Adverse effects of cancer therapy. Risk of secondary neoplasms.

Due to the long latency period for solid tumor induction (median 12-13 years), the radiation-induced malignancies now being observed are mostly related to the era of kilovoltage irradiation. Some tumors, such as thyroid cancer, have very low, if any, threshold dose for tumor induction. Sarcomas appear to require higher doses (approximately 100 rads) for induction. Controversy exists as to whether high doses of irradiation are less carcinogenic than lower doses due to greater cell killing at high doses. Acute leukemia has been induced by either irradiation alone or chemotherapy alone. Current intensive therapy protocols using combinations of chemotherapy and radiotherapy, or prolonged chemotherapy, are more leukemogenic, with the 4-year actuarial risk of leukemia in the range 4-17%. Immunosuppression due to various disease states or treatments had been accompanied by malignant tumors, often lymphomas in unusual sites such as the central nervous system. Unusual non-Hodgkin's lymphomas have recently been observed in patients treated for Hodgkin's disease, suggesting that some secondary neoplasms in cancer patients are related to immunosuppression.