Radiosensitivity

THE general laws governing radiosensitivity were recognized many years ago and have always been an important guide in radiotherapy. The early investigators soon observed wide differences in the reactions of various tumors and adjusted their methods accordingly. By 1915 it was possible to classify most of the well known tumors according to their sensitivity, giving the following general order: 1. Lymphoma. 2. Embryonal tumors. 3. Cellular anaplastic tumors. 4. Basal-cell carcinoma. 5. Adenocarcinoma and adenoma. 6. Desmoplastic tumors, such as squamous carcinomas, fibrocarcinomas. 7. Fibroblastic sarcoma; osteosarcoma; neurosarcoma. The law of Bergonie and Tribondeau was found to have a very uniform application and stated that tumors composed of rapidly multiplying cells, especially those of embryonal type and particularly those of the lymphoid group, are highly susceptible to radiation, and in such ratio as the above characters are developed. In recent years it has become evident that the study of radiosensitivity may be carried very much farther than at first seemed possible; that there are important variations in this property in the subgroups of the major forms of cancer; that the factors entering into radiosensitivity are very numerous and sometimes rather obscure; that the general condition of the patient has important influence on the behavior of the tumor under treatment; that there is a definite relation, not always uniform, between grade of malignancy and radiosensitivity, and that all these matters require careful evaluation before radiation therapy can be intelligently conducted, and its results predicted. Thus a new field of investigation has been opened up, and now occupies the attention of investigators, both laboratory and clinical. The Causes of Radiosensitivity High metabolism of the tumor cells was early recognized as a prominent factor in radiosensitivity. It is so nearly synonymous with rapid growth that one may usually judge of radiosensitivity by rate of growth. Increased or unstable vascularity also goes with rapid growth, so that three factors are generally combined to render rapidly growing tumors sensitive to radiation. Indeed, hypersensitivity may be encountered in this class of growths, especially when they are bulky, and much caution must be used in their treatment in order to avoid accidents, such as interstitial hemorrhage and bulky necrosis. The vulnerability of mitotic nuclei and the abundance of autolytic cell ferments are probable factors of much importance in the sensitivity of rapidly growing tumors. The embryonal character of the cells of origin renders the entire group of embryonal tumors highly radiosensitive. Their response to treatment is probably more rapid and complete than with any other group, so that radiologists have come to recognize these tumors as especially favorable.