Tumour regression as a guide to prognosis: a study with experimental animals.

An analysis has been made of the relationship between regression during therapy and the probability of local control in a group of mice treated with single doses and fractionated doses of X rays. The tumours were first generation transplants from spontaneous mammary carcinomas in C3H mice and were irradiated with single doses 3F/4 days, 9F/10 days, 9F/18 days or 15F/18 days. The size of the smallest radiation dose approached those encountered in clinical radiotherapy. A significant correlation was observed between the shrinkage during the treatment period and the local control at 150 days, for three of the four fractionated schedules. A weaker correlation was observed for shrinkage within a week after single doses, and for shrinkage during treatment with nine fractions in 18 days. It is postulated that the rate of shrinkage is an inherent characteristic of each individual tumour and does not reflect the number of cells killed. However, the shrinkage in some tumours during the course of therapy may result in more extensive reoxygenation which makes these tumours more sensitive to subsequent doses of fractionated course. The degree of shrinkage within the period of fractionated irradiation was found to be a useful prognostic guide to ultimate local control in individual tumours.