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Treatment Planning > stereo > Flashcards

stereo Flashcards

1
Q

typical stereo bone fractionations

A

24/2

30-40/5

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2
Q

max hot spot allowed in bone

A

135 % (150% of prescription dose)

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3
Q

constraints for spinal cord PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 17 Gy
3 fx = 20 Gy
5 fx = 25 Gy

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4
Q

constraints for brainstem PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 19 Gy
3 fx = 23 Gy
5 fx = 31 Gy

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5
Q

constraints for heart PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 26 Gy
3 fx = 30 Gy
5 fx = 38 Gy

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6
Q

constraints for esophagus PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 20 Gy
3 fx = 27 Gy
5 fx = 35 Gy

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7
Q

constraints for stomach PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 20 Gy
3 fx = 22 Gy
5 fx = 32 Gy

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8
Q

constraints for bowel PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 20 Gy
3 fx = 25 Gy
5 fx = 29 Gy

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9
Q

constraints for chest wall PRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 37 Gy
3 fx = 44 Gy
5 fx = 55 Gy

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10
Q

constraints for skinPRV DMAX, 2 fx, 3 fx, 5 fx

A

2 fx = 26 Gy
3 fx = 33 Gy
5 fx = 40 Gy

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11
Q

lung constraints for bone stereo

A

V20Gy < 3 %

mld < 5 Gy

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12
Q

stereo fractionations for brain

A

18-22/1
27/3
30-35/5

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13
Q

single fraction brainstem constraint

A

DMAX < 12.5 Gy

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14
Q

single fraction spinal cord constraint

A

DMAX < 13 Gy

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15
Q

single fraction brain - PTV constraint

A

< 10 cc 12 Gy

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16
Q

constraint for lens, eye, optic chiasm, 1-5 fxs

A

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,

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17
Q

constraint for hippocampi, 1-5 fx

A

D100% <6.6 Gy

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18
Q

3 fraction brain - PTV constraint

A

< 10 cc 19 Gy

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19
Q

5 fraction brain - PTV constraint

A

< 10 cc 23 Gy

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20
Q

why DIBH or DEBH for liver?

-what are OARs?

A
  • at HFX, liver less contorted with DEBH
  • all about movement, not trying to get lung to expand
  • OARs are bowel, stomach
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21
Q

liver fractionations

A

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

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22
Q

what BED do we aim for in liver

A

BED10 of 100 Gy min in PTV (apha/beta = 10 Gy)

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23
Q

lung movement for free breathing vs bresath management

A

5-7 mm

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24
Q

when do we use 4DCT?

A

For free breathing treatments (breath hold will use 3D CT)

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25
Q

max hot spot for lung and brain

A

108 % (120 % of PD)

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26
Q

define R50%

A

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
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27
Q

Define D2cm

A
  • max total dose to any point 2 cm or greater awat from PTV

- aim for about 50% for standard PTV size

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28
Q

conformity index

A
  • want < 1.2
  • treated volume/ PTV volume
  • to ensure volumes are actually conformal, use Paddick index ((union of volume)^2/(PTV * treated volume)
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29
Q

lung fractionation

A 
34/1
30/1
48/4 (peripheral)
54/3 (peripheral)
60/8
60/15
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30
Q

1 fx constraint spinal cord dmax

A

14 Gy

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31
Q

1 fx constraint esophagus dmax

A

15 Gy

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32
Q

1 fx constraint heart dmax

A

22 Gy

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33
Q

8 fx constraints, spinal cord PRV dmax

A

32 Gy

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34
Q

lung constraints, 1-15 fx

A

V20Gy< 10 %

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35
Q

lung constraint 1-8 fx

A

mld < 6 Gy

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36
Q

lung constraint 15 fx

A

mld < 14 Gy

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37
Q

8 fx constraint, esophagus dmax

A

40 Gy

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38
Q

15 fx constraint, esophagus dmax

A

50 Gy

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39
Q

does DVH have longer tail in SBRT?

A

Yes, because more heteorgeneous

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40
Q

what can exactrac do?

A

stereoscopic imaging ie 3D info from 2 planar images

  • can do IR tracking with IR camera using IR rflectors on immobilization equipment
  • couch is brain lab so can use exactrac to shift couch per exactrac match
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41
Q

when do you do single fraction lung SABR vs multi fraction?

A

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

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42
Q

size of tuning rings

A

1-4 cm depending on person and site

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43
Q

prescription dose levels for liver

A

GTV is 108 % (120% of PD)

PTV is 90 %

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44
Q

What OARs is one concerned about for SABR re-treatment in lung?

A
  • bronchi, esophagus, trachea

- major vessels seem to tolerate high dose

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45
Q

PTV margin for spine

A

ctv + 2mm

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46
Q

CTV margin for stereo

A

usually none

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47
Q

why use contrast for liver?

A

aids in tumor visualization
also need constrat free studies for planning (to avoid incorrect effective depth due to presence of high density contrast agent)

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48
Q

liver constraint

A

> 700 cc to get < 15 G

-mean liver dose (liver – GTV) has to be 13-17 Gy depending on dose

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49
Q

tricks to get nice dose fall off in ptimization

A

crop GTV and PTV- optimize separately

use tuning rings

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50
Q

issue with using 1 isocetner for multiple mets

A

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
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51
Q

what is AAPM report on SRS

A

TG42

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52
Q

approximate time estimates for implementing SRS program

A

10 weeks commercial
3 years to design hardware and software
2years to do own software
1 year do do own hardware

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53
Q

stereo time requirements for treating typical patient

A

20 % of a typical patient weekly load

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54
Q

how many cobalt beams in gamma knife/

A

201 or 192, about 30 Ci each

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55
Q

precision of gamma knife vs stereo linac

A

0.1-0.4 mm vs 1 mm

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56
Q

dose % from gamma knife to eyes, hyroid, breast, gonads

A 
eyes = 2.5 %
thyroid = 0.2 %
breast = 0.06%
gonads = 0.02%
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57
Q

dose fall-off 45-90% in different tissues for stereo

A

brain - 6%/mm
bone - 4%/mm
lung- 3%/mm

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58
Q

how big does one expect distoritions from MRI to be?

A

displacements of 20 cm or more from the center of the magnetic field can produce distortions of 4 mm in the image

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59
Q

defining characteristics of SBRT

A 
High dose per fraction
High heterogeneity (prescribe to smaller isodose)
sharp dose fall off
High conformity
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60
Q

is there a CTV in stereo?

A

not usually

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61
Q

hypothesis regarding hot spots in tumor

A

may help kill radioresistant hypoxic cells

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62
Q

how to get sharp dose fall off in SBRT?

A

non-coplanar beams
small MLC widths (2.5 mm)
lower beam energy

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63
Q

what dose grid resolution is required for SBRT?

A

2 mm or finer

2.5 mm grid = 1 % accuracy, 4 mm grid = 5 % acccuracy

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64
Q

what is often assessed for stereo plans that is not in conventional plans?

A 
conformity index 
heterogeneity index (ratio of highest dose received by 5 % pf PTV to lowest dose received by 95 % of PTV)
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65
Q

why is physicist present for first stereo plan?

A

verify TP, machine, immobilization device, isocenter, make sure localization algorithm didn’t match to the wrong vertebrae!

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66
Q

definition of stereotaxy

A

3D superposition of a fixed coordinate system onto a given organ

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67
Q

does stereo brachy exist?

A

yes

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68
Q

why do non coplanar beams help with dose fall-off?

A

POP entrance and exits super-pose-non coplanar beams help avoid this
-more compact dose distribution

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69
Q

what are advantages of circular beams over rectangular ones?

A 
easier calc of 3D dose distribution
more precise dose delivery
sharper beams
better field definition for small fields
faster dose fall-off outside volume
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70
Q

the 3 types of linac stereo

A

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

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71
Q

advtange of conical SRT

A

all points of beam entry are in upper hemisphere on patients head and all exits are in lower hemisphere; never a POP

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72
Q

be able to replicate the picture of the brains that shows tretment trajectory of gamma knife, linac, conical linac, cyberknife

A

-2 pi geometry is ideal but difficult to do in practise because of constraints on beam entry points due to equipment design

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73
Q

what is sacrificed in stereo for conformity?

A

homogeneity

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74
Q

what isodose is prescribed to for gamma kinfe? stereo linac?

A

-gamma kinfe- 50%

stereo linac- 80 or 90 %

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75
Q

pros and cons of using more isocenters

A

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

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76
Q

what is shutter error of gamma unit?

A

0.05 min- difficult to account for this as time resolution of unit is only 0.1 min

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77
Q

how does dmax change with field size in circular beams?

A
  • dmax increases with field size due to in-phantom scatter

- in standard linac beams, dmax decreases with field size due to contaminant electrons from FF, collimator jaws, and air

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78
Q

what is a shot in gamma knife?

A

elliptical region of high dose

can be 4, 8, 16 mm

79
Q

how to deal with irregular shapes in gamma knife?

A

superimpose multiple shots

80
Q

what does gamma knife inverse planning optimize?

A

shot size, location, weights

81
Q

compare gamma knife “performance” to linac SRT in terms of dose to normal brain and targeting

A
  • 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
82
Q

disadvantage of gamma knife in terms of resources

A
  • 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
83
Q

is gamma knife or linac more expensive?

A

gamma knife 7 million

linac 2.5 million (but needs more shielding)

84
Q

describe a typical gamma knife procedure

A

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
85
Q

typical gamma knife dose

A
  • 70-90Gy to 50% isodose line for neuralgia

- 15-24 Gy for mets

86
Q

what makes sharp dose penumbra for gamma knife?

A

small source to collimator distance

smaller penumbra due to proximity

87
Q

what is inhomogeneity index in SRS?

A

ratio of max dose to prescription dose

88
Q

initial gamma knife dose rate

A

3 Gy/min

89
Q

typical spine fractionations

A

24/2
30/4
35/5

90
Q

what are some contraindications for spinal SABR?

A
  • ECOG 3 or 4
  • spinal cord compression
  • spinal instability from compression fracture
  • > 9 cm length to be treated
91
Q

Do we use non-coplanar beams with spine SABR?

A

No, no clearance

92
Q

abdo stereo fractions

A

60/8 (4 fx a week)
45/5 (alternate days)
-also good for prostate, 26/2 also done in prostate

93
Q

what is an arteriovenous malformation?

A

high pressure vascular lesions that can cause cerebral hemmorhage
-happen when a group of blood vessels forms incorrectly

94
Q

what is the hippo avoidance zone?

A

5 mm expansion of hippocampus

95
Q

how is brain uveal melanoma treated?

A

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)

96
Q

where does liver get its mets from?

A

GI, breast, and lung primaries

97
Q

sites that get treated with SRT

A

-arterivenous malformations
-metastases
-uveal melanomas
-non-malignant brain tumours
NOT primary brain malignancies

98
Q

what size lesion is SRS considered for?

A

< 2.5 cm diameter

99
Q

why do we prescribe to 90%? And allow hot spots?

A

-easier to achieve a fast dose fall-off

100
Q

what skin volume is used in stereo planning?

A

5 mm wall

101
Q

is immobilization included in the body contour?

A

yes because the beam passes through it

102
Q

how much dose do non-coplanar arcs add to structures outside the plane?

A

VMAT- 5 %

103
Q

describe interplay effect

A

loss of coverage if you have motion with lots of modulation

104
Q

why do we cut out the exactrac bbs in treatment plan?

A

-sim bbs are radio-opaque; treatment bbs are radio-transparent

105
Q

max # of slices for exactrac

A

400

106
Q

do you include brain structures in optimizer?

A

-yes, but usually only include rings in lung unless OARs are involved

107
Q

advantages of using FF for cranial mets

A
  • easier to target more mets with a flat beam vs a FFF beam

- easier to verify

108
Q

what is tumor motion directly correlated to?

A

proximity to diagphram

109
Q

what is the SRS arc MU limit

A

6000 MU, could need extra arcs for single fraction lung due to this limit

110
Q

trick we can use if OAR abbuts PTV

A

move isocenter to between OAR and PTV so you get 1/2 beam block and sharpest dose fall-off

111
Q

what can you do if patient has pacemaker

A

use non-coplanar arc to get out of pacemaker plane

112
Q

describe how cyber knife works

A

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
113
Q

what type of algorithm fdoes ICRU 91 recommend for hetereogeneous densities?

A
  • type B (accounts for lateral electron transport)

- type A does not

114
Q

what is ICRU 91

A

prescribing, recording, and reporting of stereotactic treatments with small photon beams

115
Q

ICRU 91 gradient index

A

GI = Volume getting half of prscription dose/volume getting prescription dose

116
Q

differences between ICRU 50, 61, 83, 91 wrt prescriptions

A
  • 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
117
Q

2 major causes of incidents in stereo

A

errors in commissioning (small field measurements)

treatment parameter transfers (wrong SRS cones, wrong side/site)

118
Q

typical spine SABR arcs

A

3 full arcs

one has 90 degree collimator to help with sparing the cord

119
Q

Disadvantage of Cyber Knife

A

long treatment

120
Q

why not over-modulate with stereo

A
  • interplay effect
  • smaller segments- more of beam is penumbra. If using high energy, get more neutron production due to penetration through MLCs
121
Q

what does stereo rely on instead of radibiology?

A

physical principles

122
Q

how can you deal with the “donut” shape spine and trying to spare the cord?

A

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
123
Q

dose constraints for sacral plexus, 2 fx, 3 fx, 5 fx

A

20 Gy, 24 Gy, 32 Gy

124
Q

Do Cyber knives have MLCs?

A

yes, latest versions

41 leaf pairs, 2.5 mm wide at 80 cm SSD

125
Q

linac SRS cone sizes (back in day)

A

4-40 mm circular fields

126
Q

why do the cones yield sharper dose fall-off?

A

made of tungsten, strongly attenuate beam in cone

127
Q

key item regarding Cyberknife treatment

A

non-isocentric

128
Q

cone diameters in cyber knife

A

5-60 mm diameter at 80 cm SSD

129
Q

cyber knife positional accuracy

A

sub-millimeter (< 0.95 mm to < 1.5 mm depending on tracking method)

130
Q

options for collimation in Cyber Knife

A
  • fixed cones
  • MLC
  • dynamic variable aperture iris- allows for field sizes to be varied during treatment delivery
131
Q

disadvantage of cyberknife

A

only hypofractionation

132
Q

what is node in cyberknife

A

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)

133
Q

calibration point of cyberknife

A

60 mm diameter field size, 80 cm SSD, 1.5 cm depth

134
Q

how many nodes does a typical cyber knife treatment plan use?

A

> 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.

135
Q

describe the cyber knife targeting algorithm

A

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.

136
Q

difference between linac and cyberknife design

A

linac = S band (microwave)

cyberknife- X band (higher energy)- allows for a smaller, more compact build

137
Q

difference between linac and cyberknife design

A

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

138
Q

energy of cyber knife

A

6 MV

139
Q

fractionation for benign tumours

A

12/1, 25/5, 30-35/5

140
Q

fractionation for AVM

A

16-22/1

141
Q

advantage of linac over cyber knife or gamma

A

larger FS can treat multiple locations at once

can do other types of therapy (not just brain or stere)

142
Q

explain dynamic conformal arc therapy

A

leaves conform to a moving target

143
Q

CT Sim requirements for stereo

A

thinner slices (1.25 mm)

144
Q

difference between QUANTEC, HYTEC, PENTEC

A

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

145
Q

why is cyber knife only for hypofractionation?

A

field sizes it delivers would take a really long time with conventional fractionation

146
Q

find images of brain, abdomen, lung in eclipse and know the structures!

A

-Pituitary is post to chiasm
-hippos are sup to ear and eye structures
optic nerves connect eyes to chiasm
-spinal cord and brainstem are post to center of skull
-Know spleen, stomach, pancreas, aorta, vena cava, esophagus, trachea, bowel, liver, heart, chest wall, breast, lungs, prostate, bladdr, rectum, seminal vesicles, breast nodes, illiac nodes, obturator nodes, vagina, uterus

147
Q

radiation toxicity to spinal cord is called what?

A
  • myelopathy

- causes weakness, loss of senstation

148
Q

radiation toxicity to lung causes what?

A

pneumonitis

149
Q

side effects of cranial radiation

A 
tinnitus
fatigue
headache
nausea
gait disturbance
brain function issues
hearing loss
150
Q

radiation toxicity to bowel causes what?

A

enteritis- inflammation of bowel- diarrhea, nausea, vomiting, cramps

151
Q

radiation toxicity to rectum causes what?

A

proctitis- inflammation of rectum - rectal bleeding, pain, diarrhea, tenesmus (urge to defecate),

152
Q

radiation toxicity to stomach causes what?

A

enteritis- inflammation of bowel- diarrhea, nausea, vomiting, cramps

153
Q

radiation toxicity to bladder causes what?

A 
radiation cystitis (inflammation of baldder)
incontinence, blood in urine (hematuria), retention,
154
Q

why might a stereo plan look less compact and more “splayed”

A

if we limited beam entry points

155
Q

trick for planning multiple sites and concerns about dose overlap

A

make the isodose of one plan a structure, and avoid it on the other plan

156
Q

why do spine SABR over conventional SABR?

A

better pain control

157
Q

why 6x vs 10x for stereo?

A
  • 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
158
Q

pre-SRS QA test

A

WL

  • test that lasers match to mechanical isocenter
  • • Line up “isocentre phantom” to lasers. Detect IR markers of this phantom to check calibration of the IR system isocentre (i.e. optical isocentre with mechanical isocentre).
  • misalign iceberg phantom (slightly off isocenter)
  • verify that the IR-based automatic repositioning shift software and x-ray imaging marker detection is working properly (ie after shift, check that lasers line up to appropriate place)
  • o Acquire images of this phantom (which is now properly aligned) with both tubes. Check that hidden target (tungsten sphere inside the phantom) is at the cross hairs. This test verifies that the ExacTrac imaging isocentre and the mechanical isocentre of the linac coincide.
  • perform WL to verify mechanical = radiation isocenter

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
159
Q

BED of 48/4, 60/8,54/3,60/15 fir lpha/beta o 10 Gy

A

48/4 adnd 60/8 - BED 105 Gy
60/15- BED 84 Gy
54/3- BED 151 Gy

160
Q

advantage of usingDIBH in lung

A
  • less motion
  • may pull tumor away fom mediatinum
  • improves V20Gy
161
Q

why do we use xactrac for spine matching in lun even though research shows spine is nt a good surrogate for lung?

A
  • exactrac with brain lab bed allows for rotations

- also CBCT after exactrc

162
Q

What is Veff

A

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

163
Q

what is thecal sac?

A

membranous sheath that surrounds spinal cord and cauda equina. Provides nutrients and buyoncy to the cord

164
Q

how is spine SBRT set up?

A

set up with exactrac, verify with kV orthos

165
Q

liver and kidney mean dose limits for all spine SBRT fractionations

A

5 Gy

166
Q

what have studies shown for WBRT vs SRS?

A

-SRS had same control as WBRT and less side effects for < 10 mets

167
Q

cranial stereo imaging required at treatmentex

A

exactrac and verify with kV orthos

168
Q

for a shift treshold of 1 degree and 1 mm, what is max allowable distance from isocenter (in a multi-met plan)

A

1mm/ (tan 1 degree)= 5.7 cm

169
Q

max lesion size for cranial stereo

A

5 cm

170
Q

typical fractionations for non-malignant brain tumours

A

25/5
50/25
50.4/28

171
Q

how does radiation fix AVMs?

A

-leads to fibrosis that has same effect of separating the vein and artery as surgery would

172
Q

explain mutual information

A

used for image registration
intensity-based algorithm that applies a transformation to the non-localized set of images, calculates the mutual information and repeats this process iteratively until the mutual information is maximized (i.e., the amount of information they contain about each other is maximized; the ability to predict the grey value in one image given that the grey value in the other image is known. This method can cope with images produced in different imaging modalities, and can handle the fact that, for example, bone may have low intensity in T1 MRI but high intensity in CT

173
Q

how do you calculate mutual information?

A

plot image 1 signal vs image 2 signal for each pixel as scatterplot
overlay a 2D grid over scatterplot to convert it to a histogram
count how many pts land in each square
mutual signal is high when signal in 2D histogram is concentrated into a few bins and low when signal is spread across many bins

174
Q

what prescription isodose line is used for stereo with cones?

A

50%

175
Q

can you do multiple mets with one isocenter using cone linac stereo?

A

no

176
Q

benefit of cone vs MLC

A

MLC has larger penumbra due to leakage through leaves and they are farther away from the patient (larger geometric penumbra)

177
Q

pre-treatment imaging for lung SBRT with/without breath hold

A
  • if using BH, cannot use Exactrac becaue IR relfectors on RPM will interfere with exactrac
  • with BH, use fluoro daily to assess breath hold quality and follow with CBCT
  • for FB, use exactrac and match to spine. Verify with CBCT. Exactrac IR makrers are attached to patient skin to detect changes in position during treatment
178
Q

PTV/PRV for SRS/SRT vs SBRT

A

2 mm for SRS/SRT (except hippocampus which is 5 mm), 5 mm for SBRT

179
Q

downside of E2E test

A

If process fails you don’t know where it happened

180
Q

additional tests for gamma knife

A

radiation survey
radiation leakage
wipe testice
timer accuracyand linearity test

181
Q

shift tresholds for stereo

A

CBCT lung or liver- 3 mm
Exactrac cranial- 1 mm, 1 degree
bone and lung exactra- 1.5 mm, 1 degree

182
Q

ideal conformity index

A

1
for conventional definition, CI is worse if larger
if using Padick index larger CI than 1 is more conformal

183
Q

what do frameless cranial stereo treatments use?

A

fiducials implanted in the skull

184
Q

does WL tell you anything about imaging system?

A

NO

image center could be off of radiation center0 we don’t test this with wl

185
Q

explain synchrony respiratory motion tracking in cyber knife

A

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

186
Q

max field size for cyberknife with MLCs

A

12 cm x 10 cm

187
Q

3 ways that cyber knife can track motionop

A

optically, stereoscopic x-rays, or combination of both (synchrony)

188
Q

explain what frequency is in S band or Xband

A

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.

189
Q

difference between klystron and magnetronk

A

klystron- electrons move linearly

magnetron- electrons follow a spiral path from cathode to anode

190
Q

issue with X band waveguide

A

need more heat dissipation

191
Q

disadvantage of synchrony in cyber knife

A

requires regular breathing pattern

192
Q

show isodose lines for a target with prescription at 90% vs 100%

A

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

193
Q

latest version of gamma knife

A

Icon, has kV on board imaging

Treatment Planning (17 decks)

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