717 / 1245
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