Period of risk hypothesis after prophylactic radiotherapy.

A critical review is given of the period of risk hypothesis (Collins's law) in regard to treated medulloblastoma. Exceptions to the law are seen to be of two types. The first type involves recurrence of the primary tumor, either local or disseminated, and is normally observed close to the limit of the risk period. The second type usually presents a malignant astrocytoma at a considerable multiple of the risk period. The relevant pathological and cellular kinetic data are reviewed to answer the question whether glial tumor recurrence is due to dedifferentiation of the primary cell line or is secondary to the therapeutic irradiation.