MIGHT INTRINSIC RADIORESISTANCE OF HUMAN TUMOR-CELLS BE INDUCED BY RADIATION

Survival measurements were made on six human tumour cell lines in vitr o after irradiation with single doses of X rays. Doses up to 5 Gy were used giving surviving fractions down to 20%, but the majority of the measurements were made at doses < 1 Gy. These six cell lines have very different intrinsic radiosensitivities: HT29, Be11, and RT112 are rad ioresistant with surviving fractions at 2 Gy (SF2) between 60 and 74%, while MeWo, SW48, and HX142 are radiosensitive (SF2 = 3-29%). For all the cell lines, response over the dose range 2-5 Gy showed a good fit to a Linear-Quadratic (La) model. However, HT29, Be11, and RT112 cell s shelved a significant increase in X-ray radiosensitivity at doses be low < 1 Gy compared with the prediction extrapolated from a LQ model f itted to the data at higher doses. The LQ model also slightly underpre dicted the effect of low-dose X rays in MeWo cells, but the response o f SW48 and HX142 cells was well described by the LQ model at all doses , with no evidence of increased low-dose effectiveness. The most plaus ible explanation for this phenomenon is that it reflects an induced ra dioresistance so that low doses of X-rays In vitro are more effective per Gy than higher doses, because only at higher doses is there suffic ient damage to trigger repair systems or other radioprotective mechani sms. It follows that variation in the amount of inducible radioresista nce might explain, in part, differences in intrinsic radiosensitivity above > 1 Gy between cell lines: cells would be intrinsically radiosen sitive because they have a diminished inducible response.