Histological study of the regrowth of a human melanoma xenograft exposed to single dose irradiation.

The aim of this investigation was to find the location and the histological characteristics of cells that were the source of tumour regrowth after single dose irradiation. A human melanoma xenograft was irradiated with a single dose of 25.0 Gy which gives local tumour control in nearly 50% of treated animals. Serial histological sections were made from tumours removed during the first two weeks after irradiation. During the first week the perivascular organization of tumour parenchyma disappeared and the central part of the tumour became necrotic. The occurrence of vascular stasis, thrombosis and endothelial cell changes indicated that radiation injury to the vascular system was involved in the disappearance of the tumour cords. Tumour cells that remained histologically intact were located in subcapsular areas. The incidence of normal mitotic figures increased, and the fraction of abnormal mitoses and the incidence of micronuclei decreased 5 to 7 days before macroscopical regrowth was usually detected. It is concluded that cells which are the source of tumour regrowth were located in the subcapsular areas at the time of irradiation. Radiation injury to the tumour vascular system was an important factor in the necrotization of the tumour centre after treatment.