Tumor Induction in Mice Locally Irradiated with Carbon Ions: A Retrospective Analysis

Tumor induction in mice legs that were locally irradiated with carbon ions was compared to tumor induction by γ rays after single and fractionated irradiation. A total of 250 tumors were induced in 1104 mice that received carbon-ion doses of 5 through 65 Gy. A total of 77 tumors were induced in 371 mice that received γ-ray doses of 45 through 95 Gy. Of 91 carbon-ion induced tumors examined histologically, 97 percent were malignant, and sarcomas such as malignant fibrous histiocytoma (47%) and fibrosarcoma (32%) were most frequently observed. Malignant fibrous histiocytoma was also the most frequently observed tumor (12 out of 20 tumors; 60%) after γ-ray irradiation, followed by carcinomas (25%) such as adenocarcinoma and squamous cell carcinoma. Neither dose fractionation nor linear energy transfer affected tumor induction for carbon ions and γ rays. Dose responses were linear for carbon ions and γ rays, and showed no saturation up to 65 Gy of carbon ions and 95 Gy of γ rays. The relative biological effectiveness of carbon ions was 2.2 for tumor induction and 1.9 for early skin reaction. We conclude that risk of secondary tumor induction by carbon-ion radiotherapy would not be seriously higher than anticipated.