The immune defenses can be impaired by dysfunction or a reduction in the number of white blood cells. Unlike chemotherapy that has a systemic action (i.e. in the whole body), radiotherapy has a localized action.. Therefore, it does not strongly decrease the number of circulating blood cells (cytopenia), unlike some chemotherapy drugs that have a direct toxic effect on blood cells (red blood cells, white blood cells, platelets). Nevertheless, in specific situations, a decrease in blood cells can be observed, particularly in patients who are undergoing or had previously received chemotherapy. Moreover, in pelvic or spinal irradiation, radiotherapy can decrease the number of cells at sites of blood cell production, located in the bone marrow of vertebrae and pelvis (iliac crests). However, only in exceptional cases this depletion has a clinical impact.
