Polypoid Gallbladder Neuroendocrine Tumor Diagnosed As Benign Polyp Before Surgery: A Case Report

Gallbladder neuroendocrine tumors (GB-NETs) comprise only 0.5% of all NET cases, and their biology has been incompletely characterized. In the present study we report the case of a 50-year-old male patient with GB-NET who was admitted to Naito Hospital with diarrhea as the main complaint. At initial diagnosis, serum carcinoembryonic antigen (CEA) and carbohydrate antigen 19-9 (CA19-9) levels were within the normal range. Abdominal ultrasonography and contrast-enhanced computed tomography (CT) revealed gallbladder adenomyomatosis and cholecystitis, and an 8-mm pedunculated polypoid lesion was found in the neck of the gallbladder using drip infusion cholecystocholangiography-CT. As it was considered a benign polyp, laparoscopic cholecystectomy was performed. Pathological examination revealed a polypoid lesion that comprised NET cells with a cord-like or ribbon-like arrangement, and the cells exhibited positive immunostaining for chromogranin A and synaptophysin. In addition, immunohistochemical staining showed a Ki-67 index (i.e., proliferation index) of <1%, and no necrosis or mitotic figures were observed in the background. Based on these observations, we diagnosed the following: GB-NET, G1, 10x12 mm in size and located in the gallbladder neck. According to the World Health Organization 2010 classification, NET G1 is a well-differentiated tumor, with the tumor cells having a low proliferative potential [Ki-67 index <= 2%; mitotic figure number <2 (/10 HPF)]. It is regarded as a low- to mild-grade malignancy. Low-grade GB-NET occurs relatively rarely, and no clear guidelines have been formulated regarding its surgical treatment, such as minimal surgical excision margins or lymph node dissection. Detailed treatment recommendations should be developed after systematic studies of additional cases of GB-NET.