Overall survival of pelvic cancer patients depends on control of systemic disease. If local radiation therapy depletes bone marrow function to such an extent that systemic therapies must be withheld, chances of metastatic failure increase significantly. This may be more significant for this group of patients because approximately one third of adult bone marrow is located in the pelvic region. Strategies to minimize toxicities would benefit a range of pelvic cancer patients including gynecologic, anal, rectal, and prostate. New chemoradiation combinations improve outcomes for these disease sites, but come at the cost of higher levels of toxicity. As many as 40% of cervical cancer patients miss at least one chemotherapy cycle due to hematologic toxicity and 36% of anal cancer patients experience grade 3 or 4 hematologic toxicity when undergoing chemoradiation therapy. A clinical trial of concurrent chemoradiation therapy for rectal cancer was terminated due to toxicity, including hematologic toxicities. Concurrent chemoradiation therapy shows promise for advanced stage prostate cancers, but it also increases grade 3 and 4 toxicities. To successfully limit hematologic toxicities for pelvic cancers, it is extremely advantageous to avoid irradiating the highly proliferative compartments of the pelvic bone marrow. However, the complex structure of the pelvis makes it difficult to assess the efficacy of radiation therapy (RT) planning strategies to avoid areas critical to hematopoiesis. Uptake of [18F]fluorothymidine imaged with positron emission tomography (FLT PET/CT) can be an accurate and sensitive tool for identifying and monitoring the effects of chemoradiation on proliferative pelvic bone marrow. Clinically validating the utility of FLT PET/CT imaging for identifying active bone marrow in the design of bone marrow sparing RT-plans and the important bone marrow assessment time points would provide a method to reduce acute and chronic hematologic toxicities for pelvic cancer patients.