The bone marrow is a dose limiting cell renewal tissue. One factor that limits wider application of chemotherapy and irradiation is the toxicity associated with prolonged myelosuppression. Through the years, image techniques have allowed the identification of hematopoietically active regions of bone marrow that could be spared with advanced radiotherapy techniques such as intensity modulated radiotherapy and proton therapy. Limiting the radiation dose to proliferating bone marrow might reduce the incidence and severity of hematologic toxicity, improve tolerance to radiation fields, chemotherapy, could prevent hospitalization and decrease the need for transfusions or granulocyte stimulating factors.