Some authors have argued that FNA is superfluous, because mass lesions in the head and neck always require excision.12,13 Such an approach would result in the unnecessary excision of a number of lesions (up to one third of salivary gland masses) that in fact do not necessitate surgery. Some patients have a non-neoplastic lesion (e.g., chronic sialadenitis, granulomatous disease, lymphoepithelial cyst); others have a benign neoplasm, and for those who are poor surgical candidates, knowledge of its benignity provides reassurance that surgery can be avoided. Still others have a malignancy for which surgical excision is not appropriate (e.g., lymphoproliferative disease, metastasis). Even when surgery is indicated, FNA guides preoperative strategy (e.g., partial or total parotidectomy, facial nerve resection, neck dissection), providing reassurance to both the patient and surgeon.^2,7,12 Surgical management is often influenced by cytologic findings. High-grade malignancies are treated more aggressively than low-grade malignancies and benign neoplasms. In the superficial parotid gland, the most common site of salivary gland neoplasms, benign tumors and low-grade malignancies are treated with superficial parotidectomy alone, whereas high-grade carcinomas are treated with total parotidectomy requiring facial nerve sacrifice. Furthermore, lymph node neck dissection and neoadjuvant therapy are often indicated for high-grade tumors.

Aspirations are performed using a 25 or 23 gauge needle, small enough to reduce the risk of tissue trauma but large enough to obtain an adequately cellular sample. Evaluation of smears at the time of the procedure can guide the number of passes required by assessing adequacy and also facilitate appropriate triage of the specimen, like allocation of material for flow cytometry (in the case of lymphoid lesions) or cell block preparation (in the case of diagnostically challenging lesions like oncocytic, spindle cell, and clear cell tumors).

Although FNA was first described in the 1930s,14–16 salivary gland FNA is a relatively new discipline, having gained wide acceptance only in the past 40 years.^{1,2,5–8,17–27} Meta-analysis of published series indicates 96% sensitivity and 98% specificity for neoplasia, whereas distinction between benign and malignant neoplasms has 79% sensitivity and 96% specificity.²⁸ Because accuracy varies greatly among published studies, the utility of salivary gland FNA is highly practitioner-dependent.²⁸ For nondiagnostic or indeterminate aspirates, results improve with repeat FNA²⁹ and ultrasound guidance.³⁰ Limited data indicate that core biopsy offers improved accuracy but at a cost: increased requirement for anesthesia, more discomfort for the patient, and a greater risk of complications, including nerve damage, tumor spillage, and needle tract seeding.^28,31–33

Criteria for assessing adequacy have yet to be established. False-negative results are principally the consequence of inadequate sampling of the lesion,³⁴ most frequently those that are cystic. Cystic neoplasms (e.g., Warthin tumor [WT], low-grade mucoepidermoid carcinoma, metastatic squamous cell carcinoma) are the most likely causes of false-negative diagnoses. Cysts frequently yield nonspecific fluid not representative of the underlying lesion. When a cystic salivary gland lesion is aspirated, any residual mass should be resampled after fluid is withdrawn. Surgical excision is indicated if the cyst does not resolve with aspiration or if it recurs.

False-negatives resulting from an interpretation error are most common with low-grade mucoepidermoid carcinoma, adenoid cystic carcinoma, and non-Hodgkin lymphoma. False-positive diagnoses are seen with cystic lesions, particularly WT, which sometimes contains atypical squamous cells, and pleomorphic adenoma (PA), in which nuclear atypia or stromal spheres result in an incorrect diagnosis of carcinoma ex pleomorphic adenoma or adenoid cystic carcinoma, respectively. Multiple passes help to minimize sampling and interpretative errors resulting from variation in cytologic atypia and cellular constituents within a tumor.

Complications are infrequent. Tumor seeding of the needle tract is extremely rare.12,35 The most common complications are bleeding, infection, and facial nerve pain. In some cases, FNA leads to partial or, rarely, complete infarction of the neoplasm.^36–38 Particularly susceptible neoplasms are oncocytoma, WT, and acinic cell carcinoma.^12,39 With use of a 25 gauge needle, significant infarction or hemorrhage is seen in up to 10% of cases, but such changes rarely hinder histologic diagnosis.³⁹

Both Romanowsky-type and Papanicolaou stains are important, because many neoplasms contain a combination of epithelial and stromal components. Air-dried Romanowsky-stained smears highlight diagnostically useful features of the stromal component that are poorly visualized in alcohol-fixed preparations of lesions such as PA basal cell tumors, and adenoid cystic carcinoma. Romanowsky stains also aid in the evaluation of lymphoid lesions. Papanicolaou-stained preparations are especially useful for evaluating nuclear features and cytoplasmic differentiation.

Either smears or liquid-based preparations can be utilized,40–42 but conventional smears are preferred. With liquid-based preparations, extracellular constituents are less prominent, cellular shrinkage is greater, and tissue fragmentation is more pronounced, with possible decreased sensitivity and specificity.⁴⁰ Although the role of immunohistochemical stains is limited, a cell block is valuable for histochemical stains such as periodic acid–Schiff (PAS) and mucicarmine. Cell block preparations also better demonstrate architectural patterns and some cellular features, particularly serous acinar differentiation.⁴³ In challenging cases, cytogenetic evaluation is also valuable. Characteristic chromosomal translocations have been identified in PA mucoepidermoid carcinoma, adenoid cystic carcinoma, mammary analogue secretory carcinoma, and clear cell carcinoma.^44–49

Diagnostic Overview

Precise classification of salivary gland neoplasms by FNA is possible for many of the commonly encountered lesions but remains problematic for a number of the less common entities. Salivary gland FNA, in fact, poses a number of challenges to the cytopathologist. First, there are more than 35 salivary gland tumors of epithelial type,50,51 many of which are rare, placing familiarity with them out of reach for most practitioners. Second, most salivary gland malignancies are low-grade, displaying few overt cytologic features of malignancy. Third, the less common, high-grade malignancies are readily recognizable as malignant but are difficult to distinguish from one another. Fourth, some benign tumors (e.g., basal cell adenoma) have a malignant counterpart (e.g., basal cell adenocarcinoma) that is morphologically identical except that there is an infiltrative growth pattern, something that cannot be assessed cytologically. Finally, many different salivary gland tumors share similar cellular constituents; it is the architectural relationship and relative abundance of these constituents that ultimately determine the tumor type. With some tumors such as PA there is great variability not just within a given tumor but from one tumor to the next.

Fortunately for the cytopathologist, there are two mitigating factors. First, the two most common neoplasms, PA and WT, which together account for greater than 80% of salivary gland tumors, have distinctive cytomorphologic features and are readily identified. Second, conservative excision is used for both benign tumors and low-grade malignancies. By contrast, radical surgical approaches with combined-modality therapy are reserved for high-grade malignancies. Thus, although a specific diagnosis may not be feasible, low-grade neoplasms usually can be distinguished from high-grade ones, and then an appropriate differential diagnosis is sufficient for clinical management. This chapter provides a diagnostic approach and suggested reporting terminology for such cases.

The large group of so-called basaloid neoplasms are a special case, however. These neoplasms encompass the entire spectrum of biologic behavior, from benign neoplasms through low-grade malignancies to the aggressive solid variant of adenoid cystic carcinoma. A precise cytologic diagnosis is not possible with many FNA specimens showing basaloid cells only. A frozen section is often necessary to guide appropriate surgical management. This and other diagnostic dilemmas are listed in the following box. Suggested diagnostic approaches are discussed in detail in the remainder of the text.

Diagnostic dilemmas in salivary gland fine-needle aspiration

• basaloid neoplasms (especially basal cell adenoma and the solid variant of adenoid cystic carcinoma)

• oncocytic lesions (especially oncocytoma and acinic cell carcinoma)

• mucus-containing cysts (especially mucoepidermoid carcinoma, mucocele, and mucinous metaplasia)

• high-grade carcinomas (including mucoepidermoid carcinoma, salivary duct carcinoma, and carcinoma ex pleomorphic adenoma)

• clear cell neoplasms

• spindle cell lesions (especially myoepithelioma and schwannoma)

Clinical information, such as knowledge of a previous malignancy, is often helpful. Aggressive signs, symptoms, and imaging findings, such as rapid growth, pain (suggestive of neural invasion), and infiltrative growth on radiological studies, are indicative of malignancy. Most salivary gland neoplasms are firm and painless, although WT has a characteristically doughy consistency.

The presence of cystic change and bilaterality can also help narrow down diagnostic possibilities.

Tumors that are often cystic

• Warthin tumor (WT)

• mucoepidermoid carcinoma

• acinic cell carcinoma

Lesions that are sometimes bilateral

• sialadenitis

• amyloidosis

• lymphoepithelial cyst

• Warthin tumor (WT)

• acinic cell carcinoma

• lymphoma

The epidemiologic features of salivary gland neoplasms are also helpful. Most salivary gland neoplasms are more common in women, but WT, salivary duct carcinoma, and metastatic squamous cell carcinoma occur more frequently in men. Whereas 68% to 85% of parotid gland tumors are benign, 80% to 90% of sublingual and minor salivary gland neoplasms are malignant.⁵² Some tumors are almost site-specific (e.g., WT in the parotid gland, polymorphous low-grade adenocarcinoma [PLGA] in the minor salivary glands of the palate).

Attention to the constituents of an aspirate is the key to identifying the neoplastic, inflammatory, lymphoid, or cystic nature of a lesion. Hypercellular specimens are typical of neoplastic lesions. Inflammatory cells are prominent in sialadenitis and cystic lesions. Stone fragments are diagnostic of sialolithiasis. Abundant lymphoid cells can be seen in a variety of salivary gland lesions, not all of them lymphoid in nature.

Numerous lymphocytes are seen in:

• intraparotid or periparotid lymph node

• lymphoma

• lymphoepithelial cyst

• Warthin tumor (WT)

• mucoepidermoid carcinoma

• acinic cell carcinoma

• lymphoepithelial carcinoma

The presence and character of matrix material provide important diagnostic information. Mucin (pale magenta in Romanowsky preparations, translucent blue or purple on Papanicolaou smears) suggests a mucoepidermoid carcinoma, mucocele, retention cyst, or mucinous metaplasia. A chondromyxoid matrix is characteristic of PA and stromal spheres are typical of adenoid cystic carcinoma, but neither finding is entirely specific. Such findings, along with a more detailed impression of the cell type(s) seen (myoepithelial, duct-lining epithelial, basaloid, oncocytic, mucinous, squamous, acinic, sebaceous, and clear cells) enable one to refine the differential diagnosis.

A variety of crystalloids are seen in the salivary glands.53,54 Of importance, none of them is specific for any particular salivary gland lesion or neoplasm. Tyrosine crystalloids are floret-shaped and often encountered in pleomorphic adenomas, but they can be seen in other lesions, both benign and malignant (Fig. 11.1A).^53–58 Amylase crystalloids (“nontyrosine crystalloids”) are polygonal, platelike, or needle-shaped and are most often seen in benign, non-neoplastic conditions, especially infections and cysts.^59–62 (Fig. 11.1B).

Figure 11.1 Salivary gland crystalloids.
A, Tyrosine crystalloids are clustered in flowerlike arrangements (hematoxylin and eosin [H & E] stain). B, Amylase crystalloids, needle-shaped or platelike, are occasionally encountered in salivary gland fine-needle aspirations (FNAs), usually in association with inflammatory conditions (Papanicolaou stain; Courtesy of Dr. David Kaminsky, Palm Springs Pathology Services, Palm Springs, CA).

The Normal Aspirate

In some instances, often as a result of sampling error, the FNA specimen shows only normal salivary gland elements.

Cytomorphology of normal salivary gland elements

• rounded clusters of serous or mucinous acinar cells

• flat sheets and tubules of ductal cells

• adipose tissue

Aspirates of normal salivary gland tissue are sparsely cellular, composed of acinar cells, ductal cells, and admixed adipose tissue 4,63 (Fig.11.2A). Occasionally, naked acinar nuclei that mimic lymphocytes and scattered myoepithelial cells are present. Acinar cells are usually arranged in cohesive, grapelike clusters. Ductal cells are smaller and less conspicuous, arranged as tubules or honeycomb-like flat sheets. The acinar cells are of serous type in the parotid gland, a mixture of serous and mucinous types in the submandibular gland, and predominantly mucinous in the minor salivary glands. The serous-type acinar cells (Fig. 11.2B) are evenly spaced, with basally placed nuclei. They are large, pyramidal cells with abundant foamy and granular, basophilic cytoplasm and small, eccentrically placed, round to oval nuclei with indistinct nucleoli. Normal (and neoplastic) acinar cells have characteristic coarse, basophilic, PAS-positive and diastase-resistant cytoplasmic zymogen granules. In contrast with the pyramidal serous cells, mucinous cells are columnar, with pale cytoplasm that indents a bland nucleus. Ductal cells come in several varieties. Intercalated duct cells are uniform, small, and cuboidal, with scant, dense cytoplasm and uniform nuclei; occasional large branching ductal fragments are present. Ductal cells derived from the larger striated ducts are oncocytic, whereas those from the collecting ducts are columnar and ciliated. Ductal cells sometimes exhibit mucinous or squamoid changes. Mature lymphocytes can also be seen, owing to the abundance of intraparotid and periparotid lymphoid tissue. A mixture of ductal cells, adipose tissue, and acinar cells distinguishes normal salivary gland tissue from acinic cell carcinoma, which usually consists of a monomorphous population of neoplastic acinar cells.

Figure 11.2 Normal salivary gland.
A, Normal acinar cells are aggregated in grapelike bunches (arrow) with admixed adipose tissue and an occasional flat sheet of ductal cells (arrowhead) (Romanowsky stain). B, Acinar cells have uniform, round nuclei and abundant vacuolated cytoplasm (Papanicolaou stain).

Up to 20% of salivary gland aspirates yield only normal tissue.5,63 A finding of only normal salivary gland elements in the FNA specimen warrants clinical correlation to exclude the possibility of sampling error. Other explanations for a normal elements–only result include a prominent but normal salivary gland, sialadenosis, hamartoma, and lipoma.

Non-Neoplastic Conditions

Acute and Chronic Sialadenitis

In acute sialadenitis, aspiration is rarely performed, because the disorder is usually diagnosed clinically as a postoperative complication, viral or fungal infection, or secondary bacterial infection due to an obstruction such as that caused by sialolithiasis 64–67 (Fig. 11.3). There is usually no discrete mass, and in most cases the condition involves the parotid gland.

Figure 11.3 Sialolithiasis with acute sialadenitis.
Stone fragments (arrow) are blue, irregularly shaped, jagged structures of varying sizes that are diagnostic of sialolithiasis. The presence of numerous neutrophils signifies an accompanying acute sialadenitis (Romanowsky stain).

Cytomorphology of acute sialadenitis

• neutrophils, necrotic debris

• stone fragments (if sialolithiasis also present)

• scant ductal cells

Fine-needle aspirates from patients with acute sialadenitis show abundant neutrophils, necrotic cells, and fibrin.⁶⁴ Small groups of ductal cells, some with reactive atypia, are present. A malignant neoplasm is excluded based upon the hypocellularity and limited atypia. When an infectious etiology is suspected, a portion of the material should be sent for a microbiologic work-up. If there is a clinical suspicion of a neoplastic process, reaspiration after treatment and resolution of the acute infection may potentially be helpful.

As with acute sialadenitis, FNA in chronic sialadenitis can be associated with pain. Chronic sialadenitis is more likely to manifest as a clinically discrete mass, often in the submandibular gland. Common causes include sialolithiasis and radiation therapy for head and neck cancer (usually squamous cell carcinoma).

Cytomorphology of chronic sialadenitis

• scant cellularity

• small sheets and tubules of basaloid ductal cells with sharp borders

• paucity of acinar elements

• blood, proteinaceous debris, mature lymphocytes

• fragments of fibrous tissue

Aspirates of chronic sialadenitis are sparsely cellular. Clusters of small, basaloid ductal cells with sharp borders are admixed with blood, proteinaceous debris, mature lymphocytes, and small amounts of fibrous tissue (Fig. 11.4). Acinar cells are sparse or absent.⁶⁴ Sialolithiasis can be diagnosed when stone fragments are identified (see Fig.11.3). Atypical squamous metaplasia, mucinous metaplasia, radiation atypia, abundant histiocytes, extracellular mucin, crystals, and (rarely) psammoma bodies may be present.⁶⁸ Chronic sclerosing sialadenitis (Kuttner tumor) is a form of chronic sialadenitis affecting the submandibular gland and manifesting clinically as a firm mass that can be mistaken for a neoplastic process.50,69–71 The morphologic findings are nonspecific and similar to those with a conventional chronic sialadenitis. In some cases, lymphoid cells are abundant and lymphoma must be excluded.⁷¹ In a subset of chronic sclerosing sialadenitis cases, an increased proportion of IgG4-positive plasma cells is seen, and the condition is a form of systemic IgG4-related disease.⁷²

Figure 11.4 Chronic sialadenitis.
A, Sparsely cellular smears show small cohesive clusters of basaloid ductal cells that can be mistaken for the cells of a basaloid neoplasm (Romanowsky stain). B, The corresponding histologic specimen shows marked fibrosis, chronic inflammation, acinar atrophy, and residual ductal elements (hematoxylin and eosin [H & E] stain).

Differential diagnosis of chronic sialadenitis

• normal salivary gland

• lymphoepithelial sialadenitis (LESA)

• Warthin tumor (WT)

• basaloid neoplasms

• mucoepidermoid carcinoma

• squamous cell carcinoma

Normal salivary gland has a greater proportion of acinar to ductal cells than chronic sialadenitis. Chronic sialadenitis is distinguished from lymphoepithelial sialadenitis (LESA) by the absence of lymphoepithelial islands and germinal center fragments. Unlike WT, chronic sialadenitis lacks cohesive groups of oncocytes. The basaloid ductal cells of chronic sialadenitis can resemble the cells of basaloid neoplasms but are less numerous and arranged in smaller groups than those of neoplasms. The intermediate cells of a mucoepidermoid carcinoma resemble the basaloid ductal cells of chronic sialadenitis, but mature squamous cells and mucus cells are absent. Chronic sialadenitis can be separated from a squamous cell carcinoma by virtue of its scant cellularity; the absence of marked atypia, mitotic activity, and necrotic tumor cells; and the paucity of isolated epithelial cells.

Granulomatous Sialadenitis

Granulomatous sialadenitis can be caused by infection (fungal, mycobacterial, toxoplasmosis, or cat scratch disease), sarcoidosis, cyst rupture, and rarely neoplasia (Hodgkin lymphoma, T-cell lymphoma, or metastatic carcinoma).73–75 Aspirates are characterized by a variable background of granular, necrotic cell debris and inflammation, together with aggregates of epithelioid histiocytes, frequently with multinucleated forms. Epithelioid histiocytes have abundant eosinophilic cytoplasm and cytologically bland, elongated, folded nuclei with indistinct nucleoli. Asteroid bodies, Schaumann bodies, and calcium oxalate crystals can be present in sarcoidosis, but these findings are not specific, and an infectious etiology must be excluded by special stains and/or microbiologic cultures.⁷⁶

Sialadenosis

Sialadenosis is a non-neoplastic, noninflammatory enlargement of the salivary gland that more commonly affects the parotid gland and is often bilateral.64,77 It results from acinar cell hypertrophy, and has been associated with a wide range of etiologies, including endocrine abnormalities like diabetes mellitus, nutritional deficiencies, alcoholism, cirrhosis, and certain drugs, especially antihypertensives.^50,65,77

Aspirates of sialadenosis appear normal except that the constituent acinar cells are significantly larger than normal acinar cells, and inflammatory cells tend to be absent. Normal acinar cells are approximately 50 μm in diameter, whereas acinar cells of sialadenosis can measure up to 100 μm.50,64,65,77 In practice, these size differences are difficult to assess. Clinical correlation to exclude a discrete mass or neoplastic lesion is important before making a diagnosis of sialadenosis.

Lymphoepithelial Sialadenitis

LESA has been known by a variety of names, including Mikulicz disease, benign lymphoepithelial lesion, and myoepithelial sialadenitis.78,79 Resulting in part from the discovery that the cells composing the lymphoepithelial islands (formerly called epimyoepithelial islands) of this disorder are almost entirely epithelial, the term lymphoepithelial sialadenitis has emerged as the preferred designation for this disorder.^78,80 LESA most frequently affects women and manifests as diffuse, often bilateral, enlargement of the parotid gland, as well as the submandibular gland in a minority of cases. Minor salivary glands show chronic inflammatory changes related to LESA but lack the lymphoepithelial islands. LESA is believed to be an autoimmune disorder and is seen in virtually all patients with Sjögren syndrome. Approximately 50% of patients with LESA do not have Sjögren syndrome, however. They may have some other connective tissue disorder or no disease whatever.⁸⁰

Cytomorphology of lymphoepithelial sialadenitis

• cellular aspirate

• mixed population of lymphocytes, plasma cells, tingible-body macrophages

• germinal center fragments

• lymphoepithelial islands

Aspirates are cellular and show a mixed population of mature lymphocytes, plasma cells, tingible-body macrophages, germinal center fragments, and characteristic lymphoepithelial islands: large, cohesive sheets of pale, overlapping, ductal-type cells infiltrated by lymphocytes (Fig. 11.5). The ductal cells often exhibit reactive and squamous metaplastic changes. Acinar cells are rarely present.

Figure 11.5 Lymphoepithelial sialadenitis (LESA).
A, Lymphoid cells, predominantly small lymphocytes, are usually abundant (Papanicolaou stain). B, The characteristic lymphoepithelial islands are composed of reactive epithelial cells admixed with lymphocytes (Papanicolaou stain).

Differential diagnosis of lymphoepithelial sialadenitis

• chronic sialadenitis

• simple lymphoepithelial cyst

• HIV-associated cystic lymphoepithelial lesions

• Warthin tumor (WT)

• extranodal marginal zone B-cell lymphoma

In chronic sialadenitis, the aspirate tends to be sparsely cellular, with fewer lymphocytes and germinal center fragments, and the characteristic lymphoepithelial islands of LESA are lacking. Simple lymphoepithelial cysts and human immunodeficiency virus (HIV)–associated cystic lymphoepithelial lesions are cytologically similar to the cystic form of LESA and are discussed further in the next section. WT is distinguished from LESA in that the former contains oncocytic epithelium. Perhaps most important, extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) should be suspected if large numbers of monocytoid B cells are encountered. Ancillary studies to assess for clonality (flow cytometry, immunocytochemistry) can be invaluable in such circumstances.

Non-Neoplastic Cysts

Cystic lesions account for approximately 5% of salivary gland FNAs. A wide variety of non-neoplastic and neoplastic lesions can be cystic.⁸¹ Non-neoplastic cysts, both congenital and acquired, occur either adjacent to or within the salivary glands.^50,64 Diagnostically, these cysts can be broadly categorized into squamous-lined cysts and mucus-containing cysts.

Squamous-Lined Cysts

Squamous-lined cysts include congenital cysts, encompassing dermoid and branchial cleft cysts. Also in this category are the sporadic simple lymphoepithelial cysts, usually found within the parotid glands of middle-aged men.⁸⁰ Simple lymphoepithelial cysts are not related to HIV infection or Sjögren syndrome. Typically unilateral and solitary, they probably arise from either entrapped salivary duct tissue within intraparotid lymph nodes or branchial cleft remnants.⁸⁰ By contrast, HIV-associated cystic lymphoepithelial lesions are usually multiple and often bilateral.⁸² Squamous-lined cysts yield clear to turbid yellow-brown fluid.

Cytomorphology of squamous-lined cysts

• cellular aspirate

• histiocytes

• keratin debris and anucleate squames

• small clusters of squamous (or columnar) cells

• mixed population of lymphocytes

• with or without germinal center fragments with tingible-body macrophages

• absence of lymphoepithelial islands (except in HIV-associated cases and cystic LESA)

The aspirate findings are nonspecific, showing a mixed population of lymphocytes, often with germinal center fragments, tingible-body macrophages, keratin debris, squamous or columnar cyst lining cells, histiocytes, and proteinaceous debris 83,84 (Fig. 11.6A). Within this group of lesions, finding epithelial cell clusters with interspersed lymphocytes (called lymphoepithelial islands) is indicative of either a cystic LESA or an HIV-associated cystic lymphoepithelial lesion. Clinical and radiographic correlation is helpful in this distinction.

Figure 11.6 Squamous epithelial-lined cyst compared to squamous cell carcinoma (SQC).
A, In a benign squamous-lined epithelial cyst, degenerated and anucleate squamous cells with minimal nuclear atypia are present. These findings are nonspecific (Papanicolaou stain). B, In contrast, keratinizing SQC has marked nuclear atypia, bizarre cell shapes, and necrosis (Papanicolaou stain).

Differential diagnosis of squamous-lined cysts

• pure lymphoid lesions (intraparotid lymph node, lymphoma)

• cystic salivary gland neoplasms (especially Warthin tumor, acinic cell carcinoma, and mucoepidermoid carcinoma)

• cystic metastatic squamous cell carcinoma (SQC)

The presence of an epithelial component distinguishes these cysts from a lymph node. Lymphoid elements do not help in the distinction from a cystic salivary gland neoplasm, because some neoplasms have a prominent lymphoid infiltrate. A lymphoma, especially of MALT type, can be excluded using flow cytometry, which demonstrates a polyclonal population of lymphoid cells in these benign cystic lesions. Distinction from a Warthin tumor (WT) can be especially difficult, because the oncocytes of a WT can show extensive squamoid differentiation. Identification of a salivary gland neoplasm rests on the recognition of the characteristic cell type(s) associated with that tumor (e.g., oncocytes, acinar cells, mucus, intermediate, and squamous cells). Most important, a cystic metastatic SQC must also be considered. Although squamous-lined non-neoplastic cysts can exhibit reactive atypia, metastatic SQC (Fig. 11.6B) contains necrotic cells, mitotic activity (including atypical mitoses), and more severe nuclear atypia. Nevertheless, a cystic well-differentiated SQC is occasionally difficult to distinguish from a benign squamous-lined cyst. Beware of diagnosing a nonmidline developmental cyst in an older adult. Such aspirates should be thoroughly screened to exclude malignant features. Even in the absence of definitive evidence of malignancy, it is prudent to report benign-appearing squamous-lined cysts descriptively and include the differential diagnosis of a developmental and lymphoepithelial cyst. An accompanying explanatory note can emphasize the need for clinical correlation to exclude a more significant lesion and a recommendation that any persistent mass be excised to exclude malignancy. The tonsil is often the primary site for SQC manifesting as a cystic lesion. Because tonsillar SQCs are frequently associated with human papillomavirus (HPV) infection, HPV testing of the aspirate can be helpful both in confirming the diagnosis and in identifying a likely primary site.^85,86

Mucin-Containing Cysts

The umbrella term mucin-containing cysts refers to a heterogeneous group of lesions that include a malignant neoplasm (mucoepidermoid carcinoma), inflammatory conditions (chronic sialadenitis with mucinous metaplasia), and acquired cysts. Acquired cysts, comprising the mucocele and the retention cyst, occur more commonly in the submandibular and sublingual glands than in the parotid.⁵⁰ Mucoceles are pseudocysts because they lack an epithelial lining, whereas retention cysts are lined by squamous, columnar, or oncocytic epithelium. Both result from obstruction, usually by stones.

Cytomorphology of mucin-containing cysts

• sparsely cellular

• extracellular mucin

• histiocytes

• amylase crystalloids (nonspecific)

• scattered inflammatory cells

Aspirates are sparsely cellular and composed of histiocytes, some with intracellular mucin (“muciphages”), granular debris, extracellular mucin, amylase crystalloids (see Fig. 11.1B), and inflammatory cells (Fig. 11.7). Occasional metaplastic epithelial cells and normal salivary gland cells are also seen.

Figure 11.7 Mucus-containing cyst.
The combination of histiocytes and magenta-colored extracellular mucin (arrow) is a nonspecific finding. This example proved to be a retention cyst, but identical findings can be seen in a low-grade mucoepidermoid carcinoma (Romanowsky stain).

Differential diagnosis of mucin-containing cysts

• mucocele

• retention cyst

• chronic sialadenitis with mucinous metaplasia of ducts

• mucoepidermoid carcinoma

Chronic sialadenitis with mucinous metaplasia of ducts has a similar appearance to the acquired cysts; ciliated columnar cells suggest the diagnosis. The nonspecific constellation of findings described herein is also seen in some low-grade mucoepidermoid carcinomas, which, like mucoceles and retention cysts, contain muciphages. The following features, typical of a low-grade mucoepidermoid carcinoma, can help distinguish it from one of its benign mimics: a residual mass after aspiration, greater cellularity, more severe cytologic atypia, and at least an occasional cluster of intermediate and epidermoid cells.

Whenever a specimen contains extracellular mucin but minimal (or even absent) cellular atypia, an atypical interpretation is desirable nevertheless. Mucoepidermoid carcinoma MEC can be mentioned in the differential diagnosis, with an educational note. This diagnostic approach helps avoid a false-negative interpretation in the case of a low-grade mucoepidermoid carcinoma.

Sample report of a mucin-containing cyst

ATYPICAL.

Histiocytes (and epithelial elements) in a background of abundant mucin (see Note).

note: The differential diagnosis includes a mucocele, mucus retention cyst, chronic sialadenitis with mucinous metaplasia, and a mucoepidermoid carcinoma. If the lesion persists or recurs, excision should be considered.

Amyloidosis

Amyloidosis is a rare, often bilateral cause of focal or diffuse salivary gland enlargement.87–89 Characterized by variable amounts of acellular, eosinophilic extracellular material, the FNA amyloidosis specimen stains pale red with the Congo red stain and exhibits a characteristic apple-green birefringence under polarized light. The remainder of the smear is often hypocellular, with scant or absent acinar cells and scattered groups of ductal cells such as are seen in chronic sialadenitis. Amyloid is also seen in association with an extramedullary plasmacytoma.⁹⁰

Benign Neoplasms

Pleomorphic Adenoma

PA is the most common tumor in all salivary glands, both in children and in adults. Two thirds of parotid tumors and 50% of all salivary gland tumors are PAs. The most commonly encountered site is the superficial parotid, often in the tail of the gland at the angle of the jaw. On physical examination, the nodule is firm, reflecting the abundance of chondromyxoid matrix. The aspiration is typically painless.

Cytomorphology of pleomorphic adenoma

• epithelial cells

• myoepithelial cells

• chondromyxoid matrix

• tyrosine crystalloids (nonspecific)

The epithelial cells are recognizable by their cohesive groupings, usually in a honeycomb pattern. When present as individual cells, epithelial cells are indistinguishable from myoepithelial cells. The myoepithelial cells have a variety of appearances: spindle-shaped (the most common), epithelioid, clear cell, and plasmacytoid. The plasmacytoid morphology is particularly common in palatal tumors. Unlike epithelial cells, myoepithelial cells are commonly found individually, within matrix material, in loose clusters, or in larger, haphazardly arranged clusters. The matrix material is the most characteristic finding.91,92 Fibrillary and “chondromyxoid” in appearance, it stains pale purple or blue in Papanicolaou-stained preparations and can be difficult to see and to distinguish from other extracellular material such as mucin (Fig. 11.8A). It is best appreciated on air-dried preparations stained with a Romanowsky-type stain, which gives it an intense magenta color that is “metachromatic,” meaning that the dye in the stain changes color through its interaction with the matrix material (Fig. 11.8B). Although the matrix of adenoid cystic carcinoma stains with a similar intensity, the features are otherwise distinctive.^91,93 The characteristic fibrillary nature of the matrix in a PA can be appreciated by its frayed, indistinct margins. Myoepithelial cells are readily identified embedded within it.

Figure 11.8 Pleomorphic adenoma (PA).
A, Pale blue matrix, myoepithelial cells, and sheets of ductal cells are identified at low power (Papanicolaou stain). B, In the air-dried preparation, magenta-colored fibrillary matrix has frayed edges. Isolated plump spindle-shaped myoepithelial cells are embedded within the matrix (Romanowsky stain).

The proportions of the cellular constituents are extremely variable from one PA to another, and within an individual tumor—hence the importance of sampling different areas of the tumor with multiple passes. In at least two thirds of cases, the typical constituents are readily appreciated, without unusual features, and the diagnosis is straightforward.⁹⁴

Pitfalls Associated with Pleomorphic Adenomas

Sparse or absent matrix. The absence or paucity of matrix may preclude a definitive diagnosis of PA,92,95,96 but an appropriate differential diagnosis can still result in appropriate clinical management. The differential diagnosis includes basal cell adenoma or adenocarcinoma (in the case of an epithelial-rich lesion) and myoepithelioma (in the case of a myoepithelial-rich lesion).

Adenoid cystic–like matrix. Cylindromatous areas, raising the possibility of adenoid cystic carcinoma, are encountered in approximately 5% of PAs ⁹⁴ (Fig. 11.9A and B). If a well-sampled lesion is otherwise typical for a PA, and these cylindromatous areas are rare, this finding is of no concern. If extensive, the diagnosis of adenoid cystic carcinoma, or the rare adenoid cystic carcinoma ex pleomorphic adenoma,⁹⁷ should be considered.

Figure 11.9 Pleomorphic adenoma (PA) with adenoid cystic-like foci.
A, Lucent cylinders are present in a background of myoepithelial cells (Papanicolaou stain). B, In the corresponding histologic specimen, cylinders are also seen. The findings in the aspirate and resected specimen were otherwise typical of a PA (hematoxylin and eosin [H & E] stain).

Cytologic atypia. Focal, mild cytologic atypia is seen in 20% of PAs ⁹⁴ and causes little concern. Severe atypia, however, particularly if accompanied by necrosis or abundant mitoses, suggests the possibility of carcinoma ex pleomorphic adenoma. Most carcinomas ex pleomorphic adenoma are cytologically overtly malignant, and the challenge is recognizing the preexisting PA. Extensive atypia without overt malignant features (Fig.11.10) justifies the diagnosis of a PA with atypia, with an accompanying comment that the finding could be indicative of early malignant change.

Figure 11.10 Pleomorphic adenoma (PA) with atypia.
Atypical cells with enlarged nuclei and prominent nucleoli are present in an aspirate that is otherwise typical of a pleomorphic adenoma. When few and far between, such cells are not concerning for malignancy. More extensive atypia, as in this case, merits the interpretation PA with atypia. The resected specimen revealed a noninvasive carcinoma ex PA (hematoxylin and eosin [H & E] stain).

Squamous or mucinous metaplasia. These changes are of little concern when present focally in an otherwise typical PA. If such changes are extensive, the differential diagnosis includes mucoepidermoid carcinoma, possibly arising in a preexisting PA.98,99 Mucoepidermoid carcinoma MEC ex pleomorphic adenoma is exceedingly rare and usually a high-grade malignancy.¹⁰⁰

Myoepithelioma

Myoepithelioma is a rare and unusual salivary gland tumor that can be considered a monomorphic variant of pleomorphic adenoma (PA). The clinical features and natural history of myoepitheliomas and PAs are similar.⁵⁰

Cytomorphology of myoepithelioma

• loose aggregates and isolated cells

• spindle cell, clear cell, epithelioid, or plasmacytoid morphology

• lack of honeycomb-like sheets of epithelial cells

• lack of chondromyxoid matrix

The myoepithelial cells of a myoepithelioma are identical to those of a PA. The absence of chondromyxoid matrix material and epithelial cells distinguishes a myoepithelioma from a PA. A smear composed exclusively of myoepithelial cells, however, often results from incomplete sampling of a myoepithelial-cell–rich pleomorphic adenoma ⁹⁵ (Fig. 11.11) and can result in a misdiagnosis of myoepithelioma. Although this error has no clinical consequence, a generic interpretation of a “myoepithelial cell-rich neoplasm” (accompanied by a differential diagnosis) is prudent in such circumstances.

Figure 11.11 Myoepithelial cell–rich neoplasm.
Loosely cohesive, spindle-shaped myoepithelial cells are the sole constituents of this aspirate. The resected specimen proved to be a myoepithelial cell-rich pleomorphic adenoma, but the aspiration findings were indistinguishable from those of a myoepithelioma (Papanicolaou stain).

A spindle cell myoepithelioma can be difficult to distinguish from a schwannoma.95,101–103 Both can have nuclear palisading, although this is more common in a schwannoma, in which rows of palisaded nuclei are called Verrocay bodies. Immunohistochemistry is helpful; although S-100 is diffusely positive in schwannoma and often positive in myoepithelial cells, myoepithelial cells also stain for one or more of the following: p63, keratins, smooth muscle actin, glial fibrillary acidic protein, and calponin. Of these, p63 has the greatest value among the current immunohistochemical markers of myoepithelial differentiation. An important caveat: p63 also stains cells showing squamous differentiation (e.g., the epidermoid cells of a mucoepidermoid carcinoma).

Plasmacytoid myoepithelial tumors 104,105 (Fig. 11.12) resemble plasmacytomas, but myoepithelial cells lack the “clock-face” chromatin and perinuclear hof of plasma cells. Immunohistochemistry and serum electrophoresis are useful in difficult cases.