High dose rate radiosensitization: a clinical pilot study of 27 cancer patients.

In laboratory experiments, it was reported by some authors that radiation response increases with the dose rate, although other workers have reported that this relationship does not exist. Up to the present, this problem has remained controversial. The present clinical pilot study was designed with the aim of comparing the effect of high and low dose rates. The study was carried out on 27 patients who suffered from various types of malignancy. Twelve patients with advanced but relatively comparable bilateral metastases in the neck region, and 15 patients with multiple pulmonary metastatic foci were studied. Radiation was given at a high dose rate (1112 rad/min) when irradiating one side of the neck or one of the metastatic lesions in the lung, while radiation was given at a low dose rate (400 rad/min) on the opposite side of the neck or a separate lesion in the lung. All other aspects of radiation, however, were identical for all lesions. The total dose varied from 4000-7000 rad/4-7 wk. Five fractions per week and a daily dose of 200-300 rad was given. It was observed that the high dose rate gave rise to a higher radiosensitivity than the low dose rate. Tumor regression rates were calculated by measuring the dimensions of the lesions. The tumor regression rate in the high dose rate group was universally higher than that in the low dose rate group. The radiation response in the high dose rate group was about 5.2-37.6%, averaging 18%, higher than that in the low dose rate group. In conclusion, a high dose rate of radiation elicits a higher radiosensitivity, and hence, a possible higher cancericidal effect.