Pathology of thymoma-where are we today?

Abstract

Thymoma is the commonest epithelial neoplasm arising from thymus gland. Tumour is slow growing and in the absence of metastasis, surgery is the treatment of choice. Complete resection and bland morphology are important prognostic features. However, a significant proportion of these tumours tend to recur. These recurrent tumours, advanced thymomas and thymic carcinomas require platinum-based combination chemotherapy and radiotherapy. Efforts are being made to explore additional treatment modalities to control disease with the aim of improving survival. Number of thymoma cases worldwide is small in comparison to lung cancers. As a result, fewer studies have been carried out to enhance our understanding of molecular events responsible for the initiation, maintenance, and progression of thymomas. Inspite of this there are advances in understanding the pathology of thymic epithelial neoplasms including genetics, PD-L1 and molecular testing which has bearing on the prognosis, post-surgical management, and testing algorithm. Similar to pulmonary pathology, thymic epithelial tumours will require adequate tumour sampling to carry out ancillary testing. Mutational analytical tests include EGFR, RAS, BRAF, RET, AKT1, PIK3CA and T53 genes. If adequate sample is available (upto100 cells), PD-L1 testing should be considered for immunotherapy in recurrent/ advanced thymomas and thymic carcinomas. This list is likely to expand in future with increasing emphasis on molecular testing to support treatment with newer therapies.

Keywords: Thymoma; genetics; personalized medicine.

Figure 1

Thymoma (B3): complete membrane staining with PD-L1 is seen in more than 90% of tumour cells, positive for high expression (A,B: ×10, C,D: ×20; A,C: HE staining, B,D: IHC staining).

Figure 2

Thymoma (mixed B1 and B2): tumour cells are negative, while background normal thymus shows scattered positive lymphocytes with incomplete membranous staining (A,B: ×10, C,D: ×20; A,C: HE staining, B,D: IHC staining).

Figure 3

Protocol for histological management of a mediastinal mass. The figure shows the histopathology laboratory protocol for dealing with a biopsy from a mediastinal mass.