Nodal metastases in small rectal neuroendocrine tumours.

AIM: Rectal neuroendocrine tumours (NETs) are the most common type of gastrointestinal NET. European Neuroendocrine Tumour Society guidelines suggest that rectal NETs measuring ≤10 mm are indolent with low risk of spread. In practice, many patients with lesions ≤1 cm do not undergo complete tumour staging. However, the size of the lesion may not be the only risk factor for nodal involvement/metastases. The aim of this study was to determine if MRI ± nuclear medicine imaging alters tumour stage in patients with rectal NETs ≤10 mm. METHODS: Patients referred to a tertiary NET centre between 2005 and 2020 who met the inclusion criteria of a rectal NET ≤10 mm, full cross-sectional imaging, primarily an MRI scan and, if abnormal findings were identified, a subsequent 68 Ga-DOTATATE positron emission tomography scan were included. All patients were followed up at our institution. RESULTS: In all, 32 patients with rectal NETs 10 mm or less were included in the study: 16 women; median age 58 years (range 33-71); 47% (n = 15) were referred from bowel cancer screening procedures. The median size of the lesions was 5 mm (range 2-10 mm). 81% (n = 26) were World Health Organization Grade 1 tumours with Ki67 <3%. Radiological staging confirmed nodal involvement in 25% (8/32); two cases had distant metastatic disease. Lymphovascular invasion was present in 3% (1/32) of patients but none demonstrated peri-neural invasion. CONCLUSION: This study demonstrates that small rectal NETs can develop nodal metastases; therefore it is important to stage these tumours accurately with MRI at baseline and, if there are concerns regarding potential lymph node metastases, to consider 68 Ga-DOTATATE positron emission tomography imaging.