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brachy > TG137 > Flashcards

TG137 Flashcards

1
Q

What is TG137?

A

permanent interstitial brachy for prostate cancer

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2
Q

issue with edema

A

target volume delineation depends a lot on time of imaging after implant because of edema from surgery

  • this effects the assessment of dose delivery
  • edema itself leads to large temporal change in delivered dose
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3
Q

common isotopes for permanent prostate LDR

A

I-125

Pd-103

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4
Q

what does TG-137 recommend for imaging for post-implant evaluation?

A

2-3 mm slice axial CT images

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5
Q

are seeds expected to move over time?

A

Yes, due to swelling of prostate and after swelling has gone down

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6
Q

Is MRI or CT more useful?

A

soft tissue better with MRI

seeds better with CT

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7
Q

when should imaging for dosimetry evaluation be done?

A
  • day of implant and at an optimal time for respective nucleotides (ex 1 month for I-125)
  • latter is available on dose-response data in literature
  • pre-implant prostate volume should also be obtained
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8
Q

common doses

A

125-145 Gy

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9
Q

I-125 dose rate

A

7 cGy/h

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10
Q

common OARs

A

urethra, rectum

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11
Q

planning targets and constraints

A

CTV: V100% > 95%
CTV: V150% < 50%
Rectum: D2cc < Rx, D0.1cc < 150%
Urethrea: D10<150%, D30< 130%

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12
Q

what are some biophysical models that are used for prostate implants?

A

-analytic expression of BED based on linear-quadratic cell inactivation model (Dale BED)
-EUD
-TCP determined by Poisson probability of inactivating all tumour cells with the average surviving cells calculated according to Dale’s BED.
More advance models taking into account cell repopulation and different temporal patterns of dose delivery have been developed

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13
Q

what biophysical model is recommended by TG-137?

A

Dale BED model

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14
Q

what parameter values are recommended in Dale BED model?

A

alpha = 0.15 Gy-1, beta = 0.05 Gy-2, alpha/beta = 3.0 Gy, Tp = 42 days (potential doubling time) and repair half-life of 0.27 hours

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15
Q

what do the biophysics models ignore?

A

heterogeneity corrections

interseed shielding

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16
Q

where should PDR patients be careful?

A

around children, pregnant women, urinating, intercourse, airport and funeal should be detectors

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17
Q

Using CT/MR/US

A
  • volumes can vary a lot depending on the modality
  • time of imaging (especially if after implant) is significant
  • differences in optimal plan depending on what modality you use

-hard to delineate prostate with CT (bladder and prostate often mixed together if overlap) i.e. not great base and apex definition

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18
Q

main recommendations from TG137

A

-postimplant evaluation should be done at optimal time for specific radionuclides
0encourage use of a radiobiological model with a specific set of parameters to facilitate relative comparions of treatment plans reported by different institutions using different loading patterns or radionuclides

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19
Q

sources used in LDR

A

125I
103Pd
131Cs

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20
Q

staging of prostate cancer

A

low risk: stage < 2B, gleason < 6, PSA < 10
intermediate: stage < 2b, gelason = 7, PSA within 10 and 20
high risk: stage = 2c, gleason >8, PSA >20

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21
Q

options other than brachy for early stage prostate cancer

A
  • EBRT
  • cryoablation
  • hyperthermia
  • radiofrequency ablation
  • hormones
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22
Q

3 ways cure rates in prostate brachy are measured

A
  • overall survival
  • disease-specific survival
  • biochemical control
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23
Q

coverage index

A

CI = 100(V100-Vt)/Vt where Vt is target volume and V100 is volume that gets 100 % of prescription dose

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24
Q

why do we want D90> 140 Gy?

A
  • significant increase in freedom from biochemical failure from studies
  • showed that age, EBRT, type of implant didn’t significantly affect biochemical failure, just D90 did
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25
Q

what correlatives with urethral toxicity?

A

V150
V200
prostate size

26
Q

what is clearly seen in MR images

A
  • base and apex definition
  • prostate-rectal interface

-however, seed localization is performed either on CT or planar images and fused with MR images (seeds are seen better on CT)

27
Q

endorectal MR imaging

A
  • used to determine both the seed position and delineate anatomy
  • removes need for fusion between different modalities
28
Q

prosate swelling

A

edema

trauma from needles causes the prostate to swell to max size soon after insertion and then slowly start shrinking again

29
Q

delta

A

edema magnitude
delta = Vt/Vo, Vt is is volume at an appropariate time for imaging and Vo is initia; volume
delta = 100% (Vt/Vo-1)

edema resolution can be modelled by a single exponential decaying function

30
Q

typical edema values

A

30% post implantation
50% within a day of implantation
10% 30 days after implantation

31
Q

edema half life

A

~ 10 days

32
Q

how would knowing edema characteristics help dosimetry?

A

could do the post-implant dosimetry at a specific time and calculate differences due to edema characteristics over the implant’s lifetime

-simulation sudies show there is an optimal time for each radionuclide to minimize dosimetric erroes due to edema

33
Q

optimal post-implant dosimetry time for each LDR radionuclide

A

131 Cs 10 days
103 Pd 16 days
125 I 42 days

max error from dosimetry performed at these times would be less than 5 % regardless of edema characteristics

unfortunately if the dosimetry is bad, it is hard to change it unless you add more dose

34
Q

3 things to report in prostate brachy

A

pre-implant prostate volume
implant day dosimetry
post-implant dosimetry at nominal optimal dosimetry time for the radionuclide

35
Q

modified uniform and modified peripheral loading

A

more peripheral seeds

less central seeds

36
Q

how does the seed strength affect the dose?

A

-higher strength provided better dose coverage and better urethral protection

therefore using less, higher strength seeds is viable. Could be specific to implantation technique in the study

37
Q

why is dose calculated with point-source approximaion?

A
  • difficult to determine orentiation of seeds via CT imaging
  • because implanted seeds tend to align along the implant needle track, accounint for anisotropy will give more dose along transverse planes and decrease dose inferior and superior
38
Q

CTV in prostate cancer

A

CTV = GTV = whole prostate gland

39
Q

prescription doses for monotherapy

A

145 Gy for I125
125 Gy for Pd103
100-125 Gy for 131Cs
to 100% isodose

40
Q

planning objectives

A 
V100>95% for CTV
D90> 100 % for CTV
V150 < 50 % of CTV
rectum D2cc < Rx
rectum Dmax< 150%
urthtrha D10 < 150%
urethra D30 < 130%

V200, D100 are reported but dont correlate with clinical study results

41
Q

what is only OAR that can be seen reliably in both MRI and CT post-implantation?

A

rectum

42
Q

intraoprative pre-planning

A

usually determine prostate volume and number of seeds required before the implantation date so seeds can be ordered

intraoperative preplanning can be done if the instituion has enough seeds and they can use do this step on implantation day

43
Q

interactive planning

A

plan is made based on images (like EBRT) and updated based on implanted needle positions the day of.
-based on implanted needle position, won’t account for seed movement after deposition

44
Q

dynamic dose calculation

A

like interactive planning but planning is based on deposited seed location (rather than needle location) using image guidance
-motion of prostate and edema are accounted for

-used in HDR because needles are imaged. Not yet used in LDR because hars to see the seeds with TRUS

45
Q

prescribed initial dose rates for the 3 LDR nulcleotides

A

125I: 7 cGy/h
103Pd: 21 cGy/h
131Cs: 30 cGy/h

delivering 80% of total dose varies from 22 days for 131Cs to 140 days for 125I

46
Q

why is characterizing the biology in LDR important?

A
  • diverse spatial and temporal dose and dose rate vriations

- cell repopulatio and sublethal repair can be significant over the course of LDR

47
Q

what factors have been studied using the Dale model?

A
  • effectviness of LDR vs HDR
  • effect of mixing sources with different half lives
  • impact of tumour shrinkage
  • impact of edema
  • effect of combining brachy with EBRT
48
Q

limitation of Poisson model

A

underestimates tumor control rate when tumor-cell repopultion occurs during treatment

-Zaider and Minerbo derived a more general TCP formalism capable of dealing with cell repopulation

49
Q

factors to consider in radiobiological modelling

A

-hypoxic cells
cell-cycle effects
radiation-induced apoptosis

-radiobiological modelling is intrinsically organ specific (alpha/beta, tissue architecture etc)

50
Q

Explain what the parameters in Dale’s BED equation for PDR prostate brachy are

A

Do- initial dose rate
lambda- decay constant of radionuclide
u - time constant for sub lethal damage repair (inversely proportional to repair half time)
Teff- effective treatment time for an implant
D(Teff)- total dose delivered by the imlant within time period of Teff

51
Q

How is Teff defined?

A
  • rate of cell inactivation wquals rate of cell repopulation
  • 2 processes compete in LDR: cell repopulation and cell inactivation
  • As the treatment time elapses, the rate of cell inactivation from the instantaneous dose rate becomes exponentially smaller whereas the rate of cell repopulation stays the same

depends on T1/2 (half life of radionuclide), Tavg = 1.44 T1/2 and D (total dose delivered to full decay of radionuclide)

52
Q

BED for temporary implant with source dweel time less than Teff?

A

use the actual souce dwell time

53
Q

for the same prescribed dose, is BED larger for radionuclides with shorter half lives?

A

yes

54
Q

issues with using Teff

A

BED would be different when calculating at different time points- even wehn looking at relative effects of a radionuclide vs another. Time point matters

55
Q

for different types of nuclides, is it the addition of the BED from each independent one?

A

No, the expression is more complex than that

56
Q

how does edema change the BED equation?

A
  • dose rate no longer decays exponentially with time because the distance between the sources and the tumor is now time dependent due to the prostate swelling and then resolving
  • edema resolution model is incorporated into the dose rate decay
57
Q

How has the Dale BED been adpated?

A
  • for mixtures of nuclides
  • for edema
  • for inhomogeneous dose distribution
58
Q

how to adapt Dale BED to consider inhomogeneous dose distributions?

A
  • partition tumor volume into small subvolumes such that dose rate distribution in each subvolume is uniform
  • add them up
59
Q

TCP model based on Poisson statistics

A

probability of cure TCP = exp(-{N})
wheer {N} is the expectation value of the reamaining tumor cells at the end of the treatment. For N, can substitute in expression for survival fraction

60
Q

alpha/beta for prostate cancer

A

-low (~ 1-4 Gy)

61
Q

recommended radiobioligcal models for LDR prostate calcs

A

beta = 0.05
alpha = 0.15
Tp= 42 days
repair half life = 0.27 hour

brachy (17 decks)

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