Pathological discrepancies in the diagnosis of thymic epithelial tumors: the Tallinn-Lyon experience

On the Histologic Classification of Thymoma

The classification of thymoma continues to be a source of controversy in pathology. The difficulties in histologic classification are evident from the number of proposals that have been offered over the years, as well as for the continuous changes and modifications introduced by the World Health Organization to their classification system over the past 20 years. We analyze here some of the issues involved in the classification of these tumors and the difficulties encountered for practicing pathologists in deciphering the "letters and numbers" system devised by the World Health Organization. We would like to propose an alternate approach to thymoma histologic classification that capitalizes on the basic observation of their cytologic features and incorporates the pattern of growth resulting from the interplay of the tumor cells with other cellular constituents as a secondary characteristic. The proposed histologic classification provides a simplified, reproducible means of histologically categorizing these tumors and can be easily understood by most practicing pathologists in simple and clear morphologic terms.

DNA-Methylation Analysis as a Tool for Thymoma Classification

BACKGROUND

Thymomas are malignant thymic epithelial tumors that are difficult to diagnose due to their rarity and complex diagnostic criteria. They represent a morphologically heterogeneous class of tumors mainly defined by "organo-typical" architectural features and cellular composition. The diagnosis of thymoma is burdened with a high level of inter-observer variability and the problem that some type-specific morphological alterations are more on the continuum than clear-cut. Methylation pattern-based classification may help to increase diagnostic precision, particularly in borderline cases.

METHODS AND RESULTS

We applied array-based DNA methylation analysis to a set of 113 thymomas with stringent histological annotation. Unsupervised clustering and t-SNE analysis of DNA methylation data clearly segregated thymoma samples mainly according to the current WHO classification into A, AB, B1, B2, B2/B3, B3, and micronodular thymoma with lymphoid stroma. However, methylation analyses separated the histological subgroups AB and B2 into two methylation classes: mono-/bi-phasic AB-thymomas and conventional/"B1-like" B2-thymomas. Copy number variation analysis demonstrated methylation class-specific patterns of chromosomal alterations.

INTERPRETATION

Our study demonstrates that the current WHO classification is generally well reflected at the methylation level but suggests that B2- and AB-thymomas are (epi)genetically heterogeneous. Methylation-based classifications could help to refine diagnostic criteria for thymoma classification, improve reproducibility, and may affect treatment decisions.

Interobserver variation in the classification of thymic lesions including biopsies and resection specimens in an international digital microscopy panel

Aims

Thymic tumours are rare in routine pathology practice. Although the World Health Organization (WHO) classification describes a number of well‐defined categories, the classification remains challenging. The aim of this study was to investigate the reproducibility of the WHO classification among a large group of international pathologists with expertise in thymic pathology and by using whole slide imaging to facilitate rapid diagnostic turnover.

Methods and results

Three hundred and five tumours, consisting of 90 biopsies and 215 resection specimens, were reviewed with a panel‐based virtual microscopy approach by a group of 13 pathologists with expertise in thymic tumours over a period of 6 years. The specimens were classified according to the WHO 2015 classification. The data were subjected to statistical analysis, and interobserver concordance (Fleiss kappa) was calculated. All cases were diagnosed within a time frame of 2 weeks. The overall level of agreement was substantial (κ = 0.6762), and differed slightly between resection specimens (κ = 0.7281) and biopsies (κ = 0.5955). When analysis was limited to thymomas only, and they were grouped according to the European Society for Medical Oncology Clinical Practice Guidelines into B2, B3 versus A, AB, B1 and B3 versus A, AB, B1, B2, the level of agreement decreased slightly (κ = 0.5506 and κ = 0.4929, respectively). Difficulties arose in distinguishing thymoma from thymic carcinoma. Within the thymoma subgroup, difficulties in distinction were seen within the B group.

Conclusions

Agreement in diagnosing thymic lesions is substantial when they are assessed by pathologists with experience of these rare tumours. Digital pathology decreases the turnaround time and facilitates access to what is essentially a multinational resource. This platform provides a template for dealing with rare tumours for which expertise is sparse.

Accuracy of pathologic diagnosis for thymic epithelial tumors: a brief report from an Italian reference Center

Objectives - Rare tumors are diagnostic challenges for pathologists. Consultation or referral to Centers with expertise is crucial for the right diagnosis. This is particularly true for Thymic Epithelial Tumors (TETs), whose treatment strategies vary according to histological subgroup. We aimed at evaluating the accuracy of TET pathologic characterization in an Italian reference Center. Materials and methods - All the cases with diagnosis or suspicion of TETs, which underwent a pathological second opinion at Istituto Nazionale dei Tumori (INT), Milan, between 2015 and 2019 were retrospectively reviewed. All cases had been pathologically characterized through immunohistochemistry (IHC). Descriptive statistics were used for qualitative variables. Concordance was estimated through Cohen's kappa (k). Results - Out of 278 cases of TETs diagnosed in INT, 72 were referred to INT for a pathologic revision. The INT revision changed the diagnosis in 41 cases (56.9%), with a potential therapeutic shift in 32 (44.4%). In particular, 20 cases of thymoma were reviewed as a different subtype of thymoma (19/20) or lymphoma (1/20); nine cases of thymic carcinoma were reviewed as thymoma. On the other hand, three cases of lung carcinoma were reviewed as thymic carcinoma (2/3) or thymoma (1); eight cases of carcinoma Not Otherwise Specified were reviewed as thymic carcinoma; one case of lymphoma was reviewed as thymoma. Concordance between pathologists was moderate for thymoma (74.7%, k 0.447), inferior for thymic carcinoma (60.5%, k 0.139). Conclusion - A significant proportion of cases referred to INT for a presumptive TET received a different characterization. A potential shift in therapeutic indication was not rare. This underlines the importance for TETs to get a second pathological diagnosis by an expert pathologist and supports the need for networks on rare cancers.