[Normal tissue tolerance to external beam radiation therapy: bone marrow].

Bone marrow is one of the major dose-limiting tissue for radiotherapy. It is composed of many sub-units with complex regulatory mechanisms implying cytokines and growth factors, dispersed throughout the skeleton, each acting with a semi-autonomy but are unified into an integrated system that responds to ionizing radiations as one critical organ. A better knowledge of the complexity of this tissue's distribution and physiology is fundamental to understanding and forecasting the consequences of radiation-induced bone marrow injury. According to cancer characteristics, the volume of hematopoietic bone marrow included within radiation fields and the dose it receives vary in a very significant way, and finally the impact on blood cell count varies in widely different ranges. Furthermore, to predict the overall risk of therapy-induced hematological toxicities, it is necessary to take into account the possible contemporary administration of other cytotoxic drugs (before and/or during radiation therapy). Conversely, the hematological toxicity of usually well-tolerated chemotherapies can be increased, if the patient has a history of radiotherapy. Although the importance of minimizing the volume of active bone marrow exposed to ionizing radiations is well established, so far, no consensual recommendation exists about the dose-volume relationship between bone marrow irradiation and hematological tolerance. Data have recently emerged from trials studying the interest of IMRT for treatment of pelvic malignancies which confirm that reducing bone marrow exposure to irradiation prevents the rise of hematological toxicities during and after radiation therapy, even if some questions remain unanswered on how to define the contours of bone marrow volume.