TG74

Question 1

What is TG74 on?

Answer 1

collimator output factor

Question 2

what influences Sc?

Answer 2

head scatter: primary collimator, FF, secondary collimator, chamber
-monitor backscatter
-source obscuring at small fs

Question 3

scatter factor

Answer 3

ratio of total absorbed dose to primary dose

Question 4

monitor backscatter range

Answer 4

2% to 5% for largest to smallest field

Question 5

what dominates the output ratio for very small fields at low energies

Answer 5

source obscuring effect

Question 6

shape of mini-phantom

Answer 6

cylindrical (but not cylindrical around the chamber- think of it as chamber is plugged into curved side of cylinder)

Question 7

build-up cap materials for Sc

Answer 7

lucite, graphite, brass
-may have to apply correction factor for high Z

Question 8

challenge for output factors with IMRT

Answer 8

One has to determine Sc for points not only inside of the beam collimation but also outside the beam collimationunder the blocks