Metastatic 5-mm rectal neuroendocrine carcinoma.

Many small rectal neuroendocrine tumours (NET) are excised at endoscopy without prior knowledge of their metastatic potential. Although main risk factors for lymph node metastases are tumour size, invasion of the muscularis propria and lymphovascular infiltration, lesions smaller than 10 mm can also metastasize. A 63-year-old woman was referred to our hospital reporting evacuation difficulties. Colonoscopy revealed a submucosal nodule smaller than 1 cm at the lower rectum. The histological diagnosis of the biopsy specimen was NET G1 (WHO 2010). Endorectal ultrasonography demonstrated a 7-mm hypoechoic lesion on the submucosal layer (Fig. 1, arrow) with one swollen lymph node close to the lesion (Fig. 1, arrowhead). Thoracic and abdominal CT detected no distant metastasis. After submucosal injection of methylene blue dye around the tumour for node identification, an en-bloc full-thickness rectal wall transanal excision including both the lesion and lymph node was performed (Fig. 2). Pathological examination demonstrated a 5 mm NET, confined to the submucosal layer, with one lymph node metastasis (pT1aN1, stage IIIB). The tumour was positive for NSE, synaptophysin and cromogranin A, whilst it was negative for somatostatin, PSA and serotonin. The Ki-67 labelling index was 1.5% (G1). Based on these findings and according to ENET guidelines, the patient underwent low anterior rectal resection with total mesorectal resection, J-pouch coloanal anastomosis and loop ileostomy. There was no residual tumour in the rectal wall, but metastasis of neuroedocrine tumour was found in 1 out of 15 lymph nodes identified with a Ki-67 labelling index of 1.1%.