The Effect of Alterations in Haematocrit on Tumour Sensitivity to X-rays

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Abstract
Hypoxic cells in human tumors probably contribute to the failure of radiotherapy in some sites. Changes in the O2 carrying capacity of the blood, such as in anemia, have been shown to influence tumor response. The effect of acute and chronic changes in hematocrit [Hct] on the radiosensitivity of 3 mouse tumors (EMT6, KHT and RIF-1) were studied. Alterations in Hct were achieved by bleeding followed by retransfusion. When radiation was preceded immediately by an acute reducion in Hct (anemia), radiosensitivity was markedly reduced in each tumor. An acute rise in Hct (polycythemia) increased or decreased X-ray sensitivity depending on its severity. The optimum Hct for maximum sensitivity was always found to be at a level 5-10% above normal. When the time between induction of anemia and irradiation was increased, simulating a progressively longer duration of anemia, marked changes in radiosensitivity of all the tumors were observed. A short duration of anemia resulted in a resistant tumor with each cell line, but the resistance was gradually lost as the anemia was prolonged, even though no recovery in Hct occurred. The rate of recovery to normal radiosensitivity varied from 24 to 72 h in the different tumors. Therefore, only Hct changes which occurred within 1-3 days of a dose of radiation affect the radiosensitivity of these tumors.