Patients with distal rectal adenocarcinoma will be submitted to an initial whole body PET/CT prior to the preoperative chemoradiation therapy with 5-fluorouracil and 5040 cGy. Six weeks and 12 weeks after the preoperative treatment is finished the PET/CT will be repeated. At 1 and at 2 years after preoperative treatment is concluded another PET/CT will be performed. The colorectal surgeon evaluating the response to chemoradiation therapy will be blinded to the results of the PET/CT, as the radiologist will be blinded to the response assessment. PET/CT results will not be used to determine treatment strategy, unless metastatic disease or other disease is diagnosed. Patients with a complete clinical response at 8 weeks will be rigorously followed while patients with an incomplete clinical response at 8 weeks will be submitted to radical surgery. PET/CT results will be compared to current radiological studies and final pathological reports.