Cell contact as a possible contribution to radiation resistance of some tumours

This letter is stimulated by the recent communication of Dewey (1971) concerning the radiation survival-curve of Harding-Passey melanoma cells. Subsequently, Hornsey (1972) has suggested that, although the melanoma curve in vitro is relatively radiosensitive, with a D0 of 85 rads, the wide shoulder with an extrapolation number of 25 and Dq of 274 rads may be very significant in determining the curability of this tumour. Furthermore, she noted that the few available published values for Dq or D2-D1 of normal and tumour tissues in vivo are on the average a factor of at least two greater than the values for most single cells irradiated in vitro. This suggests, as do recent results from our laboratory, that the melanoma cells may be even more resistant in situ than when cultured as single cells.