Neuroendocrine carcinomas (NECs) are classified as a subgroup of neuroectodermal neoplasms with epithelial differentiation, which are heterogeneous and vary from low to high grade malignant. They can widely occur in different organs, particularly the lungs and larynx [1] . According to the World Health Organization (2017), NECs of the larynx are classified as well, moderately, and poorly differentiated types [2] . Two subtypes of poorly differentiated NECs are recognized as small cell and large cell NEC [2] . Most NECs of the head and neck region occur in the larynx and salivary glands. The oral cavity is an unfrequent location of primary NECs [1] [3] [4] . Interestingly, an NEC combined with a squamous cell carcinoma (SCC) component is referred to as a composite tumor [5] . To the best of our knowledge, there have only been three cases of NEC combined with SCC in the oral cavity reported to date [6] [7] [8] . Although some cases of NEC and SCC overlap have been reported, the possible role of neuroendocrine differentiation in head and neck SCC remains unclear [7] . In this report, we describe a rare case of a primary NEC combined with SCC of the soft palate, and discuss the etiology as well as the histopathological and immunohistochemical features of the tumor.

A 59-year-old Japanese man, with a history of smoking, presented to our hospital with a chief complaint of a painful mass of the right side of the soft palate that first appeared 2 months beforehand. An intraoral examination revealed an irregular reddish mass, measuring approximately 5 × 4 cm (Figure 1). An incisional biopsy was performed, which resulted in a diagnosis of cT2N0M0 SCC. Hence, tumorectomy of the soft palate to the mesopharynx was performed along with neck dissection and flap reconstruction using the forearm skin. There was no evidence of disease at 30 months after surgery.

Microscopically, the resected specimen consisted of two components (Figure 2(A) and Figure 2(B)). One of the components was composed of solid nests,

cords, and glandular-like structures with dispersed high mitotic activities (Figure 2(C)), in addition to small round to oval atypical cells with hyperchromatic molded nuclei, high nuclear-to-cytoplasmic ratios, inconspicuous nucleoli, and scant eosinophilic cytoplasm, as well as oval-shaped cells with abundant cytoplasm (Figure 2(D)). The histologic differential diagnosis of the first component included poorly differentiated SCC, basaloid SCC (BSCC), adenosquamous carcinoma, and NEC. The other component consisted of two phenotypic morphologies. The major area was non-keratotic consisting of dysplastic oval to spindle-shaped squamous cells with indistinct cell borders (Figure 2(E)) and large nuclei that were hyperchromatic and pleomorphic (Figure 2(F)). A few nests were composed of small and crowded basaloid cells. Nuclear palisading was observed at the periphery of the nests (Figure 2(G) and Figure 2(H)). There was no metastasis to the neck lymph nodes.

The two components clearly showed different immunostaining patterns. Nest of atypical small cells were positive for neuroendocrine markers chromogranin A (Figure 3(A)), synaptophysin (Figure 3(B)), CD56 (Figure 3(C)), and p16 (Figure 3(D)), but negative for 34βE12 and p63. Areas of differentiated squamous cells and a few basaloid nests were positive for 34βE12 (Figure 3(E)), p63 (Figure 3(F)), and p16 (Figure 3(G)), but negative for all neuroendocrine markers. Both tumor components were negative for carcinoembryonic antigen (CEA). The immunohistochemical analysis results of the tumor components are summarized in Table 1. Based on these histological and immunohistochemical features, the final diagnosis was primary NEC combined with SCC of the soft palate.

NECs constitute a heterogeneous group of neoplasms with a wide range of tissue origins, histomorphological features, and clinical symptoms [4] . In addition, Mahomed et al. attempted to classify oral NECs into typical carcinoid, atypical carcinoid, small cell, and large cell types [9] . Epidemiologically, laryngeal NECs usually occur twice as common in males than in females and have been reported in patients in the sixth to seventh decade of life [10] . Most patients are heavy tobacco users [10] . Most NECs in the head and neck region occur in the larynx and salivary glands. The oral cavity is a rare site for primary NECs, which have been reported in only 15 cases to date [3] [4] [6] [7] [8] [11] - [19] . The clinicopathological features of the reported cases of primary neuroendocrine tumor of the oral cavity are summarized in Table 2. The sites of primary NECs in the oral cavity were the gingiva, tongue, retromolar region, uvula, floor of the mouth, mandible, and buccal mucosa. The prognosis of oral NECs varies from no evidence of disease at follow-up, to local recurrence and distant metastasis [3] . Some researchers have reported composite tumors of NEC and SCC in various locations, including the lungs and larynx [5] [20] . To the best of our knowledge, there have only been three cases of NEC combined with SCC in the oral cavity

reported to date, which were located at the floor of the mouth and gingiva [6] [7] [8] .

The true origin and histogenesis of oral NECs are not clearly understood. Some researchers have proposed that NECs originate from neuroendocrine cells located at the basal layer of the oral squamous epithelium [9] [18] [21] . Another

hypothesis is that NECs are derived from primitive and pluripotent indifferent cells of either the squamous epithelium or minor salivary gland [18] [21] . NEC and SCC might arise from pluripotent cells that differentiated along two distinct pathways, or NEC could have differentiated secondarily from cells arising from a SCC [22] .

The differential diagnosis of basaloid-like carcinomas of the head and neck included BSCC, high-grade NEC, and poorly differentiated SCC. Differential diagnosis between these carcinoma types can be difficult by simple hematoxylin and eosin staining, especially when dealing with small biopsy specimens. Immunohistochemical staining provides more specific tissue antigen and tissue origin results. High molecular weight cytokeratins, such as 34βE12 and cytokeratin-5/6, can efficiently distinguish NECs from SCCs and BSCCs. Staining of SCCs and BSCCs is strongly positive for 34βE12, while NECs are negative [6] [23] . p63 is also useful to identify basal and stem cells of the squamous epithelium and could be helpful to differentiate NECs from SCCs and BSCCs, as staining is positive for SCCs and BSCCs, but negative for NECs [6] [23] . In our case, tumor cells of the squamous cell differentiated area and a few basaloid nests were positive for 34βE12 and p63, but negative for the neuroendocrine markers. Immunohistochemical analysis of NECs is usually positive for at least one neuroendocrine marker (chromogranin, synaptophysin, or CD56) [2] [3] [6] [7] [23] . In our case, nests of atypical small cells were positive all of neuroendocrine markers (chromogranin A, synaptophysin, or CD56), while negative for high molecular weight cytokeratin 34βE12 and p63. Therefore, the final diagnosis of the present case, according to the histological and immunohistochemical features, was primary NEC combined with SCC. Adenosquamous carcinoma was excluded due to the lack of CEA immunoreactivity [24] . The possibility that the current tumor was a metastatic NEC was not favored, as an extensive clinical investigation failed to detect a primary tumor site.

In addition, recently, El-Mofty et al. [25] reported that human papillomavirus (HPV)-related oropharyngeal SCC had unique microscopic features due to the lack of keratinization and the presence of tumor cells containing bizarre nuclei

M, Male; F, Female; NA, Not available; TC, Typical carcinoid; AC, Atypical carcinoid; NEC, Neuroendocrine carcinoma; SCC, Squamous cell carcinoma; IHC, immunohistochemistry; EMA, Epithelial membrane antigen; CEA, Carcinoembryonic antigen; NSE, Neuron specific enolase; CGA, Chromogranin A; SYP, Synaptophysin; CK, Cytokeratin; LMW, Low molecular weight; CT, Calcitonin; K, Keratin; DOD, Died of disease; DUC, Died of unrelated cause; NED, No evidence of disease; AWD, Alive with disease; AWRR, Alive with recurrent relapses; mths, months.

with extensive mitotic activities. In the present case, the histomorphology of the SCC part was similar to that describe by El-Mofty et al. [25] . The NEC part of our case was comprised of small round to oval atypical cells with high mitotic activities, hyperchromatic molded nuclei, and a high nuclear-to-cytoplasmic ratio. Furthermore, in our case, the tumorcells of both components were diffuse and strongly positive for p16. Immunohistochemistry of this marker is considered to be a reliable surrogate marker of high-risk HPV infection when considering with the appropriate morphologies in oropharyngeal carcinomas [26] . Therefore, in our case, there was a possibility of HPV infection in both NEC and SCC components.

The prognosis of NEC of the oral cavity remains obscure. Variations in the location, size, extension, and differentiation of the primary tumor influence the prognosis [27] . For laryngeal NECs, the 5-year survival rates of the well, moderately, and poorly differentiated types are approximately 80%, 50%, and 15%, respectively [10] . In our case, there was no evidence of disease at 30 months after surgery.

To the best of our knowledge, the present case is the first of a composite tumor consisting of NEC and SCC of the soft palate. Further studies are needed to elucidate the histogenesis, precise treatment, and prognosis of oral NEC. Hence, more cases and future studies are needed to clarify the pathophysiology of NEC combined with SCC.

In summary, we have presented a rare case of primary NEC combined with SCC of the soft palate. The final diagnosis was concluded through the combination of morphological and immunohistochemical results. More cases of this composite tumor in the oral region are required to clarify their pathophysiology.

The study protocol was approved by the Ethics Committee of Kyushu Dental University Hospital and written informed consent was obtained from the patient.

The authors declare no conflicts of interest.

Udompatanakorn, C., Yada, N., Ishikawa, A., Miyamoto, I., Sato, Y. and Matsuo, K. (2018) Primary Neuroendocrine Carcinoma Combined with Squamous Cell Carcinoma of the Soft Palate: A Case Report and Review of Literature. Open Journal of Stomatology, 8, 90-99. https://doi.org/10.4236/ojst.2018.83008