with beam spoiler)
• Larger SSD lower dose variation (ISL smaller effect)
• Larger patient thickness larger dose variation
• AP/PA treatments will yield variations not larger than 15% in most cases.
• Lateral opposed beams = larger dose variations (homogeneity within +/-10% may be achievable with high energies, large SSD, for small e.g., pediatric patients)
-patient thickness in lat direction usually larger
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what is dose limiting organ in TBI/TMI
lung
radiation induced pneumonia
o Difficult to establish dose-response relationship because pneumonitis also associated with chemo and graft vs host disease
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methods to protect lungs in TMI/TBI
use Pb to shield lung. Verufy position using EPID, kV, light field
cross arms over chest
if lungs are shielded, may need to boost rib dose with electrons
o Limit dose rate at mid-separation at level of umbilicus to no more than 15 cGy/min to minimize lung toxicity.
o Typically want to keep max dose to lung below 12 Gy = Rx
o May also shield kidneys
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max dose to lung in TBI
12 Gy
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typical places to measure entrance and exit dose with TLD or OSLD in TBI
cheek, spine, thorax, under lung/kidney attenuator, leg, ankle
Dosimeter on inner thigh (where legs touch) is used as surrogate for midline dose.
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what are issues with TBI in-vivo dose evaluation?
o Very large fields patient close to floor/walls electron and photon backscatter to patient skin and to dosimeters on patient skin
o Correcting for tissue inhomogeneities in hand calcs (empirical, correction based approach)
o Difficulty in achieving reproducible set; patient motion over course of long treatment times.
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how can you reduce backscattered radiation from walls of treatment room?
place low Z absorber between patient and wall
258
medical issues during treatment
nausea, vomiting, diarrhea, fever, chills, headache, fatigue, anorexia, parotitis, immunosuppression making patient susceptible to infection
259
medical issues after treatment with TBI
cataracts, cardiotoxicity, pneumonitis, gonadal failure, induced menopause, sterility, graft vs. host, secondary malignancy
260
commissioning considerations for TBI
o Same basic dosimetric parameters as for conventional RT but measured under TBI conditions (i.e., at extended SSD, large FS)
- output calibration PDD or TMR
- need large enough water phantom to provide full scatter
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how to measure Sc for TBI?
Measuring Sc in air not recommended [I think should just measure overall Sc,p in phantom, correcting for limited phantom size if necessary] since with large FS, there will be scatter from walls/floor which is difficult to correct for.
-output factors aren't needed if only 1 FS is used for all treatments
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Issues with horns and large fields
• Linac beams with FF may have unacceptable horns at edge of beam such that an inverse FF may be used (made of Perspex/acrylic/polymethyl methacrylate/lucite/Plexiglas) – should be designed based on measurements at depth (not in air)
263
goal for dose profile uniformity in TBI
+/-10% variation within central 80% of field
264
issue with inverse square law and TBI commissioning
inverse square law which is valid near iso may not be valid at SSD = 3-5 m.
o ISL should be tested over the range of possible treatment distances that may be used. If deviations from ISL are >2% then dose calibrations should be performed at various distances.
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issues with water phantom with TBI commissioning
Need water phantom large enough to provide full scatter.
• AAPM TG-17 has multiplicative correction factors to adjust data measured in limited phantom sizes to data for larger/infinite phantoms (that are wide enough and deep enough to provide full scatter conditions). Correction factors are > 1 because need to add in effects of additional scatter.
o AAPM TG-17 recommends phantom no smaller than 30x30x30 cc. Water phantom should be used.
• In practice, need to shift the tank around to measure full profile
• Can add large containers of water surrounding measurement phantom
• Then for patient treatments, must correct for actual area of patient intersecting the radiation beam as well as patient thickness. E.g., doses at extremities will be reduced due to lack of scatter. Use Sp?
(ie go from small tank, to huge body of water, to patient size)
266
issue with ion chamber and TBI commissioning
Large portion of ion chamber cable irradiated (stem effect; extracameral contributions)
• Can shield the chamber to assess magnitude of this contribution. Knowing the expected decrease in dose due to shielding, the contribution due to the cable can be determined.
With low dose rate due to extended SSD, leakage signal may be considerable
o Do test run using anthropomorphic phantom using TLDs or film embedded in phantom
267
how to deal with contour variations?
RTAR method to account for missing/additional tissue. Or can use bolus/compensators (can use bolus as long as loss of skin sparing is not a problem; can use compensator instead)
268
what is recommended for lung inhomogeneity correction?
ETAR
-ETAR has improved accuracy for large fields compared to simple RTAR method.
ETAR is same as RTAR except that the field size for the TAR on the numerator is the field size at the point of interest multiplied by the weighted density of the irradiated volume (found by averaging electron densities over all pixels, weighted by their relative contribution to the dose at the point of calculation – which can be obtained from Compton scatter cross sections).
• This method corrects for scatter dose changes via use of an effective field size.
• For example, denser region corresponds to effectively larger field size.
269
what is the issue with using MU calcs instead of measuring TBI data?
o Must verify that TMR data obtained under standard conditions (at iso) are still valid at extended SSD
Do dose verification measurement in a phantom; also in vivo meas.
o Also need ESQ at point of calculation. Values are tabulated for Rando phantom for various beam energies and body sites. These values were obtained by Clarkson integration of scatter function (SAR or SMR) and finding which square field in water phantom at same depth has same scatter function.
270
considerations for shiedling if treating TBI
o Barrier that patient stands in front will have relatively large use factor.
o TBI workload must be scaled back to isocentre (patient is not treated at isocentre) – i.e., need higher dose at iso to achieve given dose at extended SSD – use ISL
o Patient scatter originating not from iso
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TBI at NSHA
o Patient lying supine on stretcher, treated with left/right horizontal beams.
o Gantry at 275 degrees – pointing slightly down because stretcher is slightly too low (want beam aimed at patient midline at level of umbilicus)
o Collimator at 45 degrees, 40x40 cm2 FS, 18 MV beam, spoiler is used
o SAD = 5 m (from target to patient midline at level of umbilicus)
o Spoiler is 45 cm from patient midline at level of umbilicus
o Tattoos on patient at umbilicus, lung. Additional room lasers to position the patient on the stretcher.
o Lung shields placed on spoiler. Designed based on CV sim projections, average lung thickness. Patient arms are down and shield exists where arms do not cover lungs.
o Light field used to check setup.
o Correction-based empirical calculation on Excel spreadsheet. Plan parameters are manually input into Eclipse.
o Compensators go on tray on linac head. Made of stacked lead sheets 0.8 mm thick.
o Lung dose limited to 12 Gy.
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Rationale for TMI vs TBI
use intensity modulation to target bone marrow rather than entire body as PTV.
o Reduce dose to OARs relative to target (increase therapeutic ratio); reduce side effects; potentially allow for dose escalation
273
NSHA's TMI method
o CT sim: scan head first then feet first and co-register these two image sets.
CT scan length = 130 cm
o Thermoplastic S-frame mask plus vaclok; 1 cm TE bolus from knees to ankles; 0.5 cm TE bolus between elbow and wrist
274
CTV for TMI at NSHA
bones excluding mandible, nasal bones, metacarpals and phalanges of hands. Brain and spinal cord may be included in CTV.
o Legs are treated using extended SSD POPs; usually two isocentres needed (4 fields). Patient is feet first for this.
o Rest of body treated with VMAT. Five isocentres along cranial caudal direction (2 arcs per iso; 10 arcs total: H&N, 2 chest, abdo, pelvis
These are optimized simultaneously using leg POPs as baseplan
o Each iso is separated out into distinct plan so that kV-kV pair can be used for matching.
Non-standard orthogonal images (e.g., 315 and 45 degrees) can be used where arms are in way of spine.
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PTV for TMI/TBI
o PTV = CTV plus soft tissues between adjacent bones
| For TBI, PTV is entire body
276
dose limiting organs in TMI/TBI
TMI: lungs, kidneys, liver, heart
TBI: only considers lungs
277
constraints in TMI for mean lung, liver, heart, kidney dose
< 70% of Rx = 8.4 Gy
278
what other machine can be used to do TMI?
tomotherapy
DVHs of PTV and OARs are very similar for helical tomotherapy and VMAT with some studies showing an increase of healthy tissue sparing by 8-18% with VMAT compared to helical tomo. Other studies show OAR dose reductions with VMAT compared to tomotherapy ranging from 4-50%
279
OARs to contour in TMI
```
lung, liver, kidney, heart
lenses, eyes, oral cavity
thyroid, parotids, esophagus
stomach, small bowel, rectum
genitalia, bladder, spleen
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TSEI
total skin electron irradiation
a special radiotherapy technique which aims to deliver a uniform dose to the entire skin of a patient while sparing all other organs
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specifications for TSEI single field
Physical specification of large stationary electron field at extended SSD:
Profiles: dose uniformity (want +/- 5% cf CAX in central 80% of field area)
Beam energy – want 4-7 MeV at patient surface (corresponding to 6-10 MeV at waveguide exit)
PDDs – want to characterize brems photon contamination contribution (tail of PDD – so must measure PDD deep enough)
Dose rate – want to achieve ~1 Gy/min so that treatment times aren’t too long
• May need to increase beam current to achieve this. Must evaluate monitor chamber linearity
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specifications for field combinations in TSEI
Also need to determine dose distribution resulting from superposition of fields. Oblique entry makes prediction of dose distribution resulting from overlapping fields difficult.
Use cylindrical tissue equivalent phantom (diameter = height = 30 cm) with film sandwiched in between layers.
Also could measure with anthropomorphic phantom with TLDs embedded in surface. Assess hot/cold spots – need shielding (e.g., eyes, nails)? Boost fields?
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considerations for TSEI
o Need ability to run electron beam without applicator
o Use scatterer to improve field uniformity
o Use energy degrader to reduce beam energy to 4-7 MeV at patient surface
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different techniques for TSEI
patient lying prone/supine on translating couch, patient standing on rotating platform, multiple static fields at extended SSD
• Stanford technique:
The Stanford technique is con- sidered the standard treatment. It consists of a six dual field technique with the six positions spaced at 60 degree intervals about patients longitudinal axis, in a 2 day cycle with 3 dual field positions treated every other day.
Esentially, if treating the patient standing up, there are 6 treatment fields to consider (2 ant/post and 4 obliques). With a SSD of 3-4 m, cannot fit entire patient body into a field. Stanford technique combines 2 treatment fields
o 200x80 cm2 FS at 3-5 m SSD
o 6 fields: ant, post and 4 obliques. Deliver 3 per fraction.
o Cover vert with 2 abutting fields (CAX of lower field below feet; CAX of upper field above head). Fields angled 10-20 degrees from horizontal to minimize brems reaching patient (brems tends to be forward directed along CAX).
o Patient stands and holds handle; alternate which hand is holding handle
o 1 mm Al scatterer plus TE energy degrader
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prescription for TSEI
• Rx: 40 Gy in 20 fx to skin surface at level of umbilicus
o Goal is to treat entire body to depth of at most 1 cm
• Mainly used for mycosis fungoides, a cutaneous T-cell lymphoma.
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Issue with vertical treatment
have to prescribe additional dose (26-28 Gy) to soles of feet
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describe the 2 rotational techniques for TSEI
classic and rotary dual
classic- large field, patient is rotated in field, need large SSD and also need scatters to get uniform dose distribution
rotaty dual- uses dual field to allow for smaller SSD
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penumbra for POP vs single field
penumbra are about 5mm larger for POP than for single field for 80/20 and 90/10, about 10 mm larger for 95/5
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penumbra for 4-field box vs single field
80/20 and 90/10 from corner of box 80 or 90% to 20 or 10 % iso is about 6 mm / 8 mm more than single field
50-10 % is about 10 mm 6x, 11 mm 10 X
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4-field using 10x vs 6 x
6x clover is more apparent
| 10 x 100% resembles more like a square (more homogeneous)
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hot spot in breast tangents with and without FIF
without: 114 % at nipple, 105-110 % at beam entry points
with: 105 % at nipple, beam entry points
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ortho shielding thicknesses
8 mm Pb layers
1- 100 kV- brings to 1 %
2-180 kV- brings to 0.6%
3- 300 kV- brings to 4.2 %
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penumbra for 4 field box, 6 MV
95-80% is 3.7 mm
80-50% (at corner) is 3 mm
50-20% is 7 mm
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penumbra for POP, 6 MV
95-80 % is 2 mm
80-50 % is 2mm
50-20 % is 6 mm
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penumbra for electrons 9 MeV
95 to 80 % is 1.5 mm
80 to 50 % is 3.5 mm
50-20 % is 6 mm
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penumbra for electrons 9 MeV
95 to 80 % is 1.5 mm
80 to 50 % is 3.5 mm
50-20 % is 6 mm
95 to 20 % at bulge is 14 mm
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during what phase are cells most sensitive to radiation
G2 and mitosis
| resistant during S phase
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is APBI BID?
yes
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angle-down technique
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avoid shoulders (hard to immobolize, unecessary dose)
come in at neck from oblique superior angles to treat target in the neck
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good place to junction beams
at the top of the target (so no cold spot in target)
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what is gamma pod?
specifically for breast treatment
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why are non coplanar beams easier to implement for brain treatment?
can enter through the skull
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trick for remembering PRV size
if above C2, use 3 mm
| if not use 5 mm
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typical MLC over travel distance
10 cm
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value of 180 kVp PDD at 10 cm depth
20%
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where are IMRT and VMAT most usefuul?
where you want high conformity due to surrounding OARs
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• How do you check if wedges are centered properly?
• First, ensure that chamber is on CAX (on collimator rotation axis): take measurements at two collimator angles 180 degrees apart – adjust chamber position until readings are equal within 1%. X and Y jaws are at different heights so expect different readings at different collimator angles if chamber not centred.
