Evaluation of early metabolic responses in rectal cancer during combined radiochemotherapy or radiotherapy alone: Sequential FDG-PET-CT findings

Background and purpose: The purpose of this study was to prospectively investigate metabolic changes of rectal tumors after 1 week of treatment of either radiochemotherapy (28 x 1.8 Gy + Capecitabine) (RCT) or hypofractionated radiotherapy (5 x 5 Gy) alone (RT). Materials and methods: Fourty-six rectal cancer patients, 25 RCT- and 21 RT-patients, were included in this study. Sequential FDG-PET-CT scans were performed for each of the included patients both prior to treatment and after the first week of treatment. Consecutively, the metabolic treatment response of the tumor was evaluated. Results: For the patients referred for pre-operative RCT, significant reductions of SUV(mean) (p <0.001) and SUV(max) (p <0.001) within the tumor were found already after the first week of treatment (8 Gy biological equivalent dose (BED). In contrast, 1 week of treatment with RT alone did not result in significant changes in the metabolic activity of the tumor (p = 0.767, p = 0.434), despite the higher applied RT close of 38.7 Gy BED. Conclusions: Radiochemotherapy of rectal cancer leads to significant early changes in the metabolic activity of the tumor, which was not the case early after hypofractionated radiotherapy alone, despite the higher radiotherapy dose given. Thus, the chemotherapeutic agent Capecitabine might be responsible for the early metabolic treatment responses during radiochemotherapy in rectal cancer.