This is a highly malignant composite or combined neoplasm formed by an adenomatous (villous or tubulo-villous) or carcinomatous (adenocarcinoma or squamous cell carcinoma) component and by a poorly differentiated (small, intermediate or large cell type) neuroendocrine carcinoma (NEC) (Figure 4 and Figure 5). This neoplasm has been reported in the esophagus [11,12,13,14,15], stomach [16,17,18,19,20,21], ampullary region [22], large bowel [23,24,25,26,27,28,29,30,31], and anorectal region [32,33].

High grade MANECs with a small cell or large cell NEC component are rare. Approximately 100 documented cases appear in the literature. Most high grade MANECs of the esophagus occur in the distal half [14,34], while those of the stomach show equal distribution in the upper and lower part [21,35,36]. High grade MANECs can also arise in either the right or the left colon [23,24,30,31].

Macroscopically, these neoplasms, independently of the site of origin, appear as polypoid masses or ulcerating stenotic lesions measuring from 0.5 to 14 cm in greatest diameter, with a mean size of about 5 cm. Histologically, the NEC component is morphologically similar to small cell or large cell NEC of the lung and corresponds to a grade 3 neuroendocrine neoplasm, according to the 2010 WHO classification [4]. The small cell component has a diffuse or nesting growth pattern and is formed by small to intermediate sized cells with scanty cytoplasm and fusiform nuclei with granular chromatin and inconspicuous nucleoli (Figure 4). Diffuse geographic necrosis is common. The mitotic rate is high ranging from 20 to 80 mitotic figures per 10 high-power fields. The large cell NEC, or high grade NEC (HGNEC) non small cell type [15], component is formed of cells with an abundant cytoplasm showing more vesicular nuclei with prominent nucleoli as well as a more prominent organoid, trabecular and palisading pattern (Figure 5). Immunohistochemically, pure neuroendocrine areas of both small and large cell neuroendocrine components are diffusely positive for synaptophysin and usually for chromogranin A, although to a lesser extent [31,37,38]. At least two out of three commonly used neuroendocrine markers (synaptophysin, chromogranin A or CD56) must be abundantly expressed to formulate a diagnosis of high grade MANEC [4]. The Ki67 labeling index is usually very high (60%–90%) [12]. Expression of appropriate or inappropriate hormonal peptides, such as somatostatin, adrenocorticotropic hormone (ACTH) or vasoactive intestinal peptide (VIP), have been detected in a few cases of MANEC [39,40,41]. p53 nuclear accumulation (Figure 6A) was detected in 7/11 of our gastric, 3/3 ampullary, and 12/12 colorectal cases [21,31]. In colorectal MANECs neuroendocrine cells generally show nuclear immunoreactivity for CDX2, especially in the large cell subtype (Figure 6B). In addition, TTF1 (Figure 6C) and ASH1 immunoreactivity has also been documented in some neoplasms [31]. The expression of these transcription factors does not show a prognostic significance and they cannot be used as site-related markers because they have also been found to be expressed in NECs of other sites such as the lung, gallbladder and urogenital system [31,42,43]. However, their expression is interesting because it indicates the phenotypical heterogeneity of the neuroendocrine component of colorectal MANECs. Like pure colorectal NECs, colorectal MANECs showing CD117 immunoreactivity (Figure 6D) and vascular invasion were associated with worse patient survival [31]. Ultrastructural analysis reveals a few small (100 to 200 nm in diameter) neurosecretory granules resembling those of immature “protoendocrine” cells of early fetal development [44].

The non-neuroendocrine component of high grade MANECs can be represented by tubulo-villous or villous adenoma [21,23,24,25,30,31], by adenocarcinoma [11,14,15,27,30,31,39,45] or, more rarely, by squamous cell carcinoma [11,12,15,31]. The squamous cell component is most often observed in esophageal and anorectal MANECs [11,12,15], whereas adenomatous or adenocarcinomatous areas are prevalently detected in gastric [17,19,21,35] and colorectal MANECs [15,23,24,25,30,31].

Prognosis of high grade MANECs largely depends on stage and tumor type. In a recent retrospective series of esophageal NECs and MANECs, better survival was registered for patients with loco-regional disease compared to patients with distant metastases [14]. In general, patients with gastrointestinal MANECs seem to have a better median overall survival than patients with pure NECs and this seems to be mainly due to the higher stage at the time of diagnosis of the latter [14,15]. A similar prognostic behavior was registered in our cases of high grade gastric MANECs (Figure 7) [21]. Conversely, in a recent investigation of colorectal MANECs we did not observe a different survival rate between patients with MANECs compared to patients with pure NECs [31], suggesting that some clinical differences between NECs and MANECs may be site-related [46].

In this category two different tumor types are included: mixed adenocarcinoma-neuroendocrine tumor and amphicrine carcinoma. In the first category the exocrine component is represented by an adenocarcinoma/carcinoma which can show different degrees of differentiation, while the neuroendocrine component is represented by a differentiated neuroendocrine tumor which can show grade 1 (NET G1) or grade 2 (NET G2) differentiation, in line with the criteria proposed in the recent 2010 WHO classification of tumors of the gastrointestinal tract [4]. Unlike high grade MANECs, the exocrine component is biologically more aggressive than the neuroendocrine one. Amphicrine carcinoma represents a peculiar tumor in which exocrine and neuroendocrine features are co-expressed by the same neoplastic cells, which show a divergent differentiation demonstrable with immunohistochemical or electron microscopic techniques.

This is a very rare neoplastic lesion containing both an adenomatous and a NET component (Figure 11). This tumor has also been referred to as glandular-carcinoid tumor [9] and has been reported in the colon and rectum and terminal ileum [9,10,91]. Four colorectal cases have been described in two studies [9,10] and we examined such a tumor, located in the rectum of a 45-year-old man. The patients were from 45 to 80 years of age. Macroscopically, neoplasms appear as polyps ranging in size from 1.5 to 3 cm. Histologically, they consist of an adenomatous component either of tubular or villous type, with low- or high-grade dysplasia, and of a NET (carcinoid) component. In some cases the neuroendocrine cells intermingle with the adenomatous glands forming a combined tumor, while in other cases the two components appear to arise as separate lesions juxtaposed to one other (collision tumor). In the case of composite tumors, the polyp had the neuroendocrine component in the center, whereas the adenomatous component occupied most of the periphery of the polyp. Neuroendocrine cells form solid nests and are of small size. They have nuclei with stippled chromatin, lacking significant atypia or mitotic activity. The neuroendocrine component was reported to be argyrophil, chromogranin A and/or synaptophysin positive [9,10]. In some cases positivity for serotonin, somatostatin, and glucagon has been detected [9]. Tumors were limited to the mucosa-submucosa and were removed by endoscopic polypectomy or trans-anal excision. Their prognosis seems excellent (Table 4) because no evidence of tumor recurrence was found in any of the cases so far reported [9,10]. The morphological and prognostic features of these rare neoplasms seem to suggest that they are low grade malignant tumors. They represent an entity not included in the 2010 WHO classification [4]. It is reasonable to propose for this specific category the term “mixed adenoneuroendocrine tumor” (MANET).