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

Our plain-English summary of Primary thymic papillary adenocarcinoma with a rare CD5/CD117-negative and PAX8/WT1-positive immunophenotype: A case report, by D. Zhu, C. Wan, J. Guo, et al (Oncology Letters, 2026).

The study

• Authors: D. Zhu, C. Wan, J. Guo, T. Zeng, W. Wang
• Journal: Oncology Letters
• Published: 2026;32(3):402
• Type of article: Case report
• Read it: PubMed · Publisher · Free full text

Abstract

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.

Copyright: © 2026 Zhu et al.

In plain English

This was a case report of one patient: a 48-year-old man at Shiyan Taihe Hospital in China who had had rheumatoid arthritis for more than 10 years. A routine examination found a mass in the front of the chest. CT showed a 3.3 by 2.0 cm well-defined, lobulated lesion, which was removed completely by video-assisted thoracoscopic surgery. Under the microscope the tumor had papillary (finger-like) structures, and remnants of shrunken thymus tissue were found at its edge. On staining, the tumor was positive for PAX8 and WT1 and focally positive for calretinin. It was negative for CD5 and CD117, markers traditionally used for thymic carcinoma, and negative for the mesothelial marker D2-40. The authors reported that their workup excluded mesothelioma and cancers spread from the lung, thyroid and genitourinary tract. They diagnosed a Masaoka-Koga stage I primary papillary adenocarcinoma of the thymus. No further treatment was given, and there was no recurrence at the 6-month follow-up. The authors named the short follow-up as a limitation. The article does not discuss asbestos.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: case report (single patient)
• Who was studied: One 48-year-old man with a more than 10-year history of rheumatoid arthritis, treated at Shiyan Taihe Hospital (Hubei University of Medicine), China
• Main results:
  • Contrast-enhanced chest CT showed a 3.3x2.0 cm irregularly lobulated anterior mediastinal mass with well-defined margins
  • Residual atrophic thymic tissue was found at the edge of the tumor
  • The tumor was diffusely positive for PAX8 and WT1 (among other markers) and focally positive for calretinin
  • The tumor was negative for the traditional thymic markers CD5 and CD117
  • The tumor was negative for the mesothelial marker D2-40
  • The Ki-67 proliferation index was about 10%
  • The authors reported that the evaluation excluded mesothelioma and metastatic carcinomas from the lung, thyroid and genitourinary tract
  • The tumor was Masaoka-Koga stage I; no adjuvant chemoradiotherapy was given, and no recurrence was seen at the 6-month follow-up
• The authors' conclusion: The authors concluded that pathologists should not exclude a primary thymic origin just because of CD5/CD117 negativity or PAX8 and WT1 positivity. In their view, broad, exclusion-based immunohistochemistry with clinical and radiological correlation is needed to avoid misdiagnosis.
• Limitations the authors state:
  • Follow-up was short (6 months)
  • The patient's long-term rheumatoid arthritis medication regimen was unknown, which limited evaluation of any effect of drug-induced immunosuppression

Topics

Medical terms in this study, explained in our Lexicon: Adenocarcinoma, Standing, Video-Assisted Thoracoscopic Surgery (VATS), Carcinoma, Mesothelioma, Thyroid Transcription Factor-1 (TTF-1), Podoplanin (D2-40).

Related studies

• A rare case of primary effusion lymphoma in HIV negative patient: Diagnostic challenges and literature review — P. Malik, S.N. Khader, et al. (2021)
• Comparative genetic analysis of a rare synchronous collision tumor composed of malignant pleural mesothelioma and primary pulmonary adenocarcinoma — T. Naka, Y. Hatanaka, et al. (2016)
• The epithelioid BAP1-negative and p16-positive phenotype predicts prolonged survival in pleural mesothelioma — A. Chou, C.W. Toon, et al. (2018)
• Immunocytochemistry and DNA-image cytometry in diagnostic effusion cytology I. Prevalence of markers in tumour cell positive and negative smears — H. Motherby, M. Kube, et al. (1999)
• microRNAs are differentially regulated between MDM2-positive and negative malignant pleural mesothelioma — R.F. Walter, C. Vollbrecht, et al. (2016)
• Rare thoracic cancers: a comprehensive overview of diagnosis and management of small cell lung cancer, malignant pleural mesothelioma and thymic epithelial tumours — D.W. Dumoulin, P. Bironzo, et al. (2023)

Browse more research in Oncology Letters, or return to medical research.