CD117, BAP1, MTAP, and TdT Is a Useful Immunohistochemical Panel to Distinguish Thymoma from Thymic Carcinoma

Insights into molecular aspects and targeted therapy of thymic carcinoma: a narrative review

Background and Objective

Thymic carcinomas are rare tumors derived from thymic epithelial cells. Owing to their rarity, the search for molecular biology has been conducted in combination with thymoma as one histological subtype, and only a few studies have exclusively focused on thymic carcinoma. Currently, no therapy is more effective than complete surgical resection, and the development of novel therapies, including targeted therapies, is hampered. In this review, we summarize the knowledge regarding altered genes and pathways in thymic carcinoma with recent preclinical and clinical targeted therapies.

Methods

We conducted a narrative review of the relevant English literature available in PubMed and Google Scholar on genomic characteristics and targeted therapies for thymic carcinoma.

Key Content and Findings

Although the literature consists of a relatively small series, it suggests that the frequently involved genes or pathways associated with thymic carcinoma are tumor suppressor genes, including TP53 and CDKN2A/B, and the receptor tyrosine kinase pathway. Targeted therapy demonstrated antitumor activity with encouraging results. However, potential predictive biomarkers have not been identified and the response to these therapies appears to be irrelevant to gene alterations.

Conclusions

Some studies have revealed the molecular characteristics of thymic carcinoma, although the results of these studies have shown a different pattern of gene alterations. The further accumulation of data would be helpful in revealing the genomic landscape and establishing molecular-targeted therapies.

Non-Mutational Key Features in the Biology of Thymomas

Thymomas (THs) are a unique group of heterogeneous tumors of the thymic epithelium. In particular, the subtypes B2 and B3 tend to be aggressive and metastatic. Radical tumor resection remains the only curative option for localized tumors, while more advanced THs require multimodal treatment. Deep sequencing analyses have failed to identify known oncogenic driver mutations in TH, with the notable exception of the GTF2I mutation, which occurs predominantly in type A and AB THs. However, there are multiple alternative non-mutational mechanisms (e.g., perturbed thymic developmental programs, metabolism, non-coding RNA networks) that control cellular behavior and tumorigenesis through the deregulation of critical molecular pathways. Here, we attempted to show how the results of studies investigating such alternative mechanisms could be integrated into a current model of TH biology. This model could be used to focus ongoing research and therapeutic strategies.

Thymic epithelial tumours: histopathological classification and differential diagnosis

The epithelial and lymphoid compartments of the thymus can give rise to a wide variety of tumours, including thymomas, thymic carcinomas, lymphoreticular proliferations, germ cell tumours, and sarcomas. While some of these have close similarity to their counterparts in other organs, both in terms of histology and immunohistochemistry, as well as molecular features, others are unique to the thymus. The epithelial tumours, which can develop in the thymus, will be discussed in this review, with a particular emphasis on resolving differential diagnosis by means of morphology, immunohistochemical profiles, and molecular diagnostics.

Dataset for reporting of thymic epithelial tumours: recommendations from the International Collaboration on Cancer Reporting (ICCR)

AIMS

Thymic epithelial tumours (TET), including thymomas and thymic carcinomas and thymic neuroendocrine neoplasms, are malignant neoplasms that can be associated with morbidity and mortality. Recently, an updated version of the World Health Organization (WHO) Classification of Thoracic Tumours 5th Edition, 2021 has been released, which included various changes to the classification of these neoplasms. In addition, in 2017 the Union for International Cancer Control (UICC) / American Joint Committee on Cancer (AJCC) published the 8th Edition Staging Manual which, for the first time, includes a TNM staging that is applicable to thymomas, thymic carcinomas, and thymic neuroendocrine neoplasms.

METHODS AND RESULTS

To standardize reporting of resected TET and thymic neuroendocrine neoplasms the accrediting bodies updated their reporting protocols. The International Collaboration on Cancer Reporting (ICCR), which represents a collaboration between various National Associations of Pathology, updated its 2017 histopathology reporting guide on TET and thymic neuroendocrine neoplasms accordingly. This report will highlight important changes in the reporting of TET and thymic neuroendocrine neoplasms based on the 2021 WHO, emphasize the 2017 TNM staging, and also comment on the rigour and various uncertainties for the pathologist when trying to follow that staging.

CONCLUSION

The ICCR dataset provides a comprehensive, standardized template for reporting of resected TET and thymic neuroendocrine neoplasms.

Current Treatment Approaches for Thymic Epithelial Tumors

Thymic epithelial tumors (TETs), including thymoma, thymic carcinoma and neuroendocrine tumors, are uncommon tumors that originate from the epithelial cells of the thymus. Nevertheless, despite their rarity, they represent the most common tumor type located in the anterior mediastinum. Therapeutic choices based on staging and histology may include surgery with or without neoadjuvant or adjuvant therapy represented by chemotherapy, radiotherapy or chemo-radiotherapy. For patients with advanced or metastatic TETs, platinum-based chemotherapy remains the standard first-line treatment; however, some new drugs and combinations are currently under evaluation. In any case, proper management of patients with TETs requires a multidisciplinary team approach to personalize care for each patient.

EZH2 and POU2F3 Can Aid in the Distinction of Thymic Carcinoma from Thymoma

Thymic carcinoma is an aggressive malignancy that can be challenging to distinguish from thymoma using histomorphology. We assessed two emerging markers for these entities, EZH2 and POU2F3, and compared them with conventional immunostains. Whole slide sections of 37 thymic carcinomas, 23 type A thymomas, 13 type B3 thymomas, and 8 micronodular thymomas with lymphoid stroma (MNTLS) were immunostained for EZH2, POU2F3, CD117, CD5, TdT, BAP1, and MTAP. POU2F3 (≥10% hotspot staining), CD117, and CD5 showed 100% specificity for thymic carcinoma versus thymoma with 51%, 86%, and 35% sensitivity, respectively, for thymic carcinoma. All POU2F3 positive cases were also positive for CD117. All thymic carcinomas showed >10% EZH2 staining. EZH2 (≥80% staining) had a sensitivity of 81% for thymic carcinoma and a specificity of 100% for thymic carcinoma versus type A thymoma and MNTLS but had poor specificity (46%) for thymic carcinoma versus B3 thymoma. Adding EZH2 to a panel of CD117, TdT, BAP1, and MTAP increased cases with informative results from 67/81 (83%) to 77/81 (95%). Overall, absent EZH2 staining may be useful for excluding thymic carcinoma, diffuse EZH2 staining may help to exclude type A thymoma and MNTLS, and ≥10% POU2F3 staining has excellent specificity for thymic carcinoma versus thymoma.

The clinicopathological significance of thymic epithelial markers expression in thymoma and thymic carcinoma

BACKGROUND

The classification of thymomas is based on the morphology of epithelial tumor cells and the proportion of lymphocytes. Type A thymomas are composed of the spindle or oval tumor epithelial cells. Tumor cells of B thymomas are epithelioid-shaped with increasing atypia. Type AB thymomas have the features of epithelial tumor cells of A and B thymomas. The diagnosis can be difficult because of the complex morphology. Some novel thymic epithelial markers have been reported in several preclinical studies, but they have not been applied to clinical practice. Here, we investigated the expression of 3 cortical and 3 medullary markers, which are thymoproteasome-specific subunit β5t (β5t), thymus-specific serine protease 16 (PRSS16), cathepsin V, autoimmune regulator (AIRE), CD40 and claudin-4.

METHODS

Immunohistochemistry was used to analyze 53 cases of thymomas and thymic squamous cell carcinomas (TSCC), aiming to explore the expression of cortical and medullary epithelial markers and their correlation with histological classification, Masaoka-Koga stage, and prognosis.

RESULTS

Our results found that for cortical epithelial markers the expression of β5t, PRSS16, and cathepsin V was higher in type AB and B thymomas than in micronodular thymoma with lymphoid stroma (MNT), and we observed a dramatic increase of β5t and PRSS16 expression in type AB compared to type A thymomas. In medullary epithelial markers, the expression of AIRE was higher in type A than in B3 thymomas. CD40 and β5t expression were associated with the Masaoka-Koga stage. High cathepsin V expression was related to a good prognosis and a longer progression-free survival.

CONCLUSION

This is the first comprehensive analysis of the role of thymic cortical and medullary epithelial markers as biomarkers for differential diagnosis and prognosis in thymic tumors. Thymic medullary epithelial immunophenotype was found to exhibit in type A, MNT, and TSCC. Type B thymomas primarily exhibited a cortical epithelial immunophenotype. Type AB thymomas showed cortical, medullary, or mixed corticomedullary epithelial immunophenotype. Our results demonstrated that thymic cortical and medullary epithelial markers including β5t, PRSS16, cathepsin V, and AIRE could be used as ancillary markers in the diagnosis and prognosis of thymic epithelial tumors.