Room: Exhibit Hall
Purpose: The goal of this study was to evaluate the head scatter factors using the different combination of ionization chambers and buildup-cap (mini-phantom) in flattened and unflattened photon beams.
Methods: An Elekta Infinityáµ€á´¹ linear accelerator(Elekta Inc.) was used for this study. Measurements were done using a 0.6 cc Farmer-type, a 0.13 cc cylindrical, 0.04 cc clindrical ionization chambers. For in-water output factors(Scp) measurements, the chamber was positioned at a depth of dose maximum, 5 gï½¥cmâ?»Â² and 10 gï½¥cmâ?»Â². The source to chamber distance was maintained at 100 cm for field sizes ranging from 2Ã—2 cmÂ² to 40Ã—40 cmÂ². For in-air output factors(Sc) measurements, a brass build-up cap with the thickness of 2.4 mm and 4.0 mm for 6-MV and 10-MV. And an acrylic mini-phantom with thickness of the 10 gï½¥cmâ?»Â² were used, respectively. The chambers were positioned at 100 cm source-to-axis distance (SAD) for field sizes ranging from 2 Ã— 2 cmÂ² to 40 Ã— 40 cmÂ².
Results: The Scp values of both FF and FFF beams measured by all ionization chambers at a depth of dose maximum had similar characteristics except for much smaller or larger fields. The differences of the Scp also varied at a deeper depth in such fields. The Sc values of FF beams measured by the Farmer, CC13 and CC04 ionization chambers with mini-phantom were within 0.3 %, 0.2 % for 6-MV, 10-MV, respectively. By the same token, the Sc values of FFF beams were within 0.3 %, 0.3 % for 6-MV, 10-MV, respectively.
Conclusion: In this study, the head scatter factors derived from the different ionization chambers with buildup-cap(mini-phantom) had similar characteristics both in flattened and unflattened photon beams except for much smaller or larger fields.