Data from previous animal experiments were analyzed retrospectively with a view to the induction of secondary tumors. The skin of 86 rats was exposed locally to multiple fractions of 200 kV p X rays at doses of 60 to 82 Gy applied in 30 or 35 fractions in 6 weeks, i.e., in the clinically relevant range of 1.9 to 2.7 Gy per fraction. The course of the early skin reaction was scored and compared to the late carcinogenic effects of the treatment using Kaplan-Meier data analysis. With increasing total dose the median duration of the early skin reaction increased, while median latency of tumor induction in the irradiated area of skin decreased from 381 days after 60 Gy to 274 days after 82 Gy. Independent of the total dose, number of fractions, and operative intervention, 84 to 100% of the animals cured by irradiation developed a secondary neoplasm inside the treated area. The majority of induced tumors were squamous cell carcinomas. In the skin covering the thorax, which had received a dose of 15 Gy in 10 fractions for elective irradiation of the lungs, no tumors were induced.
