Predictive factors of acute skin reactions to carbon ion radiotherapy for the treatment of malignant bone and soft tissue tumors

Background: The skin is considered a critical organ at risk in carbon ion radiotherapy (CIRT) for locally advanced malignant bone and soft tissue tumors (MBSTs). The predictive factors for acute skin reactions after CIRT have not been investigated. The present study aimed to identify these factors and evaluate the correlation between the severity of acute skin reactions and skin dose parameters. Methods: CIRT with total doses of 64.0-70.4Gy (relative biological effectiveness [RBE]) was administered to 22 patients with MBSTs. The skin-tumor distance (STD), maximum skin total dose (Dmax), and area of the skin receiving a total dose of X Gy (RBE) were evaluated. Results: All patients developed acute skin reactions after CIRT, including Grades 1 and 2 dermatitis in 15 (71%) and 6 (29%) patients, respectively. There was a significant difference in the STD between the two groups (P=0.007), and the cut-off value of STD for predicting Grade 2 acute skin reactions was 11mm. There was a significant difference in Dmax between the groups (P<0.001), and the cut-off value of Dmax for predicting Grade 2 acute skin reactions was 52Gy (RBE). Significant differences between the two groups were observed in terms of the area irradiated with 40Gy (RBE) (S40), and the cut-off value of S40 for predicting Grade 2 acute skin reactions was 25cm². Conclusions: In acute skin reactions after CIRT for MBSTs, STD, Dmax, and S40 were found to be significant predictive factors for acute skin reactions.