Overall, pleomorphic adenoma or mixed tumor is the most common neoplasm of the seromucinous glands of the upper aerodigestive tract.³,⁵ These tumors may develop in patients of any age and are somewhat more
common in women. Within the upper aerodigestive tract, the tumors most commonly develop in the palate, likely secondary to the large number of seromucinous glands present at this site.

Grossly, the tumors are firm and appear circumscribed. They may be noted to be mobile; however, they can involve adjacent bone, as space for growth is limited throughout the upper aerodigestive tract.⁶ Unlike their counterparts in the major salivary glands, these tumors rarely will have well-developed capsules. They should, however, remain without infiltrative margins (eFig. 6.1).⁵,⁶

Pleomorphic adenomas are especially known for their histologic heterogeneity and will display both epithelial and mesenchymal components.³,⁵,⁶ The epithelial components show both true epithelial and myoepithelial differentiation. Architecturally, the epithelial cells often form
glandular structures with lumens (Fig. 6.1; eFig. 6.2). The architecture, however, can be complex and numerous anastomosing trabeculae, small tubular glands, nests, and single epithelioid cells can be present (eFig. 6.3). The epithelial cells may be columnar, cuboidal, squamous, plasmacytoid, oncocytoid, basaloid, clear, or spindled (eFig. 6.4). Most tumors have more than one epithelial cell type scattered throughout the lesion. The stromal elements may by myxoid, hyaline, chondroid, or even osseous (Fig. 6.2; eFig. 6.5). Both the mesenchymal and epithelial elements generally show bland cytology, and mitotic figures are rare. Pleomorphic adenomas of the seromucinous glands typically show a more cellular, myoepithelial phenotype than their counterparts in the major salivary glands and have less stromal elements.⁵ Furthermore, in our experience, they often contain more prominent squamous differentiation.

The differential diagnosis for these lesions may include other salivary gland-type adenomas and malignancies such as polymorphous (low-grade) adenocarcinoma, adenoid cystic carcinoma, and, occasionally, mucoepidermoid carcinoma when a prominent squamous component is seen. The distinction between these neoplasms and other adenomas on biopsy is somewhat academic; however, even distinguishing them from malignancies is not always necessary as complete excision is recommended for all these lesions. This is the case as the benign tumors may recur without compete excision and even sometimes develop into or concomitantly harbor malignancies (carcinoma ex pleomorphic adenoma).

The immunohistochemical profile of pleomorphic adenomas depends somewhat on the dominant phenotype of the tumor, i.e., whether the
tumor shows mostly epithelial, myoepithelial, or mesenchymal differentiation.⁷ Most of the epithelial components will express various cytokeratins and show variable degrees of myoepithelial differentiation with expression of smooth muscle actin (SMA), calponin, S100, and glial fibrillary acidic protein (GFAP), CD10 and p63 (eFig. 6.6).⁶,⁷,⁸,⁹

Many pleomorphic adenomas have been shown to harbor karyotypic abnormalities with frequent abnormalities involving 8q12 and the long arm of chromosome 12.¹⁰,¹¹ These abnormalities correspond with the PLAG1 and HMGA2 genes, respectively, two genes involved in transcription.¹²,¹³,¹⁴ Both proteins can be shown to be overexpressed in some pleomorphic adenomas by immunohistochemistry.

The distinction between pleomorphic adenomas and polymorphous (low-grade) adenocarcinomas or even adenoid cystic carcinomas can be difficult on biopsy, especially when the particular pleomorphic adenoma is very cellular. The identification of a mesenchymal component would be indicative of a pleomorphic adenoma whereas the identification of perineural invasion or infiltration of surrounding tissues would be indicative of a polymorphous (low-grade) adenocarcinoma or adenoid cystic carcinoma. Pleomorphic adenomas almost always behave in a benign fashion and rarely recur.⁶,¹⁵

Other benign salivary gland-type epithelial tumors also may develop throughout the upper aerodigestive tract. These primarily include myoepitheliomas and canalicular adenomas; however, basal cell adenomas and sebaceous adenomas also have been described. One should note that
malignancy is usually determined by overall infiltration with these neoplasms and that the unequivocal diagnosis of benignancy cannot always be made based on small biopsies.

Canalicular adenomas most often involve the upper lip.¹⁶,¹⁷,¹⁸ They are usually solitary and multilobulated. They are rarely larger than 2 cm and are usually described as painless. The tumors are circumscribed but most do not have capsules. Microscopically, long rows or columns of cuboidal to columnar epithelial cells interweave throughout the tumors and lead to a “beaded” canalicular pattern (Fig. 6.3; eFig. 6.7). Cellular atypia and mitotic figures are rare (eFig. 6.8). The entrapped stroma is pink and often contains many small capillaries. Epithelial antigen expression can be detected by immunohistochemistry as can the expression of S100 protein and SOX10.¹⁹ Other markers of myoepithelial differentiation such as actins or p63 are not expressed.²⁰ After excision, the tumors rarely recur.

Myoepitheliomas resemble pleomorphic adenomas grossly and clinically but microscopically lack epithelial architecture and a significant mesenchymal component (other than myxoid stroma) (Fig. 6.4).²¹,²²,²³ In other words, they are composed entirely of myoepithelial cells. The myoepithelial cells can show a variety of histologic appearances but are usually categorized as epithelioid, spindled, plasmacytoid (rhabdoid), or clear cell (Fig. 6.5; eFigs. 6.9-6.11). The tumors can have a variable cellularity, and some tumors may have a prominent myxoid background. Immunohistochemically, the cells express cytokeratins and will show some degree of myogenous or basal differentiation that can be demonstrated with antibodies to SMA, calponin, S100, SOX10, GFAP, p40, or p63, although more specific markers of myogenous differentiation are
sometimes not seen, especially with the plasmacytoid variant (Fig. 6.6; eFigs. 6.12-6.14).⁸,²²,²³,²⁴,²⁵,²⁶ A minority of tumors have been shown to harbor EWSR1 translocation similar to soft tissue myoepithliomas and clear cell carcinomas.²⁷ Surgical resection should attempt to achieve free margins and the tumors only rarely recur.²¹,²³

The differential diagnosis for myoepitheliomas depends on the histologic phenotype. The plasmacytoid variant must be distinguished from melanoma and plasmacytoma, which can be done by showing epithelial or myogenous differentiation. Antibodies more specific for melanoma or plasmacytoma antigens such as HMB-45 or CD138, respectively, may also be helpful. Spindle cell myoepitheliomas have a broad differential diagnosis including numerous mesenchymal tumors (e.g., nodular fasciitis, schwannoma, neurofibroma, solitary fibrous tumor, melanoma, etc). Antibodies to epithelial antigens or p63 may be helpful, as may antibodies more specific to some of the mesenchymal tumors in the differential diagnosis (e.g., CD34 for solitary fibrous tumors). Finally, clear cell myoepitheliomas must be differentiated from other clear cell tumors (see below). To distinguish myoepithelioma from myoepithelial carcinoma, one must rely on both cytologic features and growth pattern (see below).

Many reported cases of basal cell adenoma of the upper aerodigestive tract appear to have been canalicular adenomas and some authors consider canalicular adenoma simply to be a variant of basal cell adenoma.²⁸,²⁹ Indeed, in reviewing the literature, it is actually difficult to calculate the number of basal cell adenomas that involved the upper aerodigestive tract that did not have a canalicular pattern. It appears that even if basal cell adenomas are distinguished from canalicular adenomas, basal cell adenomas still most frequently involve the lip when they involve the upper aerodigestive tract.⁶

Grossly, basal cell adenomas are well circumscribed.³⁰ Histologically, they are composed of basaloid cells that grow as somewhat organoid nests (the solid pattern), trabeculae, and tubules (Fig. 6.7; eFigs.
6.15 and 6.16).⁶,³⁰,³¹,³²,³³ A variable amount of moderately cellular stroma is present between the aggregates of basaloid cells. The basaloid cells themselves are bland cytologically and have scant cytoplasm (Fig. 6.8). Nuclear palisading is usually noted at the periphery of the epithelial structures, and some cells may have more abundant, eosinophilic cytoplasm and show overt squamous differentiation. Lumens may be seen within the epithelial nests and trabeculae and are often lined by columnar ductal cells. Some cases may have hyaline material surrounding the groups of basaloid cells (membranous-type or dermal analogue tumor). Immunohistochemically, all the cells will show immunoreactivity with antibodies to keratins; however, cells lining the apparent lumina stain most intensely.³⁴ Conversely, antibodies to S100 protein, SMA, and p63 highlight the peripheral cells that are juxtaposed to the connective tissues (eFig. 6.17). p63 will also highlight areas of squamous differentiation. c-kit expression can be seen with both basal cell adenomas and adenocarcinomas.³⁵ Immunostaining with antibodies to beta-catenin will show nuclear localization in most basal cell adenomas as the tumors often harbor CTNNB1 mutations.³⁶

Basal cell adenomas must be distinguished from other basaloid neoplasms such as basaloid squamous cell carcinomas, basal cell adenocarcinomas, and adenoid cystic carcinomas. Basaloid squamous cell carcinomas are high-grade, infiltrative malignancies with numerous mitotic figures and severe cytologic atypia. Basal cell adenocarcinomas are primarily distinguished from adenomas by their infiltrative borders, and thus the two cannot usually be distinguished on small biopsy. Adenoid cystic carcinomas
are infiltrative, typically have perineural invasion, and many have higher grade cytology. Furthermore, a cribriform growth pattern is not often seen with basal cell adenomas. Aside from the membranous type, basal cell adenomas do very well and almost never recur after adequate resection.⁶

Papillary neoplasms of the salivary glands that cannot better be classified as other salivary gland-type neoplasms occur predominately within the seromucinous glands of the upper aerodigestive tract. These include inverted papillomas, intraductal papillomas, and the rare sialadenoma papilliferum.³⁷ These lesions are all benign and do not develop into malignancies.

Inverted papillomas occur most frequently in the mouth either at the lip or beneath the buccal mucosa.³⁷,³⁸ These lesions present as nodules or swellings that typically measure approximately 1 cm in size. Grossly, they appear well circumscribed. Microscopically, these tumors appear to involve a large duct that may be seen to be in continuity with the surface epithelium (eFig. 6.18). The duct is filled and expanded by a papillary proliferation of predominately squamous epithelium with scattered mucous cells that is frequently covered by columnar cells (eFig. 6.19). This squamous epithelium shows maturation without atypia and usually does not keratinize (Fig. 6.9). True infiltration of the stroma should not be seen and if identified should raise the question of a malignancy.

Intraductal papillomas also most frequently involve the lip or buccal mucosa.³⁷ The lesions present as swellings or nodules and usually measure about 1 cm in size. Microscopic examination shows a large duct lumen distended by arborizing papillae that are lined by a single layer of cuboidal to columnar epithelium with occasional mucous cells, all devoid of cytologic atypia.

Sialadenoma papilliferum is extremely rare and most often involves the palate of middle-aged to older individuals.³⁷,³⁹ The lesions usually appear somewhat verrucoid or papillary grossly. The papillary structures are covered at the tips by a thickened and often keratotic squamous epithelium (Fig. 6.10). Lining the sides of these papillae and separating one from another is ductal epithelium that is often two cells thick (eFigs. 6.20 and 6.21). These epithelial cells may appear somewhat oncocytic and can show tufting. The ducts extend into the stroma and branch. Some degree of chronic inflammation is usually noted within the stroma. Tumors frequently have BRAFV600E or HRAS mutations except in cases where the ductal components appear oncocytic.⁴⁰

Cystadenomas represent between 5% and 10% of benign salivary gland-type neoplasms of the upper aerodigestinve tract and often involve the lips, buccal mucosa, and palate.⁵ The age range of patients with these lesions is large, and about twice as many occur in women. Nodular and cystic spaces can be seen on cut section.⁶,⁴¹ Histologically, the cysts are lined by a cuboidal or columnar epithelium (Fig. 6.11; eFig. 6.22). Mucinous,
oncocytic and squamous epithelia have all been described. The cytologically bland epithelium varies in thickness and papilla can often be found. Some cystadenomas have been noted to involve the larynx. Here, they have been noted to have a bland oncocytic epithelium often with papillae.⁴²,⁴³

Other benign salivary gland-type tumors described in the upper aerodigestive tract include sebaceous adenomas and oncocytic proliferations, including oncocytomas.⁴⁴,⁴⁵ Lesion similar in appearance to Warthin tumors have been described also here.⁴⁶ These tumors share the histologic features of those tumors described in the major salivary glands.

Adenoid cystic carcinoma is usually listed as the second most common malignant salivary gland-type neoplasm of the seromucinous glands of the upper aerodigestive tract.⁴⁴ They occur anywhere throughout the tract but most often involve the palate.⁵,⁴⁷,⁴⁸ They show no sex predilection and may involve patients of any age, although they usually present in middle-aged or older patients. The tumors are often painful, possibly because they frequently show perineural invasion. Patients with adenoid cystic carcinomas do rather poorly and are at a very high (up to 85%) risk for recurrence.⁴⁹ Although the disease is somewhat indolent, it is almost always deadly, and up to 90% of patients die within 15 years of their diagnosis. Adenoid cystic carcinomas also often metastasize to distant locations, especially the lung, unlike most other salivary gland malignancies.

Grossly, the tumors may appear deceptively circumscribed while they are, in fact, quite infiltrative. Microscopically, the tumors show three characteristic growth patterns: cribriform, tubular, and solid (Fig. 6.12).⁴⁷,⁴⁸ Most tumors show some mixture of these patterns. The cribriform pattern is the most common and is characterized by nests and larger islands of epithelioid cells, which contain numerous punched out circular spaces. These spaces may contain a basophilic, somewhat frothy material or dense eosinophilic material (Fig. 6.13; eFigs. 6.23 and 6.24). The frothy basophilic material may also be seen within the surrounding stroma. The stroma may appear hyalinized and can resemble the eosinophilic material
found within the cystic spaces that is believed to be basal lamina. The individual neoplastic cells are small with angulated nuclei (eFig. 6.25). Cytoplasm is usually scant and may vary from basophilic to eosinophilic to clear. Small nucleoli may be present, and apoptotic and mitotic figures are
usually infrequent. Rare ductal structures may be seen with all histologic patterns and are lined by epithelial cells. The tubular pattern of growth often is composed of cells that tend to show more distinct epithelial or myoepithelial differentiation, and the tubules will contain the same materials noted previously. Finally, the solid pattern is composed of variably sized nests of basaloid cells with no or very few cystic spaces (Fig. 6.14). This pattern is considered to be higher grade than the others, and apoptotic figures, mitotic figures, and even comedo-type necrosis may be seen (some tumors that have been described as such may actually be other tumors, however, e.g., basaloid squamous cell carcinoma and salivary duct carcinoma).⁴⁷ All patterns typically infiltrate the surrounding stroma and perineural invasion can almost always be identified (Fig. 6.15; eFig. 6.26).

The grading of adenoid cystic carcinomas is probably of no use with biopsy, although it may be important at resection.⁵⁰ Some studies, however, have shown that tumors with any solid components (grade 2) or with more than 30% solid component (grade 3) are associated with higher mortality rates.⁵¹ We believe that grading may be of limited use and that many reports discussing its predictive value predate the widespread recognition of polymorphous (low-grade) adenocarcinoma and basaloid squamous cell carcinoma.

Adenoid cystic carcinomas rarely undergo high-grade transformation or “dedifferentiation.” Of the 17 cases reported in two larger series, 11 originated in the upper aerodigestive tract, most often in the sinonasal area.⁵²,⁵³ For such a diagnosis, a conventional component of adenoid cystic carcinoma must be present with a high-grade carcinoma. The high-grade component typically lacks the biphasic differentiation seen with conventional
adenoid cystic carcinoma and can show squamous differentiation (Fig. 6.16). This can be demonstrated with immunohistochemistry as myoepithelial staining is typically lost. The high-grade component most frequently grows as solid sheets or nests, although cribriform, and micropapillary
growth has been described. Individual cells are enlarged and pleomorphic with atypical nuclei that have vesicular chromatin and prominent nucleoli. Numerous mitotic figures are present with necrosis, including comedoform necrosis within the large nests of neoplastic cells. Unlike with conventional adenoid cystic carcinoma, lymph node metastases develop in half the cases showing high-grade transformation; thus, some suggest lymph node dissections may be warranted for these patients.

By immunohistochemistry, neoplastic cells show myoepithelial and epithelial differentiation and react with antibodies to p63, S100, SMA, and pancytokeratin.²⁰,⁵⁴,⁵⁵ The staining patterns reflect the differentiation of the various cells with the peripheral cells (those juxtaposed to the stroma) showing mostly myoepithelial differentiation (eFigs. 6.27 and 6.28). Some cells within the solid and cribriform nests also show myoepithelial differentiation including those surrounding the apparent ducts. Other ducts, however, have a more epithelial phenotype and do not express myogenous antigens. The trabecular components show a much more obvious biphenotypic differentiation with the outer cells showing a myoepithelial phenotype and the inner cells showing an epithelial phenotype. The tumors are sometimes strongly immunoreactive for antibodies to c-kit (expressed by the more epithelial cells) which may be useful for distinguishing the tumors from other neoplasms, especially polymorphous (low-grade) adenocarcinomas (Fig. 6.17).³⁵,⁵⁶ Solid patterns have been noted to show more diffuse immunoreactivity. This antibody cannot be used outside of histologic context, however, and we do not believe that it provides for definitive distinction of salivary gland-type tumors in and of itself.

Approximately two-thirds of adenoid cystic carcinomas have been shown to have a recurrent cytogenetic abnormality, the t(6;9) (q22-23;p23-24) that juxtaopposes the MYB and NFIB genes.⁵⁷,⁵⁸,⁵⁹,⁶⁰ This results in the expression of the MYB-NFIB fusion protein which can be demonstrated with immunohistochemistry in more than 80% of adenoid cystic carcinomas (including those apparently lacking the translocation) using antibodies to MYB. Unfortunately, other tumors in the differential diagnosis also stain positive, including most basaloid squamous cell carcinomas.⁵⁹ Although the tumors are often strongly reactive with antibodies to c-kit, molecular abnormalities of its gene have not been noted and the tumors have not been found to be responsive to tyrosine kinase inhibitors.⁶¹

The differential diagnosis for adenoid cystic carcinoma can be extensive and includes other salivary gland and basaloid neoplasms (Table 6.2). On biopsy, the most important distinction is with basaloid squamous cell carcinoma (as discussed in Chapter 5). This is because preoperative radiation and neck dissection may be employed with basaloid squamous cell carcinoma but should not be used with most adenoid cystic carcinomas. Although much time is spent with biopsies trying to distinguish adenoid cystic carcinomas from polymorphous (low-grade) adenocarcinomas, it may not always be necessary. The distinction will be discussed, however, below. Adenoid cystic carcinoma must also be distinguished from HPV-related multiphenotypic sinonasal carcinoma.⁶² These tumors occur in the sinonasal tract and can appear morphologically and immunohistochemically similar to adenoid cystic carcinomas. The tumors, however, show a wider spectrum of growth (see below), p16 expression and HR-HPV infection.