Radiosensitivity of cells in recurrent experimental tumours and the effectiveness of tumour retreatment.

Two experimental tumour models, a rat rhabdomyosarcoma (R-1) and a rat urether carcinoma (RUC-2) have been employed to evaluate the X-ray sensitivity of tumours recurrent after primary treatments with various doses of X-rays and to correlate changes in volume responses with the cellular radiosensitivity. The responsiveness of R-1 tumours, assessed from the volume reduction as a function of the time after treatment, was less for recurrent tumours, but their growth delay was slightly increased, while the X-ray sensitivity of the tumour cells, assessed by cell survival, was equal to that of the controls. For RUC-2 tumours, however, the reduction in volume after irradiation of the recurrent tumour was larger than after primary treatment, the growth delay was increased, but cell survival curves were not significantly different from those of the controls. It is concluded that differences in volume responses between untreated tumours and recurrent tumours are largely determined by a tumour bed effect (TBE) and that changes in cellular radiosensitivity in these tumours do not play a significant part.