Ectopic Hamartomatous Thymoma-New Insights Into a Challenging Entity: A Clinicopathologic and Immunohistochemical Study of 9 Cases.

Nine cases of ectopic hamartomatous thymoma are described. The patients were 5 men and 4 women aged 34 to 52 years (mean, 43 y). All patients presented with solitary lower neck masses ranging in size from 3.5 to 8.0 cm (mean, 5.4 cm). Grossly, the lesions were circumscribed and lobulated masses with a fleshy white cut surface; cystic changes were identified in 3 cases. Histologically, the tumors were composed of varying proportions of spindle cells arranged in fascicles, mature adipose tissue, and an epithelial component composed of squamoid elements and glandular or ductal structures. In 1 case, duct-like structures dominated the histologic picture. Structures reminiscent of Hassall corpuscles were identified in 2 cases. No overt malignant changes were seen, and the mitotic activity ranged from 0 to 2 mitoses per 10 high-power fields. Immunohistochemically, the spindle cells coexpressed CK5/6, CD34, and smooth muscle actin, whereas the squamous component was positive for CK5/6 only. Bcl-2 was variably expressed in the spindle and epithelial elements, whereas Pax8 and STAT6 were uniformly negative. Clinical follow-up revealed that all patients were alive and well 2 to 5 years after diagnosis. Ectopic hamartomatous thymomas are benign neoplasms with a wide morphologic spectrum. The histologic and immunohistochemical features are reminiscent of thymic derivation and suggest possible origin from remnants of the thymic anlage. Changes in the nomenclature may be advised to more specifically designate tumor differentiation and to avoid confusion with true thymomas as these represent entirely separate clinicopathologic entities.