Brachytherapy Exam 2

point A

- 2 cm up from os/flange, 2 cm lateral from tandem - where uterine vessels cross the ureter - prescription point

esophageal brachytherapy

-for recurrent esophageal obstruction -bougie applicator goes thru mouth down throat -2 fractions, 7 Gy/fx -Ir-192, HDR -OAR healthy esophagus and heart

biliary treatment

-liver cancer patients in prep for transplant, ercp -single fraction 930 cGy -Ir-192 HDR -OAR duodenum

prostate seed implant

-permanent seeds placed to treat early stage/low grade prostate cancer -monotherapy dose of 145 cGy -I-125, LDR -Pd and Cs also used (lower rx dose due to shorter halflife) -preplanned TRUS, post-plan CT -OAR rectum, bladder, urethra

shielded cylinder

-spares previously treated healthy tissue in the case of recurrence -potential uncertainty in shield placement

breast brachytherapy

-treats early stage ductal carcinoma -daily txmt preceded by imaging to verify applicator position and integrity (SAVI) -10 fx (BID) 340 cGy/fx OR 3 fx of 700 cGy/fx -Ir-192, HDR -OAR skin, chest wall, heart, lungs, rib -volume to 90% PTV must be >90% rx

endobronchial brachytherapy

-treats recurrence or mets to clear bronchial obstruction (palliative) -flexible catheter placed via nose -2 or 3 fractions, 7 Gy/fx -Ir-192, HDR -OAR heart

standard reference point (line source)

1 cm from center, 90 degrees from line source

point B

2 cm up from os/flange, 5 cm lateral from pelvic midline (obturator nodes)

How long are eye plaques typically left on the eye?

4-7 days

typical LDR dose rate

50-60 cGy/hr

What is the standard classical prescription for LDR tandem and ovoid implant?

8,000 cGy (50-60 cGy/hr)

standard eye plaque prescription

85 Gy tumor > 5mm = apical height tumor < 5mm = 5mm height

Classical LDR gyn implant isotope

Cs-137

eye plaque isotope

I-125

thyroid ablation isotope

I-131

HDR gyn implant isotope

Ir-192

isotope for contact treatments with HDR

Ir-192

bone met isotopes

P-32, Sm-153, Sr-89, Ra-223

air kerma strength

Sk- air kerma rate * the distance from the source that air kerma was measured units = cGy-cm2/hr = U

lymphoma and liver mets radioisotope

Y-90

advantages and disadvantages of HDR prostate instead of PSI

advantage: more optimization options, multifraction, better localization and dose modeling, only the pt receives dose disadvantage: more time for pt and personnel, sometimes multifraction

air kerma rate

air kerma / time (basically dose rate) cGy/hr Gy/sec

rectal reference point

along AP line going through center of ovoids, 5mm posterior to posterior vaginal wall

F(r,0)

anisotrophy function; corrects for changes in dose rate as a result of differences in self-shielding as the point of interest travels around the source

effective half-time

bio * physical / bio + physical

What are the shapes used to describe the ideal dose distribution of a classical tandem and ovoid/ring implant?

coronal = pear sagittal = cigar

purpose of points A, B, V, rectum, bladder

describes implant in terms of dose to various points and dose to OAR

point V

directly lateral to center of sources in ovoids, on surface of ovoids (vaginal mucosa)

At what distance from a line source can we pretend it's a point source?

distance greater than 10x seed length

D(r,0)

dose rate at a point 'r' cm from center of source and angle __ from long axis of a line source (cGy/hr)

upsidedown V

dose rate constant at standard reference point (1/cm2) -multiply this by Sk to get dose at std reference point

geometric optimization

dwell time for each position is dependent on its physical location in relation to all the other source locations

advantages of breast HDR brachy over external beam

early stage disease control, lumpectomy + radiation has equivalent outcome of mastectomy, lower dose to OAR, faster, cheaper

What is the alternative treatment for medium sized uveal melanomas?

enucleation (removal of eye); same outcome as COMS plaque

what has greatest influence on whether or not a brachy implant will give good dosimetry?

geometry

GL (r,0) / GL (ro,0o)

geometry function; accounts for inverse square

What is the purpose of vaginal packing or rectal retractors?

immobilization, spares rectum (pushes rectum posteriorly)

What is a limitation with using point A as the prescription point?

it references a one size fits all applicator, rather than the individual patient's disease

objective function

least-squares optimization- requires user input of OAR and target contours, computer tries different dwell times at available locations to minimize value of the mathematical equation

What 2 source geometries are there TG-43 equations for?

point source and line source

bladder reference point

posterior edge of balloon, centered sup/inf and l/r

What three things are important for a patient to have done prior to I-131 treatment?

pretreatment scan, stop thyroid meds, no salt diet prior/during

air kerma

quantity of KE that electrons in a volume of air have gained from being irradiated by photons divided by mass of air (Gy)

gL(r)

radial dose function; accounts for absorption and scattering on transverse plane

Why is a lower total dose used for radioisotopes with shorter half-lives, but the same average energies?

shorter half life = increased dose rate = less repair

single v multichannel device

single- less likely for error when hooking it up multi- allows for better dose shaping

freiberg flap uses

skin, sarcoma, intra-op

TG-43

standardizes brachy dose calculations, makes them more accurate

Why should we use the largest possible ovoid caps or cylinder in any given patient?

to prevent overdosing nearby tissues before we can treat further tissues to the adequate dose

What are COMS plaques used to treat?

uveal melanoma

primary intracavitary treatment sites

vagina, cervix, uterus

What medium does TG-43 assume is surrounding radioactive sources to calculate dose in?

water

What needs to be done when a package of radioactive materials is received by an institution?

wipe test and inventory