Ectopic hamartomatous thymoma: a case report with immunohistochemical and ultrastructural studies.

A case of ectopic hamartomatous thymoma (EHT) arising in the supraclavicular region of a 52-year-old male is presented. The well-defined tumor measuring 1.7x1.5x0.7 cm consisted of three components: spindle cell (70%), epithelial (25%), and adipose (5%). The spindle cell component was characterized by sheet-like, haphazard and short fascicular arrangements of bland spindle cells. Neither mitotic figures nor cellular pleomorphism were found. Admixed with, and adjacent to, the spindle cell areas was an obviously epithelial component of variable appearance, ranging from glandular spaces lined by mainly cuboidal clear cells, irregularly anastomosing cords, and strands of epithelial cells to irregular solid nests of squamous epithelium with dark and clear cytoplasm. Myoepithelial cells were also observed. Immunohistochemically, the spindle cells were strongly and diffusely positive for cytokeratins and some of them were positive for BRST2, alpha-smooth muscle actin, and CD10. The tumor was negative for S-100 protein, glial fibrillary acidic protein, and CD34. Ultrastructurally, tonofilaments and desmosomes were observed in the spindle cells. The findings indicate an epithelial origin. The patient was well without recurrence or metastasis 8 months after excision. Pathologists and clinicians should be aware of the existence of ectopic hamartomatous thymoma in the supraclavicular or suprasternal region and should differentiate it from a high-grade sarcoma, such as biphasic synovial sarcoma or glandular malignant peripheral nerve sheath tumor.