brachy stuff Flashcards
constraints and objectives for permanent implant prostate PDR
CTV: V100%> 95% so D90% will be > 100%
CTV: V150% < 50%
rectum: D2cc < Rx, D0.1cc < 150%
urethra: D10< 150%, D30<130%
who is eligible for SABR prostate with fiducials?
low-intermediate risk
-36.25 Gy/5 to PTV, 40Gy/5 to CTV, on alternate days
-if patient has hip prosthesis, there would be too much artifact in the CT
seed activity
0.2-0.4 mCi for I-125
1-2 mCi for Pd-103
survey readings in brachy
Brachy: >1 mSv/hour; indicative of source completely outside of the afterloader). Normal background is <0.01 mSv/hour in the treatment room when the source is completely in the safe.
gyne 1500/3 bladder and rectum constraints
bladder 620 cGy
rectum 420 cGy
APBI brachy dose
o HDR balloon brachytherapy dose: 34 Gy b.i.d. x 5 days
o OARs (skin, lung, heart) must be far enough away; with small breast this may be difficult to achieve.
HDR monotherapy prostate experiments dose levels: 13.5 Gy x2 fx
Ir-192 vs 60Co decay
Ir-192 decays 1%/day, Co-60 decays 1%/month
initial dose rate of a permanent implant
= prescription dose/ average life of the source
average life is 1.44X the half-life
- Pd-103- 21.3 cGy/h
- I-125: 7 cGy/h
standard brachy skin prescriptions for surface applicators
Leipzig, Valencia, and eBT
40Gy/8
42Gy/6
achieves BED of 60-71.4 Gy
treatment usually delivered every other day
prescription are prescription points are all over map, some at surface, some 3-5 mm from skin surface
-most common is 3-4 mm prescription depth for Leipzig, valencia, and eBT applicators
~10%/mm gradient (PDD)- so skin surface if prescrbing to 3 mm depth is 130-150%
standard brachy skin interstitial treatment dose
30 Gy/10fx
used for lesions more than 5 mm deep (surface brachy would give unacceptable high skin dose)
valencia vs leipzig
- Valencia has FF between source and skin surface = increased treatment time than Leipzig but better dose profile
- typically made of high Z material (thus can be artifacts if CT is used for imaging)
Have to use Monte Carlo because TG43 doesn’t model the metal
does skin applicator have to be flush to skin?
yes, otherwise there would be an air gap
-dose fall off for air gap of 1 mm can be 10%
what should dose to the skin surface be limited to for skin brachy?
125% for flaps and 140% for custom molds
why use brachy for skin vs kV or electrons?
skin brachy is useful if the skin has a complicated surface
kV and electrons- need flat surface. With skin, can use freiburg flap, moulds
skin brachy can be interstitial- less dose to skin surface compared to kV. Also less dose to organs past skin with brachy compared to EBRT
Valencia/Leipzig can be tough for skin surface as PTV has to fit within 3 cm diameter device and need flat surface- however, faster dose fall-off than ortho and will “stick” to target
freiburg flap-can get more heterogeneous dose distribution compared to using kV or electrons
no brachy near eyes (skin too fragile)- skin in general is risky-can disfigure face
standard of care for gyne brachy planning
mri
can you use PET-CT bunker for HDR suite?
No, because PET-CT is shielded for 0.5 MV photons whereas Ir-192 spectrum includes photons of higher energy than 0.5 MV
is 60 cc rule in prostate more relevant for HDR or LDR?
LDR - with HDR it is easier to spare OARs with optimization
you cannot see anterior part of prostate- what can you do?
decrease US frequency to get increased penetration at expense of resolution
building new brachy program- major consideration?
what source to use
Ir-192 vs I-125 g(r) fall-off
In machester system, why is Point A relative to tandem whereas point B is relative to patient body?
point A (which represents the crossing of the uterine artery and the ureter) is best approximated relative to the uterus while point B (which represents the pelvic lymph nodes) is best approximated relative to the patient’s body. This difference can be significant when the uterus is tilted relative to the pelvis.
vault dose in this clinic
monotherapy: 21/3
11/2 brachy with 45/25 EBRT
ring and tandem: 28/4 HDR with 45/25 EBRT (for gyne not vagi)
why can’t you use vault to treat beyond 5 mm?
plan would be too hot
how does the disk size for the vault affect the plan?
large disk- difficult to get homogeneous dose at upper part because side has 2 cm diameter whereas top is only 12 mm (for 4 cm cylinder); top tends to be hot. Dr. Bowes likes to not include the first dwell position to improve homogeneity.
small disk- hard to get homogeneous dose
-larger disk = impact of IS differences along vault less significant since IS is less significant at larger distances
how is ICRU system different than manchester system?
ICRU: relate the dose distribution to the target volume rather than to a specific point. (more like ext beam)
when is vaginal cuff (vault) treatment indidcated?
post hystorectormy
endometrial cancer
1) patients with grade 1 or 2 cancers with > 50 % myometrial invasion
2) 2) patients with grade 3 cancers with < 50 % myometrial invasion
usually if invasion > 50 %, get EBRT plus brachy boost (in addition to operation)
if invasion < 50%, just get brachy boost (in addition to operation)
-studies showed that radiotherapy post hysterectomy prevented relapses, mostly in vaginal cuff
-brachy = less toxicity compared to EBRT
textbook EQD2 constraints for rectum, sigmoid, bladder in cervix treatment
75 Gy EQD2 for rectum and sigmoid
90 Gy EQD2 for bladder
2 most common indications of gyne brachy
post-operative endometrial
cervix
is cervical brachy therapy ever used as monotherapy?
rarely, if early-stage cancer
LDR dose for gyne
40 Gy in 2 insertions (2 fractions of 20 Gy)
-0.2-4 Gy/h dose rate
guidlines for dose to OARs for gyne brachy
American brachy association calculates EQD2 for patient-specific plans
-typically, for 7 Gy fraction, 4Gy to hottest 2 cc of OARs is reasonable
usual number of sources used in LDR tandem and ovoids
3 in tandem and one in each ovoid
-hard to shape dose compared to with HDR
does ABS recommend still using a tandem if doing interstitial?
yes, to prevent a cold spot
when does american brachy association recommened LDR or PDR boost for gyne?
2 applicatiors to allow for reduction in tumor volume and improved tumor coverage wth 2nd application
-first should be within 4-6 weeks of start of EBRT
-2nd should be 1-2 weeks later
how long after surgery do you do the vaginal vault?
at least 4 weeks
what if the vaginal vault cylinder is too small?
-can be air gaps or folds leading to underdosage of target
who is candidate for vagi interstitial brachy?
patients with stage I-IVA vaginal cancers or recurrent cervical, endometrial, or vulvar carcinoma in vagina with vaginal lesions > 0.5 cm thick
why use tandem and cylinder?
narrow vagina
treat varying lengths of vagina if there is spread of disease
what is included in the low risk CTV in cervical cancer? Intermediate risk? High risk?
CTV-TLR comprises the whole
parametria, the whole uterus, the upper part of the
vagina, and the anterior/posterior spaces toward the bladder and rectum
CTV-THR = CTV-Tadapt that includes the GTV-Tres, the
whole cervix, and adjacent residual pathologic tissue,
if present
CTV-TIR = The CTV-TIR represents the GTV-Tinit as superimposed on the topography at the time of brachytherapy, together with a margin surrounding the anatomical cervix border (CTV-THR) in areas without an initial GTV-Tinit
dose fall-off from vault plan
1 cm to go from 200% to 100 %, 1 cm to go from 100% to 50 %
If someone is 25 cm away from the source during an emergency, how much dose would they receive in 5 minutes while resolving the issue?
3-6 cGy
required range of G-M and ionization survey meters
G-M: 0.1-100 mR/h
ionization: 1-1000 mR/h
what correlatives with urethral toxicity?
V150
V200
prostate size
typical edema values
30% post implantation
50% within a day of implantation
10% 30 days after implantation
Miami-type applicator
vaginal cylinder plus tandem. The vaginal cylinder has six channels in this case, allowing for asymmetric dose distributions within the vagina.
how to QA the position of the dwell in the applicator
can use fluoro- see end of tube and source location
-can use film or EPID- mark end of tube and see where dose distribution is centered
how to calculate transport index
determine the max radiation level in mSv/h at 1 m distance from external surfaces of package. This value multiplied by 100 gives the TI. Values shall be rounded up to the first decimal place, except that values of 0.05 may be considered zero.
categorize I-white, II-yellow, III-yellow
o TI = 0 and max radiation level at any point on external surface <= 0.005 mSv/h I-white
o TI > 0 but <=1 and max radiation level at any point on external surface > 0.005 mSv/h but <= 0.5 mSv/h II-Yellow
o TI >1 but <=10 and max radiation level at any point on external surface > 0.5 mSv/h but <=2 mSv/h III-Yellow
o TI > 10 and max radiation level at any point on external surface > 2 mSv/h but <= 10 mSv/h III-Yellow, Must be transported under exclusive use.
- Where the TI satisfies the condition for one category but the surface radiation level satisfies the condition for a different category, assign it to the higher category.
necessary markings for transport of radioactive source
- names and adresses of consigner and consignee
- UN number (7)
- radionuclides, their chemical form, and the max activity of the contents
- category I, II, or III
- transport index (for categories II and III only)
- certificate of approval from various relevent authorities
- specifify exclusive use if relevant
assumptions of TG-43
No source-to-source shielding effects
all tissues in and around implant are water equivalent
scattering volume within patient is equivalent to that used in the concensus data sets (at least 5 cm of water-equivalent material surrounds the point of calculation)
what does F(r,theta) approach with increasing radial distance?
unity
for all angles except 0
where are concensus data for the sources taken from?
averaged experimental data are averaged with MC data
what was TG-43 specifically written for?
interstitial brachy
fundamental problem with pre TG43 protocols
based upon photon fluence around the source
in free space, whereas clinical applications require dose distributions
in a scattering medium such as a patient. Determination
of two-dimensional dose distributions in a scattering
medium from a knowledge of the two-dimensional distribution
of photon fluence in free space is easily accomplished
only for a point isotropic source. An actual brachytherapy
source exhibits considerable anisotropy
-could not handle non-point sources
TG43 solves this issue by measuring dose distribution in water equivalent phantoms
what is 1-D anisotropy factor?
-average 2D variant for each radius
overall uncertainty in dose rate at a point around a source TG43
10%
for what r does TG43 formalism using a polar coordinate system break down?
r < L/2
points are inside the source capsule
issue with lack of heterogneity corrections in TG43
high energy source: nearly same behaviour as water
low energy source: importance of PE effect increases as energy decreases
I125- difference of 9-20 % in lung vs water
predominant effect on dose
distance
FINAL extrapolation/interpolation guidelines per TG43 S1
F- linear-linear interpolation using 2 nearest adjacent points
for r< rmin, use F(rmin). For r> rmax, use F(rmax)
Phi(r) - log linear using adjacent data points. This differs from TG43U. Gives equation for r< rmin. For r> rmax, use Phi(rmax)
g(r)- log linear usding adjacent neighbours
- use nearest g(rmin) for r< rmin
- for r> rmax, there is equation with exponential function
- for gp, interpolate gl instead (as gp changes drastically with r), then multiply by ratio of gp/gl
where does TG 43 not work?
- inhomogeneous
- not enough scatter material
- at small r where line source approximation breaks down
- inter-source shielding effects and shielding from applicators
- orientation of seeds unknown (argue can use pt dose approximation or 1D anisotropic function)
- transit time not accounted for
Unit of air kerma strength
uGym2/h
air kerma strengths of Ir-192 sources
29000-41000U
I-125 air kerma strength
6.3 U (6711)
51 U (6702)
Pd-103 air kerma strength
2.6 U
Co-60 air kerma strength
15000-18000U
Cs137 Sk
56 U
planning guidelines for HDR prostate brachy
V100%> 95%
V200%<11%
Urethra D10%< 118%
Urethra Dmean < 125%
Rectum V80%<0.5cc
