Solid, cystic, and mucinous spindle cell thymomas (WHO type A): A clinicopathological and immunohistochemical study...

Seven cases of an unusual thymoma subtype are presented. Patients included five men and two women, aged 43-57 years (mean: 50). Five patients presented with non-specific symptoms of cough, chest pain and shortness of breath; two patients were asymptomatic. Diagnostic imaging revealed anterior mediastinal masses in all patients and surgical resection was accomplished via thoracotomy. Grossly, the tumors were well circumscribed with focal cystic changes, measuring 2.7 cm-4.8 cm. Six tumors were encapsulated; one was minimally invasive. Histologically, the tumors exhibited cystic changes and solid areas composed of bland spindle cells with scant thymocytes, consistent with spindle cell thymomas (WHO type A). Notably, focal mucinous differentiation was observed, including glandular structures with mucinous epithelium, scattered mucinous cells, extracellular mucin, and partial cyst wall lining by mucinous epithelium. Immunohistochemistry demonstrated that the spindle cell component was positive for pancytokeratin, CK5/6, p63 and CD20 and negative for CK7, CK20, PAX8, GATA-3, TTF-1, CDX2, STAT6 and SS18-SSX while the mucinous cells were variably positive for CK7, CK20 and CDX2; mucicarmine histochemical staining confirmed mucin content. Four patients remained alive and well at 18 months postoperatively while 3 patients were lost to follow-up. The current cases represent a previously undescribed variant of spindle cell thymoma that may pose diagnostic challenges and give rise to a wide differential diagnosis.