Corpuscular thymoma, case report and review of literature

Abstract

Aim Hassall corpuscles are concentric arrangement of keratinizing epithelial cells that are restricted to the medulla of the normal thymus. Hassall corpuscles in thymomas are unusual findings and are seen in thymomas WHO type B1-3 with medullary differentiation, but rare cases have been documented. Thymoma with abundant Hassall corpuscles (corpuscular thymoma) must be differentiated from thymic carcinoma, well differentiated keratinizing squamous cell carcinoma type. Case report We report a case of a 73-year-old male patient, presenting with cough for one year. Computed tomography (CT) scan showed a well defined anterior superior mediastinal mass. Surgical excision of the mass was performed. Gross examination revealed a large, oval shaped, well circumscribed, encapsulated, firm brown mass measuring 10 × 6 × 4 cm, and weighing 219 g, with heterogeneous pale white solid, small cystic spaces, and necrotic cut surfaces. Light microscopic examination revealed two alternating components, the first forming a delicate loose network of neoplastic epithelium equally admixed with non-neoplastic lymphocytes, and the second forming vague solid sheets with paucity of lymphocytes. Both components show abundant Hassall corpuscles, extensive areas of coagulative tumor necrosis, small cystic spaces, and cholesterol clefts. Immunohistochemical studies showed that the neoplastic epithelium is strongly and diffusely immunoreactive for HMWCK, panCK, and EMA, and focally positive for CAM5.2, and negative for CD5, and C-KIT immunohistochemical stains. Conclusion Corpuscular thymoma reported in the literature as WHO type B2 or B3. Our presented case is of type B3, Modified Masoaka stage 1. We believe that thymoma with corpuscular differentiation is a rare distinct variant of organotypical thymomas, although it is not mentioned in the 2004 WHO classification. Recognition of this rare variant of thymomas is very important to avoid confusion with thymic carcinoma and squamous cell carcinoma type. Careful microscopic examination and immunohistochemical stains are mandatory to differentiate between these entities.