Primary thymic epithelial tumours of the pleura mimicking malignant mesothelioma

Our plain-English summary of Primary thymic epithelial tumours of the pleura mimicking malignant mesothelioma, by R.L. Attanoos, F. Galateau-Salle, A.R. Gibbs, et al (Histopathology, 2002).

The study

• Authors: R.L. Attanoos, F. Galateau-Salle, A.R. Gibbs, S. Muller, F. Ghandour, S.D. Dojcinov
• Journal: Histopathology
• Published: 2002;41(1):42-9
• Cited by: 27 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Aims: To illustrate the macroscopic, light microscopic and immunophenotypic similarities that exist between primary pleural thymic epithelial tumours and diffuse malignant mesothelioma. To investigate the expression of the mesothelial markers, cytokeratin (CK) 5/6, calretinin and thrombomodulin in a series of mediastinal thymic epithelial tumours.

Methods And Results: Over a 10-year period, 64 diffuse pleural tumours of non-mesothelial histogenesis were identified in the files of referrals to the South Wales regional thoracic centre (Llandough Hospital, Cardiff). Of these, five pleural tumours were diagnosed as primary pleural thymic epithelial neoplasms. From the files of the Mesopath group, Caen, three additional cases of thymic epithelial tumours with pleural involvement were identified. The study group comprised eight cases (four males, four females) with median age at presentation of 56 years (range 19-75 years). In one case there was a history of asbestos exposure. Macroscopically, seven tumours formed diffuse pleural masses. No mediastinal abnormality or intraparenchymal lesions were seen in five cases. By light microscopy, seven thymic epithelial neoplasms showed a lobulated architecture, one appeared extensively cystic. The tumours were of varied morphological subtypes: one medullary (WHO Type A), two mixed (WHO Type AB), three predominantly cortical (WHO Type B1) and two cortical (WHO Type B1). The subtypes morphologically mimicked sarcomatoid, biphasic, lymphohistiocytoid variant and epithelioid mesothelioma. The pleural thymic epithelial tumours showed immunoreactivity with broad spectrum cytokeratin AE1/AE3 (8/8; 100%), CK5/6 (8/8; 100%), and 1/8 (13%) expressed thrombomodulin. Calretinin showed variable nuclear and cytoplasmic expression in all cases, but equivocally in the thymic epithelial cell component. In 7/8 (88%) the thymic epithelial cells exhibited focal aberrant expression of CD20. Epithelial membrane antigen (EMA) showed focal expression in the perivascular and organoid areas in 6/8 (75%) cases. Carcinoembryonic antigen (CEA) and CD34 were uniformly negative. In 4/8 (50%) cases the lymphoid cell component was of immature phenotype expressing CD99, terminal deoxynucleotidyl transferase (TdT) and lymphoid precursors had a high proliferation fraction with Ki67. In the series of 20 primary mediastinal thymic epithelial tumours tested, mesothelial marker expression revealed CK5/6 (20/20), thrombomodulin (3/20; 15%) and calretinin (0/20; 0%). Varying amounts of calretinin-positive stromal cells were present.

Conclusion: Primary pleural thymic epithelial tumours are rare but may mimic malignant mesothelioma by forming diffuse serosal-based masses. In addition, both tumours may show morphological diversity (with epithelial, spindled and mixed components present). An awareness that thymic epithelial tumours may variably express the mesothelial markers CK5/6, calretinin and thrombomodulin prevents misdiagnosis. In the distinction from malignant mesothelioma a lobulated architecture and organoid features favour a thymic epithelial neoplasm. The presence of aberrant CD20 expression in a cytokeratin-positive epithelial neoplasm and/or the presence of an immature lymphoid population (by demonstration of CD1a, CD2, CD99 and TdT) indicates a thymic epithelial neoplasm. In contrast, nuclear calretinin expression favours malignant mesothelioma.

In plain English

This article described eight cases of thymic epithelial tumours of the pleura (the lining around the lungs) that resembled malignant mesothelioma. Five cases came from Llandough Hospital, Cardiff, and three from the Mesopath group in Caen. Four patients were men and four were women, aged 19 to 75. One patient gave a history of asbestos exposure, with precise details not known. In seven cases the tumour formed a multinodular pleural mass that partly encased the lung. In five cases no disease was found in the mediastinum (the central chest) or in the lung. Under the microscope, seven tumours had a lobulated pattern. All eight stained for cytokeratin 5/6, a marker used in diagnosing mesothelioma, and one stained for thrombomodulin. Clear nuclear calretinin staining was not seen in the tumour cells. In a separate group of 20 thymic tumours from the mediastinum, all 20 expressed cytokeratin 5/6 and 3 of 20 expressed thrombomodulin. The authors concluded these tumours are rare and most probably arise from misplaced thymic tissue, and that a lobulated appearance is a clue to identifying them. They noted there is no confirmatory evidence linking thymic epithelial tumours with 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 series of eight pleural tumours, with an additional laboratory staining (immunohistochemistry) study of 20 mediastinal thymic epithelial tumours
• Who was studied: Eight cases of primary pleural thymic epithelial tumour: four men and four women, aged 19 to 75 (median 56). Five came from the files of Llandough Hospital, Cardiff, and three from the Mesopath Group, Caen. A further 20 mediastinal thymic epithelial tumours came from the Llandough archives.
• Exposure: One of the eight patients (case 3) gave a history of asbestos exposure. The article gives no details of fiber type, job or dose.
• Main results:
  • In seven of the eight cases the tumour formed a multinodular pleural mass, with pleural thickening and partial encasement of the lung on the same side.
  • In five of the eight cases no mediastinal disease or abnormality within the lung was identified.
  • Under the microscope, seven of the eight tumours showed a lobulated appearance.
  • All eight pleural tumours expressed the mesothelial marker cytokeratin 5/6.
  • Thrombomodulin was expressed in 1 of the 8 pleural tumours (case 3).
  • Unequivocal nuclear calretinin staining was not seen in the thymic epithelial (tumour) cells.
  • In the 20 mediastinal thymic epithelial tumours, all 20 expressed broad spectrum cytokeratin and cytokeratin 5/6.
  • In the 20 mediastinal tumours, thrombomodulin was expressed in 3 of 20, all of WHO Type B2.
  • The authors stated that, to date, there is no confirmatory evidence linking thymic epithelial tumours with asbestos.
• The authors' conclusion: The authors concluded that primary pleural thymic epithelial tumours are rare and most probably arise from ectopic (misplaced) thymic tissue. They added that a lobulated appearance and organoid features are an important clue to identifying them.

Topics

Medical terms in this study, explained in our Lexicon: Pleura, Mesothelioma, Calretinin, Neoplasm, Asbestos Exposure, Sarcomatoid, Biphasic Mesothelioma, Lymphohistiocytoid.

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