This is a 38-year-old male with history of polymyositis who presented with anterior mediastinal mass. Per radiology the mass measures 3.8 cm. An incisional biopsy was performed and sent to pathology. Representative sections along with immunstains are shows below.
Final Diagnosis: Thymic Hyperplasia with Lymphoepithelial Sialadenitis (LESA)-Like Features
Thymic hyperplasia (TH) with lymphoepithelial sialadenitis (LESA)- like features is a tumor like lesion characterized by benign proliferation of both epithelium and lymphoid follicles. There is a significant association with B- cell lymphoma particularly MALT lymphoma and non- Myasthenia Gravis autoimmune disorders prompting proper work up for both diseases whenever this entity is encountered. In this case, the B-cell follicles are confined within intact and tight CD21 follicular dendritic meshworks arguing against lymphoma. Cytokeratin highlights thymic epithelial hyperplasia. TdT immunostain is negative which is typical for this entity. Clinical work up for both lymphoma and autoimmune diseases are negative.