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

Abstract 

Background and purpose

The purpose of this study was to prospectively investigate metabolic changes of rectal tumors after 1week of treatment of either radiochemotherapy (28×1.8Gy+Capecitabine) (RCT) or hypofractionated radiotherapy (5×5Gy) 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 (8Gy biological equivalent dose (BED). In contrast, 1week 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 dose of 38.7Gy 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.

Keywords: Metabolic treatment response, Sequential FDG-PET-CT imaging, Rectal cancer, Combined radiochemotherapy, Radiotherapy alone