[Revision of Calibration Method of CT-Number to Stopping-Power-Ratio Conversion for Treatment Planning of Particle Therapy].

A calibration method for CT-number to stopping-power-ratio conversion was recently proposed as a revision of the Japanese de facto standard method that has been used at particle therapy centers in Japan for over a decade. The revised method deals with 11 representative tissues of specific elemental composition and density, based on a latest compilation of standard tissue data. We report here how the revision was actually implemented into clinical practice. We applied the revised method to 7 CT-scanning conditions currently in use for treatment planning. For each condition, we derived CT numbers and stopping-power ratios of the representative tissues to constitute a polyline conversion function. We analyzed the change of target water-equivalent depth by the revision for 38 beams in treatment plans for 13 randomly sampled patients. The revision caused a mean change of +0.3 mm with a standard deviation of 0.4 mm. The maximum change was +1.2 mm or +0.5% of the depth, which may be clinically insignificant. The transition to the revised method was straightforward and would slightly improve the accuracy of the beam range in particle therapy.