Primary pleural thymic epithelial tumours

Epidemiology

Primary pleural thymic epithelial tumours are rare. They may closely mimic mesothelioma, macroscopically, histologically and immunophenotypically.

Macroscopic appearances

The tumours are usually multinodular. They may encase the lung. There may be involvement of lung or mediastinum.

Histopathology

The histological pattern is similar to that of mediastinal thymic epithelial tumours and include the range of subtypes (A, AB, B1, B2) described in the WHO classification. The low-power lobulated appearance, organoid features, blood-filled cysts, stromal haemorrhage, perivascular oedema with free-floating lymphocytes and proteinaceous pools all favour a diagnosis of thymoma.

Immunohistochemistry (with mediastinal thymomas for comparison)

	epithelial component
	primary pleural	mediastinal
AE1/3	8/81	20/201
CK5/6	8/81	20/201
Calretinin	1/8 (stromal component positive in 7/8)	0/20 (stromal component positive in 7/20)1
Thrombomodulin	1/81	3/201
EMA	0/81
CEA	0/81
CD34	0/81
CD20	7/81

	lymphoid component
CD1a	5/81
CD3	8/81
CD20	8/81
CD99	5/81
TdT	5/81

Differential diagnosis

The presence of immature T-cells (CD1a+/CD2+/CD99+/TdT+) favour thymoma.

• malignant mesothelioma, epithelioid, mixed or sarcomatoid, depending on the subtype of thymic tumour; mesotheliomas also commonly positive for CK5/6, calretinin and thrombomodulin. They are distinguished by being membrane positive for EMA but CD20 negative.
  

•  
  

• WHO Type A thymic tumours;

  • sarcomatoid mesothelioma
    
  • solitary fibrous tumour (AE1/3-/CD34+)
    
  • monophasic synovial sarcoma (CK5/6-)
    
  • malignant vascular tumours (Factor VIIIRA+/CD31+/CD34+)
•  
  
• WHO Type B thymic tumours;
  • epithelioid mesothelioma, particularly lymphohistiocytoid variant
    
  • metastatic carcinoma

  • anaplastic large cell lymphoma; the epithelial cells of thymoma may be CD20+ but are CD45-

• WHO Type AB thymic tumours;

  • biphasic mesothelioma

References

¹Attanoos, R.L., Galateau-Salle, F., Gibbs, A.R., Muller, S., Ghandour, F. and Dojcinov, S.D. Primary thymic epithelial tumours of the pleura mimicking malignant mesothelioma. Histopathology 2002;41:42-9.

This page last revised 2.1.2003.

©SMUHT/PW Bishop