Ectopic hamartomatous thymoma: a clinicopathologic and immunohistochemical analysis of 21 cases with data supporting reclassification as a branchial anlage mixed tumor.

This report describes the clinicopathologic and immunohistochemical findings in 21 cases of a highly distinctive tumor with a strong predilection for the lower neck region of adult males. Our study group consisted of 20 males and one female. The patients were 28 to 79 years old (mean age, 47 years; median age, 40 years), and they presented with solitary, lobular or multilobular masses ranging in size from 2.0 to 19.0 cm in greatest dimension (mean size, 5.1 cm; median, 4 cm). The tumors principally involved the lower neck region, usually in close proximity to the sternoclavicular joint. The preoperative duration of the lesions ranged from 2 months to 30 years. Histologically, the tumors were typically well marginated and composed of plump spindled cells, delicate spindled cells, mature adipose tissue, and epithelial cells, including both squamous and glandular elements. Epithelial-lined cysts were a focal finding in most cases and measured up to 2 cm in greatest dimension. Mitotic counts for the tumors ranged from 0 to 7 mitotic figures per 50 high power fields (mean mitotic count, 1.1 mitotic figures per 50 HPFs). Our immunohistochemical analysis revealed a complex immunophenotype with a diverse keratin profile. The plump spindled cells had a myoepithelial phenotype, as evidenced by the coexpression of keratins (5, 5/6, and 14), alpha-smooth muscle actin, CD10, and to a lesser extent, calponin. No compelling evidence for thymic differentiation was observed. The patients were initially managed by biopsy or partial resection (n = 4), simple local excision (n = 16), or an unspecified procedure (n = 1). Clinical follow-up of > or =3 years was available for 10 patients (48%). Two patients had recurrent disease, but there were no metastases or tumor-related deaths. A derivation from sequestered branchial epithelium is likely, but evidence for a thymic component is tenuous, at best. Our data support reclassification of this distinctive process as a branchial anlage mixed tumor. The differential diagnosis includes conventional mixed tumors of skin adnexal or salivary gland origin, synovial sarcoma, a peripheral nerve sheath tumor variant, and cystic teratoma.