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Correspondence

Radiotherapy Department, The Churchill Hospital, Oxford

This excerpt was created in the absence of an abstract.

It has been established beyond all doubt that the presence or absence of oxygen at the time of irradiation has a dramatic effect on the sensitivity of biological material exposed to a single dose of sparsely ionising radiation, such as X or rays. The dependence on oxygen is reduced, but not removed, if the radiation used is more densely ionising, or if the dose is administered in a protracted exposure at low dose-rate (Barendsen, Arts, Van Kersen, Bewley, Parnell and Field, 1966; Hall and Bedford, 1966). It has also been established beyond reasonable doubt that most tumours in experimental animals that have reached a significant size contain foci of hypoxic cells (Powers and Tolmach, 1964).