Hybrid Carcinoma of The Parotid Gland: A Report of Three Extremely Rare Cases with An Immunohistochemical Analysis and A Review of The Literature

Abstract Background: Hybrid carcinoma (HC) of the salivary glands is defined as when two or more kinds of carcinoma exist at the same location in a single mass. Herein, we report 3 rare cases of salivary gland HC. Case Presentation: Case 1 involved an 86-year-old Japanese male. Case 2 involved a 70-year-old Japanese female. Case 3 involved a 66-year-old Japanese male. Histologically, case 1 involved a combination of salivary duct carcinoma (SDC) and squamous cell carcinoma (SqCC). Immunostaining indicated that the former was positive for gross cystic disease fluid protein (GCDFP)-15 and androgen receptor (AR), whereas the latter expressed cytokeratin (CK) 5/6 and p63. Case 2 involved a combination of SqCC and neuroendocrine carcinoma (NEC). Immunostaining indicated that the former was positive for CK5/6 and p40, whereas the latter was positive for synaptophysin and neural cell adhesion molecule. Case 3 involved a combination of SDC and epithelial-myoepithelial carcinoma (EMC). Immunostaining indicated that the former was positive for GCDFP-15 and AR, whereas the inner cells of the latter were positive for CK7, and the outer cells of the latter were positive for alpha-smooth muscle actin and p40. A transitional zone between the SDC and EMC existed in case 3. Conclusions: Therefore, we diagnosed them as parotid gland HC. This is the first report of a case of HC involving a combination of NEC and SqCC. In case 3, it was speculated that high-grade SDC arose from low-grade EMC. However, as HC is different from carcinoma with high-grade transformation, HC should be diagnosed carefully.