Nodal metastases in small rectal neuroendocrine tumours

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.

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.

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.

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.