Patients diagnosed with advanced rectal cancer (RC) are often submitted to neoadjuvant chemoradiotherapy (NACRT) treatment. There is no ideal imaging tool to measure the response of tumor after treatment.

The aim of this study was to evaluate the metabolic parameters obtained by FDG PET/CT, before and after NACRT.

This retrospective study analyzed 518 patients with advanced RC. We divided patients in 2 groups: the ones who achieved a complete clinical response after treatment (good responder group) and the ones who did not achieved complete response (poor responder group). All patients underwent pre-treatment and post-treatment FDG PET/CT, with the post-treatment scan performed on average 19 weeks after treatment completion. Among the exclusion criteria were patients who began therapy before pre-treatment PET/CT and patients who did not have both studies, pre and post treatment. We included 37 patients in the good responder group who met all inclusion criteria and were selected for analysis, and 36 patients in the poor responder group. FDG PET/CT was analyzed by two nuclear medicine physician, and qualitative and quantitative analysis were performed. We used a visual response score (VRS) in qualitative analysis: grade 0 – (not reduce/progress) grade 1 (≤ 33% reduction), grade 2 (> 33% to 66% reduction), grade 3 (> 66% reduction), and grade 4 (no uptake after treatment). The following quantitative parameters were analyzed: SUV, MTV, and TLG, with ROIs drawn with a fixed threshold of 41% and a variable threshold which determined the best delimitation of the tumoral area (best fit value).

In visual analysis of good responders group the majority of patients had a VRS of grade 4 (62%) and grade 3 (33%). For poor responders group, 45.9% patients had a VRS grade 3 , 32.4% grade 2, 16.2%grades 0 and 1 and only 5.4% in grade 4. The average reductions in the analyzed variables were calculated: For good responders there was a reduction of 83% (SUV-mean with 41%); 68% (SUV-mean - BVF); 84% (TLG -41%); 94% (TLG- BVF); 7.29% (MTV-41%); 78% (MTV- BVF). For poor responders had a lower response rate: 51% (SUV-mean with 41%); 41% (SUV-mean - BVF); 63% (TLG -41%); 75% (TLG- BVF); 37 (MTV-41%); 6,5 (MTV- BVF.)

FDG PET/CT can provide qualitative and quantitative variables in monitoring neoadjuvant radiochemotherapy response and possibly identify patients who will benefit from earlier monitoring and low-risk patients. Further analyses/studies are still needed to establish a cutoff/value for quantitative measures to aid clinicians.