Diapositive 1

b. The concept of equivalent square for quantities describing

head scatter

Output in air, Sc

: depends upon the orientation of the rectangular

fields (on CEE)

Collimator Exchange Effect (CEE):

is mainly caused by a difference

in extra-focal scattered radiation that can reach the point of interest

for the same collimator setting of the upper and lower jaw

• CEE is of the order of 1% to 2.5 % for (most) modern linacs

• for older type of linacs (e.g. Saturne) the difference in output

in air with X-Y vs. Y-X setting can amount to 6%

Points Eye View of the extended source

AAPM TG 71 report, Med Phys 41 031501, 2014