Presentation Nuclear-interaction correction of integrated depth dose in carbon-ion radiotherapy treatment planning

稲庭, 拓  ,  兼松, 伸幸  ,  原, 洋介  ,  古川, 卓司

Background: In treatment planning of charged-particle therapy, tissue heterogeneity is conventionally modeled as water with various densities, i.e. stopping effective densities Rs, and the integrated depth dose measured in water (IDD) is applied accordingly for the patient dose calculation. Since the chemical composition of body tissues is different from that of water, this approximation causes dosimetric errors, especially due to alternation of nuclear interactions. We propose and validate an IDD correction method for these dosimetric errors in patient dose calculations. Methods: Rs of the patient is converted to nuclear effective density Rn, defined as the ratio of the incidence of nuclear interactions in the tissue to that in water using a semi-empirical relationship between the two. The attenuation correction factor F, defined as the ratio of the attenuation of primary carbon ions in a patient to that in water, is calculated from a linear integration of Rn along the beam path. In our treatment planning system, a carbon-ion beam is modeled to be composed of three components. We corrected the dose contribution from primary carbon ions to IDD as proportional to F, and corrected that from lighter fragments as inversely proportional to F. We tested the correction method for some non-water materials with un-scanned and scanned carbon-ion beams. Results: In un-scanned beams, the corrected IDDs agreed with the measurements within ±1% for all materials and combinations of them. In scanned beams, the dosimetric error is significantly reduced with the correction method. The planned dose distributions agreed with the measurements within ±1.5% of target dose in target, plateau, and tail regions. Conclusions: We tested the correction method of IDD in some non-water materials to verify that this method would offer the accuracy and simplicity required in carbon-ion radiotherapy treatment planning.

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