Effect of Irradiation on the Kinetics of Cellular Proliferation in Animal and Human Tumours

The kinetics of cellular proliferation in tumours is governed by three main factors, the average duration of the cell cycle, the 'growth fraction' and cell losses. The effect of the last-mentioned factors is the main reason for the variations in rate of growth among solid tumours and for the slackening in the rate of growth observed in a solid tumour as its volume increases. In ascitic tumours the duration of the cell cycle has been found to increase considerably during tumour growth. When a cell is transplanted in the form of an ascitic tumour the mechanisms which explain the gradual slackening of the rate of tumour growth are different from those which obtain when it is grafted in the form of a solid tumour; this shows that these mechanisms depend on extrinsic factors relating to the metabolic conditions,- the cellular micro environment and intercellular contact phenomena. The phenomena which occur in an irradiated tumour are quantitatively similar to those found in normal tissue, but they may differ from the qualitative point of, view and in respect of temporal behaviour. These differences, which doubtless depend on the type of tumour, will have to be analysed before radiotherapy can be used to the maximum advantage. (author)