Dose-rate effects in the radiation response of four human tumour xenografts.

The radiation repair capability of four human tumour xenograft lines, two adenocarcinomas of the pancreas, an adenocarcinoma of the breast and a melanoma, has been investigated by means of a soft agar clonogenic assay. Dose-rate dependence studies at 150, 7.6 and 1.6 cGy/min and split-dose experiments, have been performed. Results indicate a good correlation between split-dose recovery and the dose-sparing achieved by irradiation at 1.6 cGy/min when compared to acute irradiation in 3 of the 4 tumour lines. The HX118 melanoma showed dose-sparing at dose-rates of 7.6 and 1.6 cGy/min and a substantial ability to perform recovery between split doses of radiation, whereas HX32, a pancreatic carcinoma, showed little dose-sparing and a correspondingly small degree of split-dose recovery. The HX99 breast carcinoma, was aberrant in showing the greatest split-dose recovery of the four lines but only a moderate extent of low dose-rate sparing. Data have been fitted by two recently described models for radiation dose-rate dependence; the incomplete repair model of Thames and the Lethal Potentially-Lethal (LPL) model of Curtis. Curves were fitted better by the Thames model which provided a good fit for data from 3 of the 4 xenografted human tumour lines. HX99 produced dose-rate dependence survival curves that were not well fitted by either model.