RADIATION pathology, i.e., the histologic mechanisms by which radiation destroys tumors, is of great practical importance to radiologists, who need to know what happens in a tumor when it is irradiated. Such knowledge is at present incomplete. In contrast to the inflammatory diseases, the pathologic anatomy of tumors (aside from the recent developments along endocrine and chemical lines) has been largely concerned with descriptive morphology and classification. Tumor pathology furnishes information of great value to both surgeon and radiologist, yet it consists chiefly of a collection of static pictures. Even attempts to determine histogenesis are often based on no more than a morphologic resemblance to this or that cell of origin; and attempts to determine the degree of malignancy or radiosensitivity from the histologic appearance are limited by as yet comparatively meager clinical statistics, or are largely intuitive judgments based on nothing more definite than the old law of Bergonie and Tribondeau, formulated in 1906. When a tumor is irradiated a dynamic situation supervenes, and complex histologic changes take place. These changes should be familiar to pathologic anatomists, yet we find this field of knowledge crowded with theories, hypotheses, contradictions, and analogies from other biologic test material. In reviewing the literature of the many histologic studies which have been made of irradiated tumors one is struck by the fact that the bulk of these studies were made following single large doses, i.e., the massive dose method. Much of the work was done with radium, in which the large doses close to the applicator often introduced an element of damage to normal tissues which is difficult to evaluate. Specifically, histologic studies of tumors irradiated by the newer fractional and protracted methods are scarce. It may well be that we should revise our views on radiation changes in connection with fractional methods of irradiation. The histologic changes that have been observed in irradiated tumors by the many workers in this field can be summarized in a general way, briefly, as follows: First, it is widely accepted that after a latent period of a few days a great many tumor cells begin to degenerate; that is, they die off by a process of radiation necrosis. The details of this necrosis have been worked out by many, and especially well by Ludford (1). A second type of histologic change has been generally observed in irradiated tumors, consisting of an inflammatory or reactive mechanism. Infiltrations of various kinds of leukocytes take place, as well as proliferation of fibrous connective tissue; and, also, obliteration of blood and lymph vessels in various ways. The view has become widely prevalent that radiation stimulates these natural reactive mechanisms of the body in combating tumor cells.