Interesting and probably useful analyses of the effectiveness of different schedules of radiotherapy in terms of tumor cure probability have in recent years been based on a simple model tumor (1–5) with these characteristics (stated or implied): (a) The tumor is composed of M (a finite number) viable tumor cells. (b) The lethal response of individual tumor cells can be described by the multi-target or multihit model. This assumes that the cells composing tumor tissue respond to irradiation on an individual cellular basis. That is, there is no tissue effect or cell-cell interaction that modifies that response. If the multitarget model is used, the cellular radiosensitivities for cobalt-60 irradiation are assumed to be: aerobic cells, D0 = 165 rads and m = 3 (target number); anoxic cells, D0 = 400 rads and m = 3. These values for the parameters of radio-sensitivity are assumed to be constant throughout the cell replication cycle. (c) Tumor cure or permanent local control requires that all cells be killed by...