Evaluation of absolute doses for therapeutic carbon ion beams

In order to sophisticate the radiotherapy, high accuracy knowledge of the absorbed dose delivered to the patient is essential. Despite recent progress in carbon ion therapy, accurate values for physical data such as the w value in air or stopping power ratios for ionization chamber dosimetry have not been obtained. Therefore, that causes the uncertainty in determination of the absolute doses for the carbon ion beams. For this reason, we developed and performance-tested a portable graphite calorimeter designed to measure the absolute dosimetry of various beams including heavy-ion beams, using a flexible and convenient means of measurement. The absorbed dose obtained with the calorimeter was compared with that obtained using the ionization chambers following the IAEA protocol in order to evaluate the w values in air for mono-energetic carbon ion beams of 135, 290, 400, and 430 MeV/n. The comparisons to our calorimeter measurements revealed that, using the ionization chambers, the absorbed dose comes out low by 2 to 6% in this experimental energy range and with the chamber types and calibration methods. The w values in air of the carbon ion beams were evaluated to be 35.72 J/G ±1.5% in the energy range used in this study. This value is 3.5% larger than that recommended by the IAEA TRS 398 for heavy-ion beams. Using this evaluated result, the absorbed dose to water in the carbon ion beams would be increased by the same amount. (author)