Hypoxia and tumours.

Cells in tumours become hypoxic in either or both of two ways. First, the form of tumour growth may result in some cells lying so far from the capillaries that constitute their immediate source of oxygen that it is almost used up by the metabolism of the intervening cells. Secondly, the vascular system of the growing tumours may fail to deliver enough oxygen both because of inadequacies in the vessels themselves and in the quantity of blood that flows through them. The presence of hypoxic cells in tumours is of importance because it may affect the outcome of treatment by radiotherapy. Some two and a half to three times the dose of x radiation is needed to destroy the ability of cells to proliferate in the absence of oxygen as in its abundance (Alper and Howard Flanders, 1956). These three topics will be discussed, though a fourth, not pursued, should be associated with them: this is that the vascular system of a tumour is the pathway by which cytotoxic drugs used in therapy reach their site of action, and a degree of failure in transport may diminish their effectiveness in treatment.