Clinical History

Clinical presentation:

59 year old man presented with a tumor of the left parotid gland, which has been growing in size. The tumor was noted to be in the deep lobe of parotid.

An ultrasound-guided FNA was performed. A single LBC (ThinPrep) slide was prepared.

Cytologic examination:

The ThinPrep slide showed a low to moderately cellular specimen. There were clusters and single large cells with abundant cytoplasm. Some of the clusters were loosely cohesive, while others were tightly cohesive and had a suggestion of papillary configuration. The cells had a vacuolated cytoplasm with well defined borders, round nuclei, and prominent nucleoli. The single cells in the background had similar morphology, but some of them had a histiocyte-like appearance.

A cytologic diagnosis of "Suspicious for oncocytic neoplasm with cystic change" was rendered.

Histologic Follow-up:

A total parotidectomy was performed, which revealed a 1.5 cm mass. The neoplasm had a follicular growth pattern, with variable cyst formation. Colloid-like secretions were present in the cystic spaces. A panel of immunohistochemical stains were performed and showed positive staining with S100, vimentin, mammaglobin, and EMA. PASD, DOG-1, calponin, SMA, and p63 were negative. FISH study was performed, and showed a ETV-6-NTRK3 translocation. A final diagnosis of "Mammary analog secretory carcinoma" was rendered.

Discussion

MASC is a salivary carcinoma defined by a translocation that results in ETV6-NTRK3 gene fusion. Because this neoplasm was described only recently, it remains relatively unknown outside of head and neck pathology sub-specialty practices. MASC was described by Skalova et al in 2010, as a low grade malignancy that predominately affected the parotid gland. It shares overlapping features with juvenile secretory carcinoma of the breast, including an identical translocation leading to ETV6-NTRK3 fusion oncogene. MASC can mimic other salivary gland tumors such as acinic cell carcinoma (AcCC), low grade mucoepidermoid carcinoma (MEC), and adenocarcinoma NOS.

Clinical features:

It occurs slightly more commonly in males, and average age at diagnosis is 46 years.

Parotid gland is the most common site (70%), but it also occurs in submandibular and minor salivary glands. Although MASC has a low grade appearance, its biologic behavior is not yet known. Chiosea et al found a trend towards increased lymph node metastases in MASC compared to AcCC, but no statistically significant difference in disease free survival.

Histologic features:

Typically these tumors are similar in appearance to zymogen granule-poor AcCC. Retrospective studies showed that over half of such cases (previously diagnosed as AcCC) turn out to be MASC. Generally speaking, MASC is distinguished by mucin production, S100 positivity, and lack of zymogen granules,.

These neoplasms are often circumscribed but not encapsulated. They are characterized by a follicular pattern of growth, including macrocystic, microcystic, and tubular spaces of different sizes, in addition to solid components. The degree of cyst formation varies from tumor to tumor. The cystic spaces are usually associated with papillary formation, and lined by cuboidal cells, some of which showing hobnailing. Secretions in the cysts have a bubbly to colloid-like/glassy appearance, and stain positive with PASD and mucin. The tumor cells demonstrate minimal cytologic atypia, round to oval pale nuclei, small nucleoli, and abundant pale pink microvacuolated cytoplasm. PASD-positive granules are usually lacking in the cytoplasm. Occasional intracytoplasmic mucin-positive droplets may be seen. Approximately half of the cases show peri-neural invasion.

Immunohistochemistry (IHC):

The neoplastic cells show diffuse strong positive staining with S100, HMWK, mammaglobin, GCFP-15, EMA, and Vimentin. P63 and estrogen/progesterone (ER/PR) are negative. Ki 67 proliferative index is less than 1%. The tumor is negative with myoepithelial markers such as SMA and calponin. The combined P63-/HWK+ is suggestive of a striated duct origin.

Molecular studies:

MASC Expresses ETV6-NTRK3 gene fusion either by RT-PCR or FISH. By FISH, nuclei with split red and green signals indicate ETV6 gene rearrangement, whereas chromosomes with normal ETV6 genes demonstrate a yellow signal (overlapping green and red). ETV6-NTRK3 translocation is the singular definitive diagnostic feature of MASC.

Cytologic features:

The FNA features of MASC are characteristic but not entirely distinctive. The specimens are usually of high cellularity. Sheets including papillary fragments and modest number of single cells are seen in the background. The cell clusters display irregular and jagged outer borders. The background is granular or cystic, including the presence of single cells admixed with histiocytes. The neoplastic cells are large, have oncocytic appearance and well defined cytoplasmic borders. The cytoplasm has many vacuoles (soap bubble appearance), but occasionally solitary large vacuoles are seen. No cytoplasmic granularity is appreciated. The nuclei are centrally placed, round to oval, uniform, with open chromatin and small nucleoli. No significant atypia or anisonucleosis is appreciated. The single cells in the background may have a histiocytic or plasmacytoid look, with eccentrically placed nuclei.

Differential diagnosis:

The cytologic differential diagnosis of MASC mostly includes oncocytic tumors, AcCC, and other low grade and high-grade malignancies (Table 1). IHC is not helpful in FNA. Although MASC is positive for S100 and mammaglobin, many other salivary gland tumors show S100 positivity and specificity of mammaglobin for MASC is not yet known. Definitive diagnosis of MASC relies on documenting ETV6 rearrangement.

Acinic cell carcinoma (AcCC):

This is a low grade malignancy characterized by sheets of large cells with abundant cytoplasm. The neoplastic cells have foamy/vacuolated cytoplasm with ill defined borders, eccentrically placed nuclei, small inconspicuous nucleoli, and lack significant nuclear atypia or pleomorphism. Some cellular groups may be traversed by thin capillaries. Stripped nuclei are often present in the background. Occasionally there is papillary configuration or prominent cystic component

AcCC may show similar architectural/low power appearance to MASC, including papillary structures, and eosinophilic granular and vacuolated cytoplasm. However, MASC lacks zymogen granules, and demonstrates a greater variation in size of cytoplasmic vacuoles. AcCC also shows numerous naked nuclei in the background. Clinically, an extra-parotid site and male sex favor MASC , as AcCC involves mostly the parotid gland and is seen more commonly in women. DOG-1 strong positive staining favors AcCC.

Low grade mucoepidermoid carcinoma (MEC):

MEC is commonly cystic, and is cytologically characterized by an admixture of glandular and metaplastic squamous cells. The background demonstrates mucinous material and debris. The glandular cells may have a ductal appearance (intermediate cells) or may have intracytoplasmic mucin (mucin producing cells) imparting a macrophage-like appearance. Metaplastic cells are polygonal in shape and resemble immature metaplastic squamous cells and oncocytes, having dense cytoplasm, round nuclei and prominent nucleoli. Variable cytoplasmic vacuolization may also be observed. However, keratinized epidermoid cells are not usually seen in low grade MEC.

The bland appearance of neoplastic cells in MEC, background mucin, cyst formation, and cytoplasmic vacuoles may raise the differential diagnosis of MEC vs. MASC. The mucocytes in MEC, however, tend to be univacuolated (goblet cells), whereas MASC cells are multivacuolated. Rare mucinous cells maybe found in MASC, similar to MEC, but mucinous cells do not line the cystic spaces. Also, MEC show epidermoid/squamoid differentiation and intermediate cells, and usually does not have papillary architecture. By IHC, MEC is diffusely P63 positive and S100 negative, while MASC shows negative P63 and strong S100 staining.

Pleomorphic adenoma (PA):

PA is the most common neoplasm of the salivary glands, accounting for approximately 75% of neoplasms, with a peak age of 30-40 years. It is characterized by admixture of cellular (ductal and myoepithelial cells) and mesenchymal components. The mesenchymal component may be myxoid or chondroid, and is usually admixed with spindle myoepithelial cells. The myxoid mesenchymal component stains poorly with Papanicolaou stain, appearing grey or pale green; but is better highlighted by DQ stain as dark purple material with a fibrillary appearance

PA is notorious for mimicking the cytologic appearance of other tumors, especially low grade malignancies. It may be considered in this differential diagnosis because of its bland cytology and common occurrence in major and minor salivary glands. MASC, however contains no extracellular matrix or spindle/plasmacytoid myoepithelial cells. Although mucin with entrapped MASC tumor cells may resemble the myxoid stroma of PA, mucin has more of a stringy appearance compared to the fibrillary quality of the PA stroma.

Polymorphous low grade adenocarcinoma (PLGA):

This is a low grade adenocarcinoma arising almost exclusively in the minor salivary glands (especially the palate). Aspirates consist of sheets and three-dimensional clusters of intermediate sized cells having moderate amount of delicate cytoplasm, uniform round to oval nuclei and finely granular chromatin. Nucleoli are inconspicuous. Acinar formation with overlapping of nuclei may be appreciated. Diff-Quik stain may show purple extracellular material formed as dense stromal spheres, which can lead to confusion with the hyaline globules of adenoid cystic carcinoma

PLGA may mimic MASC when it arises in minor salivary glands, and has papillary features. Also, S100 is positive in PLGA, similar to MASC. PLGA cytoplasm, however, is not vacuolated, and the background stromal spheres are not seen in MASC.

Oncocytic neoplasms, i.e. Oncocytoma and Warthin tumor:

Oncocytoma is characterized by large flat sheets of epithelial cells with abundant dense eosinophilic cytoplasm, well defined cytoplasmic border, enlarged centrally placed nuclei, and prominent nucleoli. Anisonucleosis and mild atypia are not uncommon, but marked cytologic atypia, cribriforming, and necrosis are often absent. Warthin tumor is almost always seen in the parotid gland, comprising approximately 5-10% of all parotid tumors. The aspirate has a thin watery mucoid appearance, and consists of a mixed population of lymphocytes, occasional plasma cells, and variable number of oncocytes.

These oncocytic tumors have dense granular cytoplasm, similar to MASC. The cytoplasm, however, is not vacuolated, there is less cell discohesion, and nucleoli are large and prominent. In addition, Warthin tumor has a prominent lymphoid stroma.

Salivary duct carcinoma (SDC):

It is a rare primary salivary gland malignancy characterized by its histologic resemblance to in-situ and invasive ductal carcinoma of the breast. It is considered to be one of the most aggressive salivary gland malignancies. Architecturally, SDC shows a spectrum of cytologic findings including broad flat sheets and three-dimensional tightly cohesive clusters of large monomorphic polygonal cells with abundant eosinophilic cytoplasm, round to oval hypochromatic nuclei, and prominent nucleoli. Medium-sized low columnar cells with central or eccentric hyperchromatic nuclei may be found. Occasional cribriforming and papillary configurations are seen. SDC shows moderate to severe nuclear atypia and pleomorphism, however, the atypia, can be random or focal in distribution. The presence of associated necrosis in the background constitutes an important clue to the diagnosis of high grade SDC

The presence of abundant eosinophilic cytoplasm in SDC and occasional papillary forms can mimic MASC, but it has high grade nuclear features and background necrosis that are not encountered in the latter.

Metastatic renal cell carcinoma (RCC):

Due to its bland cytology and clear vacuolated cytoplasm, RCC may be confused with MASC. Renal cell carcinoma often shows peri-vascular nesting, and nuclei with large nucleoli. Remote history of renal cell carcinoma, if present, is also helpful. IHC shows positive staining with renal cell antigen, CD10, and PAX8.

Conclusion

Salivary gland tumors that resemble AcCC and have an acinar-microcystic growth pattern, but show paucity/absence of PASD zymogen granules and strong staining with S100, are suggestive of MASC. Definitive diagnosis is established when characteristic molecular translocation (12;15) (p13;q25) ETV-6-NTRK3, is detected. MASC is considered to be a low to intermediate grade malignancy with tendency for lymph node metastases. FNA cytologic features of MASC overlap with other low grade malignancies, therefore, it is not crucial to make a definitive diagnosis pre-operatively. It is important, however, not to mistaken MASC with benign tumors such as PA and Warthin tumor, or with high grade malignancies such as salivary duct carcinoma, because of obvious differences in management of those patients. It is also important to be familiar with this new entity, so its recognition on histologic examination is facilitated, and tissue is appropriately triaged for molecular studies.

References

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