Final Diagnosis & Discussions: Fine Needle Aspiration of a Parotid Mass on 61-Year-Old Man

FINAL DIAGNOSIS: ACINIC CELL CARCINOMA

 

DISCUSSION
Acinic cell carcinoma is an uncommon malignant salivary gland tumor. The most common salivary gland neoplasm is a pleomorphic adenoma, which is benign. The most common malignant salivary gland neoplasm is a mucoepidermoid carcinoma. A large series of cases from the Armed Forces Institute of Pathology (AFIP) indicated that acinic cell carcinoma is the second most common malignant salivary gland neoplasm. Acinic cell carcinoma accounts for 15% of malignant salivary gland tumors. It usually occurs in the parotid gland, with only 20% arising in the minor salivary glands (submandibular gland, etc). Despite its malignant potential, most acinic cell carcinomas are detected early enough that with appropriate treatment - surgery and possibly adjuvant radiation therapy - there is excellent long-term disease-free survival. However, recurrences and metastases can occur many years after the initial diagnosis.

The diagnosis of an acinic cell carcinoma in FNA can be particularly challenging. A review of results from the College of American Pathologists (CAP) Interlaboratory Comparison Program revealed that 49% of acinic cell carcinoma specimens were called benign, making it the malignant salivary gland neoplasm most likely to be called benign.

The CAP results indicate that acinic cell carcinoma is most likely to be misdiagnosed as normal salivary gland tissue. Acinic cell carcinoma frequently has uniform, round nuclei with evenly dispersed chromatin, a small nucleolus and abundant cytoplasm, making it an excellent mimic of normal serous salivary cells. Furthermore, acinic cell carcinomas can form rudimentary acinar structures - recapitulating normal salivary gland acini. The distinguishing features of acinic cell carcinoma include increased cellularity, poorly formed acini, and absence of salivary ducts. Capillaries present in acinic cell carcinomas can resemble ducts, however luminal red blood cells and a thin endothelium (as opposed to a cuboidal epithelium) can be found in capillaries.

The second most common misclassification of acinic cell carcinoma is chronic sialadenitis (chronic inflammation of the salivary gland). Acinic cell carcinomas are commonly associated with a dense lymphocytic infiltrate. Rarely, as in this case, acinic cell carcinomas can form cystic spaces in which cellular debris can accumulate. When these components are sampled by FNA, the background can resemble and mimic chronic sialadenitis. This pitfall can be avoided by adequately sampling the lesion and identifying the hypercellular clusters of serous cells lacking salivary ducts. However, sometimes a definitive diagnosis can not be rendered.

Warthin's tumor may mimic acinic cell carcinoma. Both lesions can have a background of lymphocytes with cellular debris, be bilateral and some acinic cell carcinomas contain oncocytic cells. The degree of cellularity and rudimentary acinar structures of acinic cell carcinoma can be utilized in differentiating the conditions.

Mucoepidermoid carcinoma is the malignant salivary gland tumor most likely to be confused with acinic cell carcinoma by FNA. Acinic cell carcinomas can have clear cells, resembling mucous cells seen in some mucoepidermoid carcinomas. The vacuolated cells in acinic cell carcinoma are negative for mucin (absence of mucicarmine reactivity), while the mucous containing cells of a mucoepidermoid carcinoma show positivie intracytoplasmic staining with mucicarmine.

References

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