Tumor cell repopulation in the rest interval of split-course radiation treatment.

Fifty-two patients who were treated for laryngeal carcinoma stage I and II by either continuous or split-course irradiation were analysed. The 5-year cure by irradiation was the same in both groups. An isoeffect line was drawn for cured tumours and used to estimate the dose necessary to kill the regrowth of tumour cells which survived the first part of the split-course treatment. The estimated dose was 40 +/- 4 cGy per day of the rest interval. The doubling time was then calculated by means of schematic survival curves for fractionated irradiation. It was found that the tumor cells which survive the first part of the split-course treatment have the doubling times 3.5 +/- 0.5 days.