[Evaluation of the accuracy of a compensating filter based on compact NC-Mill for radiation therapy]

The present report describes the fabrication technique and dosimetry aspects of a desktop numerically controlled milling machine (NC-Mill) based a compensator system that uses lead clay (Shield cray, Reactor Experiments, Inc., U.S.A.). Effective path lengths of patients were determined for CT image sets using the ray-tracing technique and converted to compensator thickness with the equivalent TMR method. Rigid urethane foam was processed with the NC-Mill to produce a mold for filters, and the lead clay was adopted as the compensating material. The dose distribution was measured on the compensating plane of an anthropomorphic phantom and a stair-step PMMA phantom. It was found that the radiation field with inhomogeneous dose was as high as 30%+/-3% with the compensating filters. In addition, when the absorbed dose at the central axis of 52 compensating filters that were used clinically was measured, 75.0% showed an error of less than +/-3%, and 3.8% showed the maximum dose error: >+/-5%. Overall, the present system was capable of producing dose uniformity to within +/-5% for a stair-step phantom, an anthropomorphic phantom, and clinical situations.