PET-CT fusion in radiation management of patients with anorectal tumors.

To compare computed tomography (CT) with positron emission tomography-CT (PET-CT) scans with respect to anorectal tumor volumes, correlation in overlap, and influence on radiation treatment fields and patient care. From March to November 2003, 20 patients with rectal cancer and 3 patients with anal cancer were treated with preoperative or definitive chemoradiation, respectively. Computed tomography simulation data generated a CT gross tumor volume (CT-GTV) and CT planning target volume (CT-PTV) and (18)F-fluoro-2-deoxy-glucose PET (FDG-PET) created a PET-GTV and PET-PTV. The PET-CT and CT images were fused using manual coregistration. Patients were treated with three-dimensional conformal therapy to traditional doses. The PET, CT, and overlap volumes (OVs) were measured in cubic centimeters. Mean PET-GTV was smaller than the mean CT-GTV (91.7 vs. 99.6 cm(3)). The mean OV was 46.7%. As tumor volume increased, PET and CT OV correlated significantly (p < 0.001). In 17% of patients PET-CT altered the PTV, and in 26% it changed the radiation treatment plan. For 25% of patients with rectal cancer, PET detected distant metastases and changed overall management. Ten rectal cancer patients underwent surgery. When the pretreatment PET standardized uptake value was >10 and the posttreatment PET standardized uptake value was <6, 100% achieved pathologic downstaging (p = 0.047). Variation in volume was significant, with 17% and 26% of patients requiring a change in treatment fields and patient management, respectively. Positron emission tomography can change the management for anorectal tumors by early detection of metastatic disease or disease outside standard radiation fields.