Response to continuous irradiation (CI) in relation to the initial slope of the cell survival curve for tumours and bone marrow.

In vivo ultra low dose rate continuous irradiation (CI, 15 cGy h-1) was used in an attempt to accurately quantify low dose radiation response (less than 2 Gy) of both tumour and normal tissue, a parameter which is clinically relevant but difficult to assess using conventional acute radiation survival curves. Dorsally placed tumours were irradiated by housing tumour bearing mice for up to three weeks in a dedicated 137Cs unit taking care to shield the lower body. Response to CI was monitored by an excision cell survival assay. For bone marrow treatment whole body irradiation was used and BM-CFUc were assessed. In all 4 tissues studied (Lewis lung carcinoma, B16 melanoma, carcinoma MT and C57B1/6 bone marrow) cell survival decreased exponentially with dose; the three tumours had similar D0 (approximately 6 Gy), bone marrow was more sensitive (D0 1.5 Gy). Acute in vitro radiation survival curves were analysed using both the multi-target plus single hit and the linear quadratic models of radiation cell kill. Comparison of CI results with acute radiation survival curve parameters derived from each model suggest that CI offers a useful means of assessing the low dose radiosensitivity of tumours and may therefore be useful in the prediction of tumour response to clinically relevant fractionated treatments.