TG137 Flashcards
What is TG137?
permanent interstitial brachy for prostate cancer
issue with edema
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
common isotopes for permanent prostate LDR
I-125
Pd-103
what does TG-137 recommend for imaging for post-implant evaluation?
2-3 mm slice axial CT images
are seeds expected to move over time?
Yes, due to swelling of prostate and after swelling has gone down
Is MRI or CT more useful?
soft tissue better with MRI
seeds better with CT
when should imaging for dosimetry evaluation be done?
- 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
common doses
125-145 Gy
I-125 dose rate
7 cGy/h
common OARs
urethra, rectum
planning targets and constraints
CTV: V100% > 95%
CTV: V150% < 50%
Rectum: D2cc < Rx, D0.1cc < 150%
Urethrea: D10<150%, D30< 130%
what are some biophysical models that are used for prostate implants?
-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
what biophysical model is recommended by TG-137?
Dale BED model
what parameter values are recommended in Dale BED model?
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
what do the biophysics models ignore?
heterogeneity corrections
interseed shielding
where should PDR patients be careful?
around children, pregnant women, urinating, intercourse, airport and funeal should be detectors
Using CT/MR/US
- 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
main recommendations from TG137
-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
sources used in LDR
125I
103Pd
131Cs
staging of prostate cancer
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
options other than brachy for early stage prostate cancer
- EBRT
- cryoablation
- hyperthermia
- radiofrequency ablation
- hormones
3 ways cure rates in prostate brachy are measured
- overall survival
- disease-specific survival
- biochemical control
coverage index
CI = 100(V100-Vt)/Vt where Vt is target volume and V100 is volume that gets 100 % of prescription dose
why do we want D90> 140 Gy?
- 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
what correlatives with urethral toxicity?
V150
V200
prostate size
what is clearly seen in MR images
- 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)
endorectal MR imaging
- used to determine both the seed position and delineate anatomy
- removes need for fusion between different modalities
prosate swelling
edema
trauma from needles causes the prostate to swell to max size soon after insertion and then slowly start shrinking again
delta
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
typical edema values
30% post implantation
50% within a day of implantation
10% 30 days after implantation
edema half life
~ 10 days
how would knowing edema characteristics help dosimetry?
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
optimal post-implant dosimetry time for each LDR radionuclide
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
3 things to report in prostate brachy
pre-implant prostate volume
implant day dosimetry
post-implant dosimetry at nominal optimal dosimetry time for the radionuclide
modified uniform and modified peripheral loading
more peripheral seeds
less central seeds
how does the seed strength affect the dose?
-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
why is dose calculated with point-source approximaion?
- 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
CTV in prostate cancer
CTV = GTV = whole prostate gland
prescription doses for monotherapy
145 Gy for I125
125 Gy for Pd103
100-125 Gy for 131Cs
to 100% isodose
planning objectives
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
what is only OAR that can be seen reliably in both MRI and CT post-implantation?
rectum
intraoprative pre-planning
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
interactive planning
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
dynamic dose calculation
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
prescribed initial dose rates for the 3 LDR nulcleotides
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
why is characterizing the biology in LDR important?
- diverse spatial and temporal dose and dose rate vriations
- cell repopulatio and sublethal repair can be significant over the course of LDR
what factors have been studied using the Dale model?
- 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
limitation of Poisson model
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
factors to consider in radiobiological modelling
-hypoxic cells
cell-cycle effects
radiation-induced apoptosis
-radiobiological modelling is intrinsically organ specific (alpha/beta, tissue architecture etc)
Explain what the parameters in Dale’s BED equation for PDR prostate brachy are
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
How is Teff defined?
- 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)
BED for temporary implant with source dweel time less than Teff?
use the actual souce dwell time
for the same prescribed dose, is BED larger for radionuclides with shorter half lives?
yes
issues with using Teff
BED would be different when calculating at different time points- even wehn looking at relative effects of a radionuclide vs another. Time point matters
for different types of nuclides, is it the addition of the BED from each independent one?
No, the expression is more complex than that
how does edema change the BED equation?
- 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
How has the Dale BED been adpated?
- for mixtures of nuclides
- for edema
- for inhomogeneous dose distribution
how to adapt Dale BED to consider inhomogeneous dose distributions?
- partition tumor volume into small subvolumes such that dose rate distribution in each subvolume is uniform
- add them up
TCP model based on Poisson statistics
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
alpha/beta for prostate cancer
-low (~ 1-4 Gy)
recommended radiobioligcal models for LDR prostate calcs
beta = 0.05
alpha = 0.15
Tp= 42 days
repair half life = 0.27 hour
