The sensitivity of normal stroma to fractionated radiotherapy measured by a tumour growth rate assay.

The growth rate of implanted tumours has been used as an assay for radiation injury in normal stroma. The subcutaneous tissue was irradiated in an unstimulated state and was then stimulated to produce new blood vessels by the inoculation of a syngeneic tumour 3 days after the last irradiation. Steep dose-response curves were obtained with a dose resolution of approximately 1 Gy. The time between irradiation and stimulation of the stroma by the tumour implant was shown to have no effect on tumour growth rate for times ranging from 1 h to 14 days after single doses. This functional assay of stromal damage has been used after irradiation with 1, 2, 5, 10 or 20 fractions of X-rays. Isoeffect data were well fitted by a linear-quadratic equation, for which the ratio of linear to quadratic coefficients (alpha/beta) was 6.2 +/- 0.6 Gy. This is on the high end of the range of published alpha/beta values for late reacting tissues and other stromal/vascular assays, but lower than those for all early reacting tissues. Overall treatment times ranging from 1 to 11 days were tested with the 2 and 5 fraction schedules. No effect attributable to slow repair or repopulation could be demonstrated over this period.