Prospective study to assess the role of FDG PET/CT in detecting systemic metastatic spread in rectal cancers with lateral pelvic lymph nodes.

Introduction: The utility of positron emission tomography (PET) in detecting additional M1 patients over conventional staging modalities is not known in rectal cancer patients with enlarged lateral pelvic nodes.

Methods: Prospective, single center, single-arm interventional study of consecutive rectal cancer patients that had baseline lateral pelvic nodes on MRI (>10 mm) between February 2017 to December 2018. Such patients underwent PET after confirming non-metastatic status on CT of thorax and abdomen. Primary outcome measure was additional M1 sites detected on PET. A 10% distant metastasis rate was expected with 80% confidence interval (CI) set at 5% as the lower limit.

Results: 44 patients were included and the concordance between MRI and PET in detection of lateral nodes was 97.7% (43 patients). Additional sites of metastasis were detected in 5 patients (11.36%; 80% CI - 5.63%-20.6%), and there was a change in treatment plan in 7 (15.9%). The number needed to treat (NNT) for PET scans to detect additional metastatic sites and change treatment were 9 and 6 respectively. There was a change in treatment intent (curative to palliative) in 2 patients (4.5%, NNT - 22).

Conclusion: In rectal cancer patients with LPLN, the use of FDG-PET-CT over conventional staging studies led to the detection of additional extra-pelvic metastasis in 11.4% and changed the treatment plan in 15.9%. This met the pre-defined threshold to endorse the use of PET-CT in patients that match the study characteristics.