Cytopathology Case 15

Final Diagnosis & Discussions:

FINAL DIAGNOSIS: Pleomorphic adenoma

 

DISCUSSION
Pleomorphic adenoma (PA) is the most common neoplasm of the salivary glands. Two thirds of parotid tumors and 50% of all salivary gland tumors are pleomorphic adenomas. The most common site is parotid, often found in the tail of the gland at the angle of the jaw. On physical exam, the nodule is usually rubbery to firm due to the abundance of chondromyxoid matrix.

Pleomorphic adenomas (as the name indicates) are highly variable neoplasms that can demonstrate a wide variety of patterns, cellularity, and amount of associated stroma. However, the neoplasm characteristically consists of some mixture of epithelial cells, myoepithelial cells and a chondromyxoid matrix. The epithelial cells are found in cohesive groups, are usually bland in appearance, and are often in a honeycomb pattern. The myoepithelial cells can be spindle-shaped, epithelioid, clear-cell or plasmacytoid. Unlike the cohesive epithelial cells, myoepithelial cells are commonly found individually or within the chondromyxoid matrix. The chondromyxoid matrix is best seen on air dried smears stained with a modified Wright-Giemsa stain, giving an intense magenta color that has frayed (or fibrillary) edges.

Not surprisingly, given its variation in microscopic appearance, pleomorphic adenomas can mimic other tumors. The typical differential diagnoses includes basal cell adenoma, myoepithelioma, adenoid cystic carcinoma, mucoepidermoid carcinoma, and carcinoma ex pleomorphic adenoma.

Basal cell adenoma is a diagnostic possibility in cases where there is absent to sparse matrix component on aspirate smears, leaving a highly cellular, bland-appearing, epithelial-predominant specimen. Given that the management is the same as for pleomorphic adenoma, and that likely basal cell adenoma is merely a variant of PA, this is not a critical distinction. Similarly, myoepitheliomas are part of the differential diagnosis of a PA in lesions that are myoepithelial-rich with a stromal component that is not obvious. Keeping this as part of the differential diagnosis on such FNAs will result in proper clinical management.

Adenoid cystic carcinoma shares several morphologic features with pleomorphic adenoma. Both are tumors of epithelial and myoepithelial cells, but latter is benign and the former is malignant. The stromal material of these tumors is also similar as both are metachromatic. However, the chondromyxoid stroma of pleomorphic adenoma is fibrillar and has irregular, feathery outlines, while the basement membrane-like material of adenoid cystic carcinoma is more homogenous and tends to have smooth, rounded outlines. Adenoid cystic carcinomas characteristically have these rounded stromal deposits embedded within small basaloid epithelial cells.

Mucoepidermoid carcinoma is also considered in the differential diagnosis of pleomorphic adenoma. The presence of glandular cells in a background of metachromatic mucinous material, particularly if there is cytologic atypia, mucinous cells, or squamous metaplasia, may suggest a diagnosis of mucoepidermoid carcinoma. The cells of pleomorphic adenomas are not diffusely atypical, although it should be noted that focal atypia is often seen. Low-grade mucoepidermoid carcinoma usually has a predominance of well-differentiated, mucous, goblet-type cells. High-grade mucoepidermoid carcinoma resembles squamous cell carcinoma.

Finally, carcinoma ex pleomorphic adenoma is the malignant transformation of pleomorphic adenoma. Clinically, a history of recent rapid growth in a long-standing, previously slowly growing tumor is characteristic. Poorly differentiated adenocarcinoma (groups of enlarged overlapping cells with high nuclear to cytoplasm ratio, coarse chromatin, irregular nuclear contours and nucleoli) and increased mitosis are evident on the smears.

Although PA is considered a benign neoplasm it can recur. The recurrence rate of pleomorphic adenoma is almost entirely dependent on the adequacy of the primary excision. Recurrence is very high if the tumor is removed by a simple enucleation because the neoplasm has a tendency to form small inconspicuous satellite nodules in the salivary tissue surrounding the main mass. Most of these recurrences will appear during the first 18 months after surgery, but others occur later.

References

  1. Richard M. DeMay. The Art & Science of Cytopathology. ASCP Press. 1996.

  2. Edmund S. Cibas and Barbara S. Ducatman. Cytology Diagnostic Principles and Clinical Correlates. Saunders Elsevier. 2009.

  3. Juan Rosai. Rosai and Ackerman’s Surgical Pathology. 9th Edition. Elsevier. 2004. Pages 878-882.