SU-E-T-743: The Simple Monitor Unit Calculation for Irregular Field in Passive Proton Beam

Purpose: We try to apply the monitor unit (MU) calculation procedure for passively proton beam therapy. For estimation of field size factor (FSF) in the process, we used root A method for conversion from irregular field to equivalent field so far. In this time, we changed the conversion from root A method to Clarkson method. The purpose of this study is to ensure the calculation accuracy by comparative evaluation between root A and Clarkson methods. Methods: As fundamental study, we measured absolute dose with several circular field. Then we performed experimental fitting of conversion curve. With this method, the conversion from the equivalent circular field to equivalent square field is available by the assumed equation. As clinical study, we compared the measured MU with the calculated MU (root A and Clarkson method) in 28 prostate patients (76 fields) for the purpose of evaluating the feasibility of our procedure. To simplify the comparison of the calculation procedure, we built the dedicated MU calculation software. Results: As the Result of fundamental study, the assumed equation for conversion from circular field to square field was estimated. As the Result of clinical study, the differences between the measured MU and the calculated MU were within 2% in all cases. All calculated MU were calculated by our software. The differences of MU calculation between measurement MU and calculation MU (root A and Clarkson) were less than 2% for all cases. The average difference and standard deviation of MU calculation between the root A and Clarkson was 0.05±0.05%. However, the calculation accuracy of Clarkson method should be verified in the complicated cases. Conclusion: The MU calculation by use of Clarkson method was supposed and verified in this study. For prostate cases, the Clarkson method is feasible for MU calculation procedure.