Changes in cellularity induced by radiation in a solid tumour.

The growth, and cellular responses of Morris hepatoma 3924 A to a locally-administered dose of 3750 R X-rays were studied using the following parameters; (1) relative tumour volume changes; (2) tritiated thymidine (3H-TdR) incorporation into DNA; (3) tumour DNA content and (4) cellular analysis, including 3H-TdR labelling index, mitotic index, aberrant mitotic frequency and relative cell density. Before depression of tumour growth, cell proliferation is temporarily interuppted. As proliferation is reinitiated, a short-lived synhcrony and prolongation of cell-cycle traverse are reflected in (a) the labelling index and mitotic index, (b) the relative cell density, and (c) the rate of incorporation of 3H-TdR into DNA. Within 4 days after radiation, cell proliferation and 3H-TdR incorporation are significantly depressed. Simultaneously there are reductions in both the relative cell density and tumour DNA contents, and these remain depressed as the tumours initiate regression. From these studies, it is apparent that the cellular responses to radiation insult occur well in advance of measurable volume changes and are observed both in tumours that continue to regress and in those that initiate regrowth.