Development of a CT-number parameterization method and validation for Monte Carlo simulation of carbon-ion radiotherapy; A Preliminary study

Chang, W.*; Koba, Yusuke*; Furuta, Takuya   ; Yonai, Shunsuke*; Hashimoto, Shintaro  ; Matsumoto, Shinnosuke*; Sato, Tatsuhiko   

In the treatment planning system (TPS) for radiotherapy, approximate calculation by replacing all materials with water and accounting only the density variation is adopted to reduce the computational cost. On the other hand, conversion from patient CT data to elemental compositions and densities is required to conduct Monte Carlo simulation. Especially for the assessment of secondary cancer risk in carbon therapy, secondary particles produced in the nuclear reaction between incident carbons and human tissues are important so that the difference of the elemental compositions is essential. We have therefore developed a method to convert CT number to human tissues keeping the consistency with the water stopping power table embedded in TPS. We applied this conversion method to 9 different human tissues and confirmed the range of carbon beams are reproduced within 1 mm precision for all the materials.