Quantification of combined radiation therapy and chemotherapy effects on critical normal tissues

In order to determine the modification of radiation effects on critical normal tissues which occurs with combinations of radiation and cancer chemotherapy, a review of laboratory and clinical data has been carried out. Information on 10 different normal tissues is available. It is clear that the antibiotic cancer chemotherapeutic agents are the most likely to enhance radiation injury, with increased levels reported in all tissues except the central nervous system. The second most common type of injury with combination therapy appears to occur with drugs causing injury to the normal tissue on their own, such as adriamycin in the heart and methotrexate in the central nervous system. Quantification of the dose-effect factor is only available on a limited number of tissues, and, primarily, in experimental animals. From these limited data, it is clear that dose-effect factors between 1.1 and 1.8 are seen, indicating that radiation doses must be reduced by 10-80% for the same level of injury when combined with chemotherapy. The augmentation of radiation damage by cancer chemotherapeutic agents is a serious problem in a wide range of tissues, but a problem which can be dealt with by accurate knowledge as to the dose-effect factor and appropriate modification of the radiation treatment.