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Simplifying Assumption for Determining Sc and Sp

R King

R King*, A Cheung , R Anderson , G Thompson , M Fletcher , Jeff Anderson Regional Medical Ctr., Meridian, MS


SU-E-T-293 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Scp(mlc,jaw) is a two-dimensional function of collimator field size and effective field size. Conventionally, Scp(mlc,jaw) is treated as separable into components Sc(jaw) and Sp(mlc). Scp(mlc=jaw) is measured in phantom and Sc(jaw) is measured in air with Sp=Scp/Sc. Ideally, Sc and Sp would be able to predict measured values of Scp(mlc,jaw) for all combinations of mlc and jaw. However, ideal Sc and Sp functions do not exist and a measured two-dimensional Scp dataset cannot be decomposed into a unique pair of one-dimensional functions.
If the output functions Sc(jaw) and Sp(mlc) were equal to each other and thus each equal to Scp(mlc=jaw)^0.5, this condition would lead to a simpler measurement process by eliminating the need for in-air measurements. Without the distorting effect of the buildup-cap, small-field measurement would be limited only by the dimensions of the detector and would thus be improved by this simplification of the output functions. The goal of the present study is to evaluate an assumption that Sc=Sp.

Methods: For a 6 MV x-ray beam, Sc and Sp were determined both by the conventional method and as Scp(mlc=jaw)^0.5. Square field benchmark values of Scp(mlc,jaw) were then measured across the range from 2x2 to 29x29. Both Sc and Sp functions were then evaluated as to their ability to predict these measurements.

Results: Both methods produced qualitatively similar results with <4% error for all cases and >3% error in 1 case. The conventional method produced 2 cases with >2% error, while the square-root method produced only 1 such case.

Conclusion: Though it would need to be validated for any specific beam to which it might be applied, under the conditions studied, the simplifying assumption that Sc = Sp is justified.

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