Primary thymic papillary adenocarcinoma with a rare CD5/CD117‑negative and PAX8/WT1‑positive immunophenotype: A case report

Primary thymic papillary adenocarcinoma (PAT) is an exceptionally rare epithelial malignancy that poses substantial diagnostic challenges due to its non‑specific radiological presentation and highly heterogeneous immunophenotype. The present study reports a case of early‑stage PAT in a 48‑year‑old man with a long‑standing history of rheumatoid arthritis. Chest computed tomography (CT) revealed a 3.3‑cm, well‑circumscribed, lobulated anterior mediastinal nodule demonstrating homogeneous contrast enhancement, radiologically mimicking a benign thymic cyst or thymoma. Following complete resection via video‑assisted thoracoscopic surgery (VATS), histopathological examination demonstrated a cystic and solid tumor with conspicuous papillary architecture. Diagnostic interpretation was confounded by an atypical immunoprofile: The tumor was negative for the traditional thymic carcinoma markers CD5 and CD117, yet exhibited diffuse expression of paired box gene 8 and WT1, thereby closely resembling metastatic renal cell carcinoma or mesothelioma. A diagnosis was strongly supported through a rigorous process of exclusion, integrating an extended negative immunohistochemical panel (including Spalt‑like transcription factor 4, thyroid transcription factor‑1 and D2‑40) with the morphological identification of residual atrophic thymic tissue at the tumor periphery. The tumor was classified as Masaoka‑Koga stage I PAT, and the patient remained free of recurrence at the 6‑month postoperative follow‑up. The present case highlights the diagnostic pitfalls of PAT, particularly its deceptively benign imaging appearance and unusual immunophenotype. It emphasizes that CD5/CD117 negativity does not preclude a thymic origin, underscoring the necessity of a comprehensive, exclusion‑based approach for accurate diagnosis.