The effect of MRI or PET fusion in radiotherapy treatment planning on the pathological complete response rate in rectal adenocarcinoma

523 Background: A pathological complete response rate of 10% to 30% has been noted to occur following preoperative chemoradiation with CT-based treatment planning in patients with rectal cancer. Fusion of the treatment planning CT with other imaging modalities like MRI or PET may help identify tumor location and improve tumor coverage. This retrospective study sought to evaluate the effect of adding MRI or PET imaging to CT-based treatment planning and its impact on pathological complete response rates in patients with rectal cancer. Methods: A retrospective analysis was performed on 39 patients, who received neoadjuvant chemoradiation for rectal adenocarcinoma from February 2009 to September 2013. Patients were divided into two groups. The first group was treated using CT-only based treatment 3D-Conformal or IMRT planning (n=9) and the second was treated using either PET or MRI fusion with the simulation CT scan (n=30). Patients were treated to a total of 5,040 cGy in 28 fractions. Pathological complete response rates (ypT0N0M0) were assessed using postoperative pathologic reports following resection. Results: 39 patients with a median age of 62 received preoperative chemoradiation with an interval to surgery ranging from 34-162 days and a median of 70 days. Patients treated with PET or MRI fusion treatment planning showed a complete pathological response rate at the primary site of 60% and a complete lymph node pathological response rate of 70.83% compared to 22.22% at the primary site and 66.66% at lymph node sites in patients with CT-only treatment planning. In patients treated using MRI or PET fusion, middle rectal cancer showed the best complete pathological response rate at 80%, followed by lower rectal cancer at 41.66%, and upper rectal cancer at 37.5%. Conclusions: Although the sample size was small, utilization of MRI or PET fusion resulted in a higher pathological complete response rate when compared to CT-only based treatment planning, especially in middle rectal cancers. Further studies are needed to accurately identify those patients with a complete pathologic response which may ultimately alter their treatment course.