Anemia may often become a problem in the treatment of the cancer patient. There are insufficient clinical data to assess the overall importance of anemia in radiotherapy, but there is clear evidence that uncorrected anemia is detrimental to local tumor control in some sites. There may be situations, however, when the transfused, previously anemic patient is at an advantage. These patients have shown a dramatically better response than non-anemic patients when radiotherapy for cancer of the cervix was given in hyperbaric oxygen. Animal experiments suggest that adaptive processes may be responsible for this effect. There is an important difference between acute and chronic anemia in their influence on the radiosensitivity of mouse tumors; while acute anemia consistently causes radioresistance, this effect is lost as the duration of the anemia prior to irradiation is prolonged. This would suggest that anemia per se should not cause tumor radioresistance in the chronically anemic patient. Blood transfusion in previously anemic animals has been shown to produce a markedly increased tumor radiosensitivity, but again this is only transient and sensitivity returns to normal when the interval between transfusion and irradiation is extended to 24 hrs. The mechanisms responsible for tumor adaptation to anemia and blood transfusion are not known, but there is evidence that changes in diffusion distances occur within tumors in response to alterations in oxygen availability and that changes in blood chemistry through the 2,3-DPG system may alter the release of oxygen to the tissues. These are complex processes and it remains to be determined what influence they have in the treatment of human cancer. However, the animal data suggest a clear benefit of blood transfusion to restore the hemoglobin level in radiotherapy, but they also emphasize the need to irradiate immediately so that adaptive mechanisms cannot erode the effect.

Hirst, , , , , , , , (1986). Anemia: a problem or an opportunity in radiotherapy? International journal of radiation oncology, biology, physics, 1986 Nov;12(11):2009-17. https://www.ncbi.nlm.nih.gov/pubmed/3533866