TG186

Question 1

What is purpose of TG186?

Answer 1

provide guidance for early adopters of model-based
dose calculation algorithms (MBDCAs) for brachytherapy (BT) dose calculations to ensure practice
uniformity

-HU info can come from CT, but not density. Provides standard materials to use across the industry

Question 2

where does PE dominate

Answer 2

low E sources (< 200 keV)

Question 3

3 major issues with model-based dose calculation algorithms in brachy

Answer 3

• choice of dose specification medium
• voxel by voxel cross section assignment
• commissioning procedures

Question 4

Is CPE valid for treatment planning in brachy?

Answer 4

yes, except close to surfaces or metal-tissue interfaces

Question 5

what do model-based algorithms in brachy do?

Answer 5

either explicitly simulate the
transport of radiation in the actual media or employ multipledimensional
scatter integration techniques to account for the
dependence of scatter dose on the 3D geometry

Question 6

3 current alogorithms of interest in brachy

Answer 6

• collapsed cone vonsolution/superposition

- MC simulations

Question 7

scatter kernals in CC for brachy vs EBRT

Answer 7

The common approach in EBRT, where CC has long
been used,14 is to use two kernels, one for the primary dose
and one for the scatter dose, both operating on the energy released
by primary photons. In brachytherapy, the approach is
to calculate the primary dose through a direct raytrace of the
primary photons using the kerma approximation, and use a
scatter order driven process to calculate dose from first scatter
and multiple scatter separately with different kernels, using the raytrace results for the primary dose as a source term
for scattering

Question 8

CC heterogeneity

Answer 8

raytracing operations are used to scale both the primary dose and kernals for heterogeneities using O’connor

-some methods beyong scaling for high Z materials

Question 9

deterministic solutions to boltzmann equation in bracy

Answer 9

• discretize in space, angle, and energy

- iteratively solve boltzmann

Question 10

first MC code with an advanced geometry
package specific to brachytherapy sources having several
nonanalog estimators (to increase computational speed)

Answer 10

PTRAN

-PTRAN can calculate the primary dose analytically
and perform MC simulations for the scatter dose
part

Question 11

differences between Dw,m and Dm,m

Answer 11

-less than 2 % for all tissues except bone

Question 12

electron ranges for brachy

Answer 12

10 keV - 2.5 X 10^-4 cm in water
3000 keV - 1.5 cm in water
for high E, more like EBRT (use ratio of stopping powers)
at small E, electron ranges are so small that you can use ratios of mass absorption coefficients

Question 13

2-wall cavity theory

Answer 13

photons can for example create secondary electrons in the fat lipid and in water and these can have different electron ranges such that some require uen/p ratio, others ratio of stopping power ratio

Question 14

examples of heterogeneous media in brachy

Answer 14

breast tissue (adipose and glandular)
breast and prostate calcifications

Question 15

accuracy of using kV CBCT for EBRT dose calcs vs fan beam

Answer 15

a few percent off
depends on phantom size
up to 20% off for 18 MV x-rays in bone