Tumour control following single-dose irradiation of a human melanoma xenograft.

A human malignant melanoma grown in the athymic nude mouse was exposed to single doses of X-rays and tumour control (TC) was studied. The TCD50 at 70 days post-irradiation was found to be 27.4 +/- 0.6 Gy. This TCD50 is considerably lower than that predicted from the in vitro survival curve of cells from the melanoma irradiated in vivo. Studies also indicated that the tumour regrowth delay following large radiation doses possibly might be larger than indicated by the survival levels measured in vitro. Thus the radiation sensitivity of the melanoma measured in vivo appeared to be higher than that measured in vitro. This was probably not due to radiation damage to the vasculature only, as indicated by studies of the transplantability of irradiated tumours. An immune response by the nude mouse perhaps also contributed, as indicated by studies of the transplantability of the melanoma in whole-body-irradiated mice. If this was so, results from studies of the response to therapy of human tumours in the nude mouse, especially when tumour control is used as an endpoint, may not necessarily be representative for tumours in man.