stereo Flashcards
typical stereo bone fractionations
24/2
30-40/5
max hot spot allowed in bone
135 % (150% of prescription dose)
constraints for spinal cord PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 17 Gy
3 fx = 20 Gy
5 fx = 25 Gy
constraints for brainstem PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 19 Gy
3 fx = 23 Gy
5 fx = 31 Gy
constraints for heart PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 26 Gy
3 fx = 30 Gy
5 fx = 38 Gy
constraints for esophagus PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 20 Gy
3 fx = 27 Gy
5 fx = 35 Gy
constraints for stomach PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 20 Gy
3 fx = 22 Gy
5 fx = 32 Gy
constraints for bowel PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 20 Gy
3 fx = 25 Gy
5 fx = 29 Gy
constraints for chest wall PRV DMAX, 2 fx, 3 fx, 5 fx
2 fx = 37 Gy
3 fx = 44 Gy
5 fx = 55 Gy
lung constraints for bone stereo
V20Gy < 3 %
mld < 5 Gy
stereo fractionations for brain
18-22/1
27/3
30-35/5
single fraction brainstem constraint
DMAX < 12.5 Gy
single fraction spinal cord constraint
DMAX < 13 Gy
single fraction brain - PTV constraint
< 10 cc 12 Gy
constraint for lens, eye, optic chiasm, 1-5 fxs
Dmax<10 Gy
in stereo, eye and optic nerves and chiasm <25 Gy for 5 fx, 20 Gy (eye), 17.4 Gy (optic nerves and chiasms) for 3 fx,
constraint for hippocampi, 1-5 fx
D100% <6.6 Gy
3 fraction brain - PTV constraint
< 10 cc 19 Gy
5 fraction brain - PTV constraint
< 10 cc 23 Gy
why DIBH or DEBH for liver?
-what are OARs?
- at HFX, liver less contorted with DEBH
- all about movement, not trying to get lung to expand
- OARs are bowel, stomach
liver fractionations
30-60/1 to 6, treatment every other day (usually 5 fractions)
look at mean liver - GTV dose- if too high, reduce prescription dose and re-evaluate. Dose should be 13-17 Gy for highest to lowest doses
what BED do we aim for in liver
BED10 of 100 Gy min in PTV (apha/beta = 10 Gy)
lung movement for free breathing vs bresath management
5-7 mm
max hot spot for lung and brain
108 % (120 % of PD)
define R50%
ratio of 50 % isodose to PD
- volume must be no greater than a set R50%, typically ~ 5 for standard PTV volume
- R50% we aim for decreases as PTV size increases
Define D2cm
- max total dose to any point 2 cm or greater awat from PTV
- aim for about 50% for standard PTV size
conformity index
- want < 1.2
- treated volume/ PTV volume
- to ensure volumes are actually conformal, use Paddick index ((union of volume)^2/(PTV * treated volume)
lung fractionation
34/1 30/1 48/4 (peripheral) 54/3 (peripheral) 60/8 60/15
1 fx constraint spinal cord dmax
14 Gy
1 fx constraint esophagus dmax
15 Gy
1 fx constraint heart dmax
22 Gy
8 fx constraints, spinal cord PRV dmax
32 Gy
lung constraints, 1-15 fx
V20Gy< 10 %
lung constraint 1-8 fx
mld < 6 Gy
lung constraint 15 fx
mld < 14 Gy
8 fx constraint, esophagus dmax
40 Gy
15 fx constraint, esophagus dmax
50 Gy
does DVH have longer tail in SBRT?
Yes, because more heteorgeneous
when do you do single fraction lung SABR vs multi fraction?
single: tumour diamters < 3 cm, > 2 cm away from chest wall, diaphgram, and lung apex
multi: tumour diameter < 5 cm, >2 cm from proximal tracheal bronchial tree
size of tuning rings
1-4 cm depending on person and site
prescription dose levels for liver
GTV is 108 % (120% of PD)
PTV is 90 %
What OARs is one concerned about for SABR re-treatment in lung?
- bronchi, esophagus, trachea
- major vessels seem to tolerate high dose
PTV margin for spine
ctv + 2mm
CTV margin for stereo
usually none
why use contrast for liver?
aids in tumor visualization
also need constrat free studies for planning (to avoid incorrect effective depth due to presence of high density contrast agent)
liver constraint
> 700 cc to get < 15 G
-mean liver dose (liver – GTV) has to be 13-17 Gy depending on dose
issue with using 1 isocetner for multiple mets
rotation of patient can mean mets at a distance away from isocenter won’t be treated
- more severe for smaller mets
- usually want all mets within 5 cm of iso
what is AAPM report on SRS
TG42
approximate time estimates for implementing SRS program
10 weeks commercial
3 years to design hardware and software
2years to do own software
1 year do do own hardware
stereo time requirements for treating typical patient
20 % of a typical patient weekly load
how many cobalt beams in gamma knife/
201 or 192, about 30 Ci each
precision of gamma knife vs stereo linac
0.1-0.4 mm vs 1 mm
dose % from gamma knife to eyes, hyroid, breast, gonads
eyes = 2.5 % thyroid = 0.2 % breast = 0.06% gonads = 0.02%
dose fall-off 45-90% in different tissues for stereo
brain - 6%/mm
bone - 4%/mm
lung- 3%/mm
how big does one expect distoritions from MRI to be?
displacements of 20 cm or more from the center of the magnetic field can produce distortions of 4 mm in the image
defining characteristics of SBRT
High dose per fraction High heterogeneity (prescribe to smaller isodose) sharp dose fall off High conformity
how to get sharp dose fall off in SBRT?
non-coplanar beams
small MLC widths (2.5 mm)
lower beam energy
what is often assessed for stereo plans that is not in conventional plans?
conformity index heterogeneity index (ratio of highest dose received by 5 % pf PTV to lowest dose received by 95 % of PTV)
why is physicist present for first stereo plan?
verify TP, machine, immobilization device, isocenter, make sure localization algorithm didn’t match to the wrong vertebrae!
why do non coplanar beams help with dose fall-off?
POP entrance and exits super-pose-non coplanar beams help avoid this
-more compact dose distribution
what are advantages of circular beams over rectangular ones?
easier calc of 3D dose distribution more precise dose delivery sharper beams better field definition for small fields faster dose fall-off outside volume
the 3 types of linac stereo
multiple non co planar or coplanar arcs- startionary patient
conical- patient rotates on treatment chair while gantry is stationary
dynamic- gantry and patient rotate in unison
advtange of conical SRT
all points of beam entry are in upper hemisphere on patients head and all exits are in lower hemisphere; never a POP
what is sacrificed in stereo for conformity?
homogeneity
what isodose is prescribed to for gamma kinfe? stereo linac?
-gamma kinfe- 50%
stereo linac- 80 or 90 %
pros and cons of using more isocenters
pro: more conformity, less dose to OARs
con: more heterogeneity in target (shallower DVH fall-off for target), more complexity, and more scattered/leakage dose to patient
what is a shot in gamma knife?
elliptical region of high dose
can be 4, 8, 16 mm
how to deal with irregular shapes in gamma knife?
superimpose multiple shots
what does gamma knife inverse planning optimize?
shot size, location, weights
compare gamma knife “performance” to linac SRT in terms of dose to normal brain and targeting
- linac is 2-6 X more dose to normal brain
- gamma precision is 0.1- 0.4 mm vs 1 mm
- gamma confirms target area 10 X/s, with linac target area is confirmed once/10 s
disadvantage of gamma knife in terms of resources
- imaging (CT, MRI, angiography) and plan must be done same day of treatment if using invasive head frame
- only does brain
- sources must be exchanged every 5 years, dose rate decreases over time
is gamma knife or linac more expensive?
gamma knife 7 million
linac 2.5 million (but needs more shielding)
describe a typical gamma knife procedure
MRI with gadolinium and MPRage
- also do angiograms where we subtract one image from the other and move the images by one pixel to get a shadow of the anatomy. Then be blood vessels with contrast show up and we can see the AVM really well
- on same day, plan treatment on MRI and treat patient
- CBCT on gamma unit is used for registration
- stereo IR system manages motion- bed moves between slots for different isocenters
typical gamma knife dose
- 70-90Gy to 50% isodose line for neuralgia
what makes sharp dose penumbra for gamma knife?
small source to collimator distance
smaller penumbra due to proximity
what is inhomogeneity index in SRS?
ratio of max dose to prescription dose
initial gamma knife dose rate
3 Gy/min
typical spine fractionations
24/2
30/4
35/5
what are some contraindications for spinal SABR?
- ECOG 3 or 4
- spinal cord compression
- spinal instability from compression fracture
- > 9 cm length to be treated
Do we use non-coplanar beams with spine SABR?
No, no clearance
abdo stereo fractions
60/8 (4 fx a week)
45/5 (alternate days)
-also good for prostate, 26/2 also done in prostate
what is an arteriovenous malformation?
high pressure vascular lesions that can cause cerebral hemmorhage
-happen when a group of blood vessels forms incorrectly
what is the hippo avoidance zone?
5 mm expansion of hippocampus
how is brain uveal melanoma treated?
usually treated with plaque brachy.
If patient doesnt’t want to travel or have brachy, or melanoma is too close to optic nerve, then SRT is an option (60 Gy/10)
where does liver get its mets from?
GI, breast, and lung primaries
sites that get treated with SRT
-arterivenous malformations
-metastases
-uveal melanomas
-non-malignant brain tumours
NOT primary brain malignancies
what size lesion is SRS considered for?
< 2.5 cm diameter
why do we prescribe to 90%? And allow hot spots?
-easier to achieve a fast dose fall-off
is immobilization included in the body contour?
yes because the beam passes through it
how much dose do non-coplanar arcs add to structures outside the plane?
VMAT- 5 %
describe interplay effect
loss of coverage if you have motion with lots of modulation
advantages of using FF for cranial mets
- easier to target more mets with a flat beam vs a FFF beam
- easier to verify
what is tumor motion directly correlated to?
proximity to diagphram
what is the SRS arc MU limit
6000 MU, could need extra arcs for single fraction lung due to this limit
trick we can use if OAR abbuts PTV
move isocenter to between OAR and PTV so you get 1/2 beam block and sharpest dose fall-off
what can you do if patient has pacemaker
use non-coplanar arc to get out of pacemaker plane
describe how cyber knife works
robot moves and bends around patient to deliver radiation from many beam angles
- image guidance gets stereoscopic kV images, tracks tumor motion, and guides the robotic manipulator to align the treatment beam to moving tumor
- x-ray sources on ceiling, detectors on floor
what is ICRU 91
prescribing, recording, and reporting of stereotactic treatments with small photon beams
ICRU 91 gradient index
GI = Volume getting half of prscription dose/volume getting prescription dose
differences between ICRU 50, 61, 83, 91 wrt prescriptions
- ICRU 50 and 61- to a reference point
- ICRU 83- prescribes o value of D in DV
- ICRU 91- prescribes to covering isodose surface of PTV
- ICRU 50 and 62 reported only Dmin,Dmax
- ICRU 83 reported more dose levels
- ICRU 91 reported more dose levels and also CI, GI
2 major causes of incidents in stereo
errors in commissioning (small field measurements)
treatment parameter transfers (wrong SRS cones, wrong side/site)
typical spine SABR arcs
3 full arcs
one has 90 degree collimator to help with sparing the cord
Disadvantage of Cyber Knife
long treatment
why not over-modulate with stereo
- interplay effect
- smaller segments- more of beam is penumbra. If using high energy, get more neutron production due to penetration through MLCs
what does stereo rely on instead of radibiology?
physical principles
how can you deal with the “donut” shape spine and trying to spare the cord?
use half beam block. Put isocenter in middle. As the field rotates it only does half the PTV at a time; cord is blocked
- one arc at 45 degrees, with beam blocked on one end. Other arc at 345 degrees with beam blocked on other end
- third arc with 90 degree collimator to help spare cord so MLCs don’t move across the cord
dose constraints for sacral plexus, 2 fx, 3 fx, 5 fx
20 Gy, 24 Gy, 32 Gy
Do Cyber knives have MLCs?
yes, latest versions
41 leaf pairs, 2.5 mm wide at 80 cm SSD
linac SRS cone sizes (back in day)
4-40 mm circular fields
why do the cones yield sharper dose fall-off?
made of tungsten, strongly attenuate beam in cone
key item regarding Cyberknife treatment
non-isocentric
cone diameters in cyber knife
5-60 mm diameter at 80 cm SSD
cyber knife positional accuracy
sub-millimeter (< 0.95 mm to < 1.5 mm depending on tracking method)
options for collimation in Cyber Knife
- fixed cones
- MLC
- dynamic variable aperture iris- allows for field sizes to be varied during treatment delivery
disadvantage of cyberknife
only hypofractionation
what is node in cyberknife
pre-assigned points in space where the robot can stop and deliver dose. At each node, the linac can deliver radiation from multiple beam angles (12 pointing directions)
calibration point of cyberknife
60 mm diameter field size, 80 cm SSD, 1.5 cm depth
how many nodes does a typical cyber knife treatment plan use?
> 50 nodes (of 100 available)
In practice, not all nodes are available because of objects within the treatment room that obstruct the path of certain beams or prevent the robotic arm from positioning the LINAC at a particular node
95-200 beams
The robotic arm moves the LINAC sequentially through the prescribed nodes during treatment. The LINAC stops at each node, the imaging system checks the target position, and corrective changes are then made accordingly.
describe the cyber knife targeting algorithm
places patient at center of 80 cm^3 sphere
On the surface of the sphere, there are 100 equally spaced points called nodes. At each node the robot defines 12 beams of radiation that intersect various portions of the tumor volume. The robotic arm stops at each node where radiation beams of a specific prescribed dose are administered.
difference between linac and cyberknife design
linac = S band (microwave)
cyberknife- X band (higher energy)- allows for a smaller, more compact build
difference between linac and cyberknife design
linac = S band (microwaven, 3 GHz)
cyberknife- X band (higher energy, 9.3 GHz)- allows for a smaller, more compact build
accelerator cross sectional area is 10 x less in X band vs S band
energy of cyber knife
6 MV
fractionation for benign tumours
12/1, 25/5, 30-35/5
fractionation for AVM
16-22/1
advantage of linac over cyber knife or gamma
larger FS can treat multiple locations at once
can do other types of therapy (not just brain or stere)
explain dynamic conformal arc therapy
leaves conform to a moving target
CT Sim requirements for stereo
thinner slices (1.25 mm)
difference between QUANTEC, HYTEC, PENTEC
QUANTEC: Quantitative Analyses of Normal Tissue Effects in the Clinic (2 Gy fractions, only looks at NTCP)
HYTEC: Hypofractionated treatment effects in the clinic (looks at both TCP and NTCP)
PENTEC: pediatric normal tissue effects in the clinic
why is cyber knife only for hypofractionation?
field sizes it delivers would take a really long time with conventional fractionation
radiation toxicity to spinal cord is called what?
- myelopathy
- causes weakness, loss of senstation
radiation toxicity to lung causes what?
pneumonitis
side effects of cranial radiation
tinnitus fatigue headache nausea gait disturbance brain function issues hearing loss
radiation toxicity to bowel causes what?
enteritis- inflammation of bowel- diarrhea, nausea, vomiting, cramps
radiation toxicity to rectum causes what?
proctitis- inflammation of rectum - rectal bleeding, pain, diarrhea, tenesmus (urge to defecate),
radiation toxicity to stomach causes what?
enteritis- inflammation of bowel- diarrhea, nausea, vomiting, cramps
radiation toxicity to bladder causes what?
radiation cystitis (inflammation of baldder) incontinence, blood in urine (hematuria), retention,
why might a stereo plan look less compact and more “splayed”
if we limited beam entry points
trick for planning multiple sites and concerns about dose overlap
make the isodose of one plan a structure, and avoid it on the other plan
why do spine SABR over conventional SABR?
better pain control
why 6x vs 10x for stereo?
- higher energy = higher ranged secondary electrons- more difficult to get build-up. Issue in stereo for small PTVs
- also higher energy = more neutron contamination from beam interacting with MLCs
pre-SRS QA test
SIMPLIFIED:
- VERIFY MECHANICAL OF MACHINE WITH LASERS
- VERIFY LASERS (MACHINE MECH.) WITH OPTICAL ISOCENTER
- VERIFY OPTICAL AUTO REALIGNMENT TO ISOCENTER BY MISALIGNING PHANTOM AND HAVING EXACTRAC LINE IT UP
- TAKE IMAGES OF HIDDEN PHANTOM WITH X-RAY TO VERIFY X-RAY IMAGING CENTER WITH MECH/OPTICAL ISOCENTER
- VERIFY RADIATION ISOCENTER WITH MECH ISOCENTER USING WL TEST
BED of 48/4, 60/8,54/3,60/15 fir lpha/beta o 10 Gy
48/4 adnd 60/8 - BED 105 Gy
60/15- BED 84 Gy
54/3- BED 151 Gy
advantage of usingDIBH in lung
- less motion
- may pull tumor away fom mediatinum
- improves V20Gy
What is Veff
normal liver volume wich if irradiated to the Rx dose would be associaed with the same NTCP as the non-uniform dose actually delivered
-hould be 60% to less than <25% for lower to higher doses
what is thecal sac?
membranous sheath that surrounds spinal cord and cauda equina. Provides nutrients and buyoncy to the cord
liver and kidney mean dose limits for all spine SBRT fractionations
5 Gy
what have studies shown for WBRT vs SRS?
-SRS had same control as WBRT and less side effects for < 10 mets
for a shift treshold of 1 degree and 1 mm, what is max allowable distance from isocenter (in a multi-met plan)
1mm/ (tan 1 degree)= 5.7 cm
max lesion size for cranial stereo
5 cm
typical fractionations for non-malignant brain tumours
25/5
50/25
50.4/28
how does radiation fix AVMs?
-leads to fibrosis that has same effect of separating the vein and artery as surgery would
what prescription isodose line is used for stereo with cones?
50%
can you do multiple mets with one isocenter using cone linac stereo?
no
benefit of cone vs MLC
MLC has larger penumbra due to leakage through leaves and they are farther away from the patient (larger geometric penumbra)
PTV/PRV for SRS/SRT vs SBRT
2 mm for SRS/SRT (except hippocampus which is 5 mm), 5 mm for SBRT
downside of E2E test
If process fails you don’t know where it happened
ideal conformity index
1
for conventional definition, CI is worse if larger
if using Padick index larger CI than 1 is more conformal
what do frameless cranial stereo treatments use?
fiducials implanted in the skull
does WL tell you anything about imaging system?
NO
image center could be off of radiation center0 we don’t test this with wl
explain synchrony respiratory motion tracking in cyber knife
X-rays and optical tracking are used to develop a model that relates the motion of the surrogate (the LED markers on the patient) with the internal motion of the tumour (via x-ray imaging of implanted fiducial markers). Optical tracking is at 30 Hz. X-ray images are obtained every 30-60 s during treatment to check that the model is accurate (within some tolerance level).
-can also track other anatomical landmarks
max field size for cyberknife with MLCs
12 cm x 10 cm
3 ways that cyber knife can track motionop
optically, stereoscopic x-rays, or combination of both (synchrony)
explain what frequency is in S band or Xband
frequency of microwave power output by klystron
The klystron is an RF power amplifier which uses electron acceleration and deceleration to produce high power microwaves on the order of 7 MW or higher. This microwave power produced by either the klystron or the magnetron is carried to the accelerating waveguide via another waveguide. In the accelerating waveguide, electrons are accelerated.
difference between klystron and magnetronk
klystron- electrons move linearly
magnetron- electrons follow a spiral path from cathode to anode
issue with X band waveguide
need more heat dissipation
disadvantage of synchrony in cyber knife
requires regular breathing pattern
show isodose lines for a target with prescription at 90% vs 100%
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
latest version of gamma knife
Icon, has kV on board imaging
