ABSTRACT
Much of the early work concerned the optimal division of a course of radiation into individual sessions, called fractions, using patterns that could be generalized to wide populations. The work employed models of the differential response of tumor and normal tissues to the number of sessions into which the total prescribed dose is divided and the spacing of these sessions over time. It was recognized early that response and risk depended on irradiated volume and tumor volume, number of fractions and over time of the treatment course.7 Volume ranges over a continuum between approximately 100 and 103 cm3; the number of fractions ranges over integer values between roughly 3 and 40, and the overall treatment time is expressed in days in the range 3-60. The development of improved fractionation schedules based on predictive formulae remains an active area of research, whose findings might ultimately change the prescribed rules for radiation administration.8-10 (see also Section 10.5). For now, treatment is arranged so that a cumulative plan distributing doses across various tissues can be decomposed into separate additive sessions, whose tissue dose levels all fall within their allowed ranges in each session and cumulatively.
