TG-43: Brachytherapy Source Dose Calculation Formalism/ LDR Intracavitary
less error and lower risk of treatment error, smaller diameter, but does not allow for geometric variation in dosimetry
advantages and disadvantages of single channel cylinder
able to shape dose, put more on peripheral channels to avoid structures
advantages of multichannel cylinder
uterine vessels; ureter
anatomically, point A is theoretically where the ___________________ cross the _______________
obturator nodes
anatomically, point B represents the dose that is being given to the _____________ _______________
Cesium 137; Iridium 192
isotope used today for LDR gyn impants? For HDR gyn implants?
posterior edge of balloon, centered sup/inf and rt/lt
location of bladder reference point
2 cm up from top of flange along tandem and 2 cm lateral to tandem
location of point A for tandem and ovoid implants
2 cm up from top of flange along tandem, 5 cm to pelvic midline
location of point B for tandem and ovoid implants
directly lateral to the geometric center of sources in ovoids, on the surfaces of the ovoids (represents mucosal dose)
location of points V (left and right)
AP line going through center of ovoids, point is 5 mm posterior of posterior vaginal wall on that line
location of rectal reference point
air kerma strength; dose rate constant
only __________________ and ______________________ have units
Cervix, vagina, uterus (Gyn), breast (surgery)
primary treatment sites for intracavitary brachytherapy
to help shape where we want to treat with recurrent cancers, selectively shield anatomy and avoid treating same previous, but it only has one channel so there is less optimization of dose shape
purposes for treating with a shielding cylinder, advantages, disadvantages
applicator pushes the tissue farther form the source, makes a more uniform dose, and reduces the dose to the vaginal mucosa due to the inverse square law
reasoning for using the largest possible ovoid caps of cylinder in any given patient
pear shape (coronal/AP); cigar shape (lateral)
shapes used to describe the ideal dose distribution of a classical tandem and ovoid implant
1 cm from center of source, 90 degrees from center of long axis of source
standard reference point relative to a line source
air kerma
the sum of kinetic energy of all charged particles liberated per unit mass air
8000
total of ___________________ cGy to point A
about 50-60 cGy/hr
typical LDR dose rate at point A
line geometry function; fancy version of inverse square law
what is (GL (r0, θ0))/ (GL (r, θ)) ? What does is account for?
line anisotropy function; dose rate as you move from transverse plane due to source attenuation from encapsulation and the source itself
what is F(r, θ)? what does it account for?
air kerma strength; cGycm^2/hr (U)
what is Sk and what are it's units
line radial dose function; absorption and scattering on transverse plane because we exist in water, not a vacuum
what is gL (r)? What does it account for?
dose rate constant; multiple air kerma strength (U) by dose rate constant (cgy/hrU) and get cGy/hour
what is Λ? How do you use Λ to Sk to determine dose rate at the standard reference point?
infinite water phantom
what medium (material) does TG-43 assume is surrounding the radioactive sources to calculate dose in?
Point D(r) - point source; Line D(r, θ) - line source
what two models are there TG-43 equations for? what are the assumptions about the geometry of the source in each of these models?
immobilization, spare rectum, spare bladder
why intracavitary implant is packed with gauze
