Dose Course 2018_Flipping book

b. The concept of equivalent square for quantities describing

head scatter

Points Eye View of the extended source

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%

AAPM TG 71 report, Med Phys 41 031501, 2014