Risk of secondary cancer from carbon ion arc therapy in two anatomical sites

PURPOSE Carbon ion arc therapy (CIAT) offers biological and dosimetric advantages over intensity-modulated carbon ion therapy (IMCT) but raises concerns about increased low-dose exposure and secondary cancer risk. This study compared radiation-induced secondary cancer risks between CIAT, IMCT, and photon-based volumetric modulated arc therapy (VMAT) for lung and head & neck cancer patients. METHODS Treatment plans for 20 patients (10 lung cancer, 10 head and neck cancer) were generated using IMCT, CIAT, and VMAT. Plan quality was evaluated through conformity index (CI), homogeneity index (HI), and organ-at-risk doses. Secondary cancer risk was assessed using the organ equivalent dose (OED) framework with linear and mechanistic models. The Wilcoxon signed-rank test was used for comparisons. RESULTS Both IMCT and CIAT showed superior dose conformity (CI: 0.80-0.84) versus VMAT (CI: 0.69-0.72; p < 0.05). CIAT further lowered the maximum spinal cord and brainstem doses. CIAT reduced contralateral lung secondary cancer risk by 90% (OED ratio: 0.11), ipsilateral lung by 17%, and brainstem by 60-67% versus VMAT. IMCT offered slightly lower ipsilateral lung risk than CIAT, while both showed comparable brainstem risk. CONCLUSIONS IMCT and CIAT showed comparable secondary cancer risks with significant advantages over VMAT, with CIAT showing particular promise for critical structure sparing.