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In monitor unit (MU) independent verification by calculation for irregular field (MLC field) using multileaf collimator in X-ray therapy, it has become common to use collimator scatter factor (S c ) and phantom scatter factor (S p ) instead of total scatter factor (S c, p ). It is usually expressed as S c, p (A)=S c (A)×S p (A), and the field size A is considered but the depth d is not. S c is data of in-air output, and measure with a mini-phantom at constant depth to remove electron contamination. On the other hand, S p is obtained from measurement data of S c, p and S c , and can be expressed as S c, p (d, A)=S c (constant depth, A)×S p (d, A) at an arbitrary depth d, thus S p depends on the depth of S c, p . Therefore, S p needs to consider depth. In addition, a linear accelerator equipped with the tertiary MLC has two field sizes, that are collimator field by upper and lower collimators and MLC field by tertiary MLC below them. In MU independent verification by calculation, it is often used that the estimated value of S p obtained by converting MLC field to equivalent square field and referring to data of S p in square field. To convert the MLC field to equivalent square field, a conversion formula from sector radius r to equivalent square field L by Clarkson's sector integration (Clarkson method) is used. In this study, using 24 types of MLC fields to evaluate estimation accuracy due to the difference of conversion formula in Clarkson method, we estimated value of S p using r=0.5611L of B-Clarkson method and using r=0.5580L of A-Clarkson method. And the difference with the measured value of S p obtained by measuring S c, p and S c in the same MLC fields was compared. While, to evaluate estimation accuracy due to the different depths using these Clarkson methods, the difference between estimated value and measured value of S p similarly obtained at depth of 5, 10 and 15 cm was compared. As results, estimated value of S p using A-Clarkson method than using B-Clarkson method was close to measured value, and it was the same trend at depth of 5, 10 and 15 cm. Therefore, it was suggested that estimation accuracy of S p by A-Clarkson method is higher than B-Clarkson method when verifying beams with different depths in MU independent verification by calculation for MLC field.

Investigation of Estimation Accuracy of Phantom Scatter Factor by Clarkson Method Considering Depth in MLC Irregular Field