PART 1: RIGHT SUBMANDIBULAR GLAND, EXCISION (15 GRAMS)
Mucoepidermoid carcinoma is a malignant glandular epithelial neoplasm composed of various proportions of mucous, epidermoid and intermediate cells with columnar, clear cell, and oncocytic features (5). It is the most common salivary gland tumor accounting for about 2% to 16% of all salivary gland tumors and 12% to 29% of all salivary gland malignancies (2, 4). More than half occur in major salivary glands with the parotid gland being most commonly affected at 45%. The incidence tends to peak at approximately the fourth decade, with a slight female predominance. It is also the most common salivary gland malignancy in childhood.
Mucoepidermoid carcinomas may exhibit varying degrees of differentiation and biologic behavior. Macroscopically, low-grade tumors are usually well-circumscribed and unencapsulated or partially encapsulated firm, cystic masses. Clinically, patients describe a slowly growing, painless swelling. High-grade tumors on the other hand, are poorly circumscribed, solid masses with infiltrative borders that may be fixed to adjacent tissues. Patients usually experience a rapidly enlarging, painful mass due to facial nerve involvement.
On gross examination, both solid and cystic areas may be apparent on cut section. Cystic spaces usually contain mucinous or hemorrhagic material and solid areas tend to be grayish-white or tan in color. Microscopically, epidermoid/squamous cells are polygonal in shape with copious eosinophilic cytoplasm and ovoid, vesicular nuclei. They form solid nests with intercellular bridging and infrequently present with individual cell keratinization which is only rarely identified in high grade tumors and areas of inflammation. Mucous cells have pale to slightly basophilic cytoplasm with compressed nuclei and frequently predominate in low-grade tumors. Mucin pools and /or a foreign body giant-cell reaction may occur if mucous secretions escape into adjacent tissues. Intermediate cells have a small, centrally located nucleus with scant cytoplasm and vary in size.
Various grading schemes have been proposed which include specific histologic features that have prognostic significance. Although histopathologic grading criteria still remains controversial, the Armed Forces Institute of Pathology (AFIP) scoring system is widely accepted and is based on five histologic features: intracystic component, neural invasion, necrosis, mitotic activity, and anaplsia (3) (Table 1). Cellular anaplasia is defined as nuclear pleomorphism, increased nuclear/cytoplasmic ratio, prominent or multiple nucleoli, and hyperchromasia. This scoring system is only applicable to parotid and minor salivary gland tumors and does not predict the outcome of submandibular gland neoplasms which have significant metastatic potential irrespective of histologic grade( 3, 1). However, Brandwein et al. noted that there is considerable grading disparity among skilled pathologists and the scoring criteria proposed by AFIP tend to downgrade mucoepidermoid carcinomas. Therefore, a modified grading schema to include additional criteria such as lymphovascular and bony invasion, as well as, the pattern of tumor invasion in the form of small nests/islands which enhances both predictability and reproducibility (Table 1).
Table 1: Mucoepidermoid carcinoma scoring system: AFIP vs. Brandwein
Courtesy of Mayo Clinic, Joaquin J. Garcia MD
Well-differentiated glandular or cystic structures lined by a single layer of mucus-secreting cells are usually present in low-grade tumors. Small cystic spaces lined by predominantly columnar cells with adjacent solid areas of intermediate and epidermoid cells is characteristic of intermediate-grade tumors. Papillary cystic infoldings, focal invasion, mild cellular pleomorphism, and few mitotic figures may also be seen. Solid nests or sheets of cells composed of primarily epidermoid cells with a scant cystic component is common in high-grade tumors. They also exhibit obvious invasion, as well as, necrosis, marked cellular pleomorphism and increased mitoses.
Differential diagnosis: necrotizing sialometaplasia, pleomorphic adenoma, cystadenoma, squamous cell carcinoma and clear cell tumors.
Contributed by Richard Freij, MD