Abstract: Undifferentiated large cell carcinoma of the thymus with a Castleman disease (CD)-like reaction is a thymic carcinoma accompanied by an inflammatory reaction closely resembling the morphological features of CD. This disease is extremely rare and distinctive, only five cases have been documented in a single report, and all five cases were associated with a reaction like the hyaline vascular type CD. For the first time, we report two cases of undifferentiated large cell carcinoma of the thymus with a plasma cell type CD-like reaction. The two cases presented similar histological findings and immunoprofiles. Undifferentiated large cells were arranged in nests and cords within hyperplastic follicles, mimicking pseudogerminal centers. Abundant plasma cells were distributed in the interfollicular areas. The tumor cells were positive for CK-pan and BRG1 staining but negative for CD5, CD117, CK5/6, p63, p40, and EBER. Therefore, the diagnoses of squamous cell carcinoma, lymphoepithelial carcinoma, or micronodular thymic carcinoma with lymphoid hyperplasia were excluded. Even though the carcinoma cells showed high-grade nuclear pleomorphism with prominent nucleoli, these two cases presented indolent clinical courses, which were consistent with the previous report.
Keywords: Castleman disease-like reaction, immunohistochemistry, prognosis, thymic carcinoma, undifferentiated large cell carcinoma