Primary squamous cell carcinoma of the parotid gland (pSCCP) has long been recognized as a separate entity and is included in the WHO classifications of salivary gland tumors. However, it is widely accepted among head and neck pathologists that pSCCP is exceptionally rare. Yet, there are many publications describing series of pSCCP and data from SEER and other cancer register databases indicate erroneously an increasing incidence of pSCCP. Importantly, pSCCP and metastatic (secondary) squamous cell carcinoma to the parotid gland (mSCCP) have nearly identical histological features, and the diagnosis of pSCCP should only be made after the exclusion of mSCCP. Moreover, all of the histological diagnostic criteria proposed to be in favor of pSCCP (such as, for example, dysplasia of ductal epithelium) can be encountered in unequivocal mSCCP, thereby representing secondary growth along preexistent ducts. Squamous cell differentiation has also been reported in rare genetically defined primary parotid carcinomas, either as unequivocal histological squamous features (e.g., NUT carcinoma, mucoepidermoid carcinoma), by immunohistochemistry (e.g., in NUT carcinoma, adamantinoma-like Ewing sarcoma, basal-type salivary duct carcinoma, mucoepidermoid carcinoma), or a combination of both. Another major issue in this context is that the International Classification of Diseases (ICD) coding system does not distinguish between primary or metastatic disease, resulting in a large number of patients with mSCCP being misclassified as pSCCP. Immunohistochemistry and new molecular biomarkers have significantly improved the accuracy of the diagnosis of many salivary gland neoplasms, but until recently there were no biomarkers that can accurately distinguish between mSCCP and pSCCP. However, recent genomic profiling studies have unequivocally demonstrated that almost all SCCP analyzed to date have an ultraviolet light (UV)-induced mutational signature typical of mSCCP of skin origin. Thus, mutational signature analysis can be a very useful tool in determining the cutaneous origin of these tumors. Additional molecular studies may shed new light on this old diagnostic and clinical problem. This review presents a critical view of head and neck experts on this topic.

• Keywords
• Cancer registration, Diagnostic misclassification, Metastatic cutaneous squamous cell carcinoma, Primary parotid squamous cell carcinoma, Salivary gland carcinoma, UV mutational signature,
• MeSH
• Squamous Cell Carcinoma of Head and Neck pathology   diagnosis   MeSH
• Humans MeSH
• Biomarkers, Tumor analysis   MeSH
• Skin Neoplasms * pathology   diagnosis   MeSH
• Parotid Neoplasms * pathology   diagnosis   MeSH
• Carcinoma, Squamous Cell * pathology   diagnosis   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Review MeSH
• Names of Substances
• Biomarkers, Tumor MeSH

• This ESMO–EURACAN Clinical Practice Guideline provides key recommendations for managing salivary gland cancer. • The guideline covers clinical and pathological diagnosis, staging and risk assessment, treatment and follow-up. • Treatment algorithms for parotid, submandibular, sublingual and minor salivary gland cancer are provided. • The author group encompasses a multidisciplinary group of experts from different institutions and countries in Europe. • Recommendations are based on available scientific data and the authors’ collective expert opinion.

• Keywords
• ESMO–EURACAN Clinical Practice Guideline, diagnosis, follow-up, salivary gland cancer, treatment,
• MeSH
• Adult MeSH
• Medical Oncology MeSH
• Humans MeSH
• Salivary Gland Neoplasms * diagnosis   therapy   MeSH
• Follow-Up Studies MeSH
• Check Tag
• Adult MeSH
• Humans MeSH
• Publication type
• Journal Article MeSH
• Practice Guideline MeSH
• Geographicals
• Europe MeSH

The salivary gland section in the 5th edition of the World Health Organization Classification of Head and Neck Tumours features a description and inclusion of several new entities, including sclerosing polycystic adenoma, keratocystoma, intercalated duct adenoma, and striated duct adenoma among the benign neoplasms; and microsecretory adenocarcinoma and sclerosing microcystic adenocarcinoma as the new malignant entities. The new entry also includes mucinous adenocarcinoma subdivided into papillary, colloid, signet ring, and mixed subtypes with recurrent AKT1 E17K mutations across patterns suggesting that mucin-producing salivary adenocarcinomas represent a histologically diverse single entity that may be related to salivary intraductal papillary mucinous neoplasm (IPMN). Importantly, the number of entities in the salivary chapter has been reduced by omitting tumors or lesions if they do not occur exclusively or predominantly in salivary glands, including hemangioma, lipoma, nodular fasciitis and hematolymphoid tumors. They are now discussed in detail elsewhere in the book. Cribriform adenocarcinoma of salivary gland origin (CASG) now represents a distinctive subtype of polymorphous adenocarcinoma (PAC). PAC is defined as a clinically, histologically and molecularly heterogeneous disease group. Whether CASG is a different diagnostic category or a variant of PAC is still controversial. Poorly differentiated carcinomas and oncocytic carcinomas are discussed in the category "Salivary carcinoma not otherwise specified (NOS) and emerging entities". New defining genomic alterations have been characterized in many salivary gland tumors. In particular, they include gene fusions, which have shown to be tightly tumor-type specific, and thus valuable for use in diagnostically challenging cases. The recurrent molecular alterations were included in the definition of mucoepidermoid carcinoma, adenoid cystic carcinoma, secretory carcinoma, polymorphous adenocarcinoma, hyalinizing clear cell carcinoma, mucinous adenocarcinoma, and microsecretory adenocarcinoma.

• Keywords
• Classification, Gene fusion, Neoplasm, Salivary gland, WHO, World Health Organization,
• MeSH
• Adenocarcinoma * pathology   MeSH
• Adenoma * genetics   pathology   MeSH
• Humans MeSH
• Adenocarcinoma, Mucinous * MeSH
• Biomarkers, Tumor genetics   MeSH
• Salivary Gland Neoplasms * genetics   pathology   MeSH
• Salivary Glands pathology   MeSH
• World Health Organization MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Biomarkers, Tumor MeSH