The adaptive response of mouse tumours to anaemia and retransfusion.

We have developed exchange transfusion methods to alter the hematocrit of tumour-bearing mice. The effects of anaemia and its correction by blood transfusion on the radiosensitivity of two mouse tumours (SCCVII/St and RIF-1) were studied using excision, in vivo/in vitro assay. An acute reduction in haematocrit caused a high degree of radioresistance equivalent to an increase in the hypoxic fractions by factors of 10 (SCCVII/St) and 30 (RIF-1). As the duration of the anaemia was prolonged, radioresistance was lost until within about 6 h normal radiosensitivity was observed even though the anaemia persisted. The restoration of the normal haematocrit by red blood cell transfusion after 24 h of anaemia caused increased radiosensitivity equivalent to a reduction in the hypoxic fraction by factors of 5 (SCCVII/St) and 10 (RIF-1), but again the effect was transient and normal radiosensitivity was re-established within 24-48 h of retransfusion. Measurements of 14C misonidazole (MISO) binding to RIF-1 tumours after these procedures indicated changes in the number of hypoxic cells which were qualitatively almost identical to those using the cell survival endpoint, leading us to believe that changes in oxygenation were responsible for the altered radiosensitivity. We feel that transfusion procedures could be used to advantage in the radiotherapy of some cancers.