Neuroendocrine neoplasms (NENs) are composed a group of tumors exhibiting neuroendocrine phonotypes and are divided into three main categories according to the 2010 World Health Organization classification system: well-differentiated neuroendocrine tumors (NETs); grade 1 and 2, Ki-67 ≤ 20% and/or mitotic count ≤ 20 per 10 high-power fields, poorly differentiated NECs; and grade 3, Ki-67 > 20% and/or mitotic count > 20 per 10 high-power fields and MANECs [11]. Eighty percent of tumors arising in the extrahepatic biliary tract (EHBT) are well-differentiated AC; NENs are uncommon [12–21] and MANECs even more so. MANEC refers to a composite tumor characterized by coexisting glandular and neuroendocrine elements, with each accounting for > 30% of the lesion. Because of its histological complexity, the characteristics of MANEC remain poorly elucidated [9].

MANECs predominantly occur in the colon, appendix, and stomach where neuroendocrine cells are diffusely distributed [22, 23]. However, MANECs arising from the EHBT are extremely rare, with a total of 10 cases reported in the medical literature since the introduction of WHO category in 2010 (Table 1). Zhang et al. [9] reviewed the 38 cases of NEN in the EHBT, and MANECs constituted only 9 (23.7%) of the cases identified.

A high rate of misdiagnosis occurs with biliary NEN because its imaging results can appear similar to those of CCA. A well-vascularized, hypodense, and heterogeneously enhanced lesion is observed in CT scans. The common characteristics are lymph node enlargement and upstream bile duct dilation. In magnetic resonance images, biliary NENs mostly appear as nodular (45%) and intraductally growing (45%) shapes and less frequently as periductal infiltration (9%) [24]. In positron emission tomography, high glucose metabolism is usually found in NEN, especially in poorly differentiated NEC [25]. Because of the paucity of tissue obtained from ERCP brush cytology, MANEC is seldom diagnosed preoperatively. The AC component of MANEC is generally detected at the tumor surface, while the neuroendocrine component is found in the deep stroma, infiltrating the stromal and vascular tissues and lymph nodes [21]. Therefore, ERCP may fail to reach the neuroendocrine component, which is embedded in a deeper portion of the tumor [7].

Making an accurate preoperative diagnosis of biliary NEN is extremely difficult due to its indefinite clinical and imaging characteristics; as a result, most MANECs of the bile duct are initially thought to be ACs or NETs [6, 7, 26]. A previous study examined 274 cases of surgically resected biliary tract cancer specimens and reported that 13 of 53 extrahepatic bile duct cancer cases contained neuroendocrine cells and 2 were newly diagnosed as MANEC [21]. Our patient was also first diagnosed with CCA by brush cytology, probably because the AC component was localized in the superficial layer and the NEC component was located in the deeper portion.

Therefore, to make a correct pathologic diagnosis of MANEC, a surgically resected specimen with immunohistochemical staining for neuroendocrine markers may be essential for the correct diagnosis [6–9]. Of the commonly used neuroendocrine markers, two of the most reliable are synaptophysin and chromogranin. Synaptophysin, with its small clear vesicles in tumor cells, and chromogranin, with its large neurosecretory granules, are usually stained diffusely in NEN [9]. CD56 (NCAM) is also used as a neuroendocrine marker [21]. In our case, NEC components were diffusely positive for synaptophysin and CD56 on immunohistochemical staining.

The prognosis of biliary MANEC is generally poor. The natural history of these tumors is still under debate with some reporting the NEC component showing more aggressive behavior, whereas others have concluded that, if the NEC component is well-differentiated, prognosis depends on the AC component [8]. However, the NEC component is said to have a greater effect on prognosis. Zhang et al. [9] reported significant variation by pathological type in the survival outcome of patients with NEN in the EHBT. The median overall survival for patients diagnosed with NET, NEC, and MANEC was 100, 7.7, and 16.6 months, respectively. Additionally, old age and tumor recurrence were found to negatively affect clinical outcomes. The Ki-67 staining index and mitotic count are crucial for tumor grading, as defined in the classification systems [11]. Harada et al. [21] reported that NEC components showed higher proliferative activity on Ki-67 immunostaining, compared to AC components, suggesting that neuroendocrine components, particularly NEC, in biliary MANEC could determine prognosis. In this case, the Ki-67 index of the NEC component was 30%, which was higher than that of the AC component.

The treatment algorithm for MANEC is not well established [7–9]. Surgery may be a mainstay for the treatment of MANEC, and adjunctive therapy with chemotherapy, radiotherapy, and somatostatin analogs can be considered according to the NEC type [7–9]. The chemotherapy regimen selection for MANEC remains a major clinical dilemma, since it is complicated by a mixture of distinctive malignant histologies. It is reasonable to treat MANEC in accordance with the more aggressive component of the tumor. MANECs containing a well-differentiated NET component and AC component should be treated as ACs. MANECs containing a poorly differentiated NEC component should be treated as NECs. A NEN shown to possess the receptor for somatostatin (SSTR) is a good candidate for treatment with a somatostatin analog. In this case, SSTR2 and mTOR were positive in the NEC component upon immunohistochemical staining, indicating the use of somatostatin analogs as adjuvant therapy. Therefore, after tumor resection, the pathological differentiation and diagnosis of NENs is important for chemotherapy [21]. Adjuvant therapies have been seldom attempted in patients with biliary MANEC because clear, consensus-based evidence is lacking. However, adjuvant chemotherapy may be justified, as recurrent events were noted in 2/9 patients (22.2%) [9]. In this case, S-1 was administered 3 months postsurgery as adjuvant chemotherapy because the AC component was dominant in the tumor. Further studies are required to tailor chemotherapy strategies and to determine which component to target to obtain the best therapeutic benefits.