Morphological difference between pleural mesothelioma cells in effusion smears with either BAP1 loss or 9p21 homozygous deletion and reactive mesothelial cells without the gene alterations

Sarcomatoid mesothelioma diagnosed in a patient with mesothelioma in situ: a case report on morphologic differences after 9-month interval with details analysis of cytology in early-stage mesothelioma

BACKGROUND

Overlapping morphological features of mesothelial cells have been rendered it difficult to distinguish between reactive and malignant conditions. The development of methods based on detecting genomic abnormalities using immunohistochemistry and fluorescence in situ hybridization have contributed markedly to solving this problem. It is important to identify bland mesothelioma cells on cytological screening, perform efficient genomic-based testing, and diagnose mesothelioma, because the first clinical manifestation of pleural mesothelioma is pleural effusion, which is the first sample available for pathological diagnosis. However, certain diagnostic aspects remain challenging even for experts.

CASE PRESENTATION

This report describes a case of a 72-year-old man with a history of asbestos exposure who presented with pleural effusion as the first symptom and was eventually diagnosed as mesothelioma. Mesothelioma was suspected owing to prominent cell-in-cell engulfment in mesothelial cells on the first cytological sample, and the diagnosis of mesothelioma in situ was confirmed by histology. Unexpectedly, sarcomatoid morphology of mesothelioma was found in the second pathology samples 9 months after the first pathological examination. Both the mesothelioma in situ and invasive lesion showed immunohistochemical loss of methylthioadenosine phosphorylase (MTAP) and homozygous deletion of cyclin dependent kinase inhibitor 2A (CDKN2A) on fluorescence in situ hybridization. The patient received medication therapy but died of disease progression 12 months after the diagnosis of the sarcomatoid morphology of mesothelioma.

CONCLUSION

Our case suggests that cell-in-cell engulfment can be conspicuous in early-stage mesothelioma with inconspicuous nuclear atypia and few multinucleated cells. In addition, the presence of MTAP loss and CDKN2A homozygous deletion are suspected to be involved in early formation to invasive lesions and/or sarcomatoid morphology. We believe that it is important to consider genetic abnormalities when deciding on individual patient management. Furthermore, cases of mesothelioma, even those of an in situ lesion, with MTAP loss and/or CDKN2A deletion should be carefully followed up or subjected to early treatment.

Update of pathological diagnosis of pleural mesothelioma using genomic-based morphological techniques, for both histological and cytological investigations

As more than 80% of pleural mesothelioma (PM) cases start with pleural effusions, diagnosis with effusion smear cytology or pleural biopsy is important. For diagnosing PM, a three-step approach is used: (1) detecting atypical cells; (2) verifying their mesothelial origin using immunohistochemistry (IHC); and (3) discriminating PM from benign mesothelial proliferations (BMP). The third step is critical for diagnosing early lesions. In small biopsy or cytologic specimens in which tumor cell fat invasion cannot be assessed, genomic-based assays, including IHC-detected BAP1 loss and fluorescence in situ hybridization (FISH)-detected homozygous deletion (HD) of CDKN2A/p16, are effective for differentiation. Both BAP1 IHC and CDKN2A FISH can equally be applied to histologic and cytologic specimens, with 100% specificity in discriminating PM from BMP. We found that methylthioadenosine phosphorylase (MTAP) loss as detected by IHC could serve as a feasible alternative in tissue and cytologic preparations for CDKN2A FISH. However, a combination including FISH was still most effective: the addition of NF2 FISH to CDKN2A FISH and BAP1 IHC yielded a greater sensitivity of close to 100% in diagnosing PM tissues. Although IHC is more feasible than FISH, owing to remaining challenges in data interpretation, caution and familiarity are warranted when diagnosing PM.

Epidemiology and Clinical Aspects of Malignant Pleural Mesothelioma

Mesothelioma is a cancer predominantly of the pleural cavity. There is a clear association of exposure to asbestos with a dose dependent risk of mesothelioma. The incidence of mesothelioma in different countries reflect the historical patterns of commercial asbestos utilisation in the last century and predominant occupational exposures mean that mesothelioma is mostly seen in males. Modern imaging techniques and advances in immunohistochemical staining have contributed to an improved diagnosis of mesothelioma. There have also been recent advances in immune checkpoint inhibition, however, mesothelioma remains very challenging to manage, especially considering its limited response to conventional systemic anticancer therapy and that no cure exists. Palliative interventions and support remain paramount with a median survival of 9-12 months after diagnosis. The epidemiology and diagnosis of mesothelioma has been debated over previous decades, due to a number of factors, such as the long latent period following asbestos exposure and disease occurrence, the different potencies of the various forms of asbestos used commercially, the occurrence of mesothelioma in the peritoneal cavity and its heterogeneous pathological and cytological appearances. This review will describe the contemporary knowledge on the epidemiology of mesothelioma and provide an overview of the best clinical practice including diagnostic approaches and management.

Diagnostic mesothelioma biomarkers in effusion cytology

Malignant mesothelioma is a rare malignancy with a poor prognosis whose development is related to asbestos fiber exposure. An increasing role of genetic predisposition has been recognized recently. Pleural biopsy is the gold standard for diagnosis, in which the identification of pleural invasion by atypical mesothelial cell is a major criterion. Pleural effusion is usually the first sign of disease; therefore, a cytological specimen is often the initial or the only specimen available for diagnosis. Given that reactive mesothelial cells may show marked atypia, the diagnosis of mesothelioma on cytomorphology alone is challenging. Accordingly, cell block preparation is encouraged, as it permits immunohistochemical staining. Traditional markers of mesothelioma such as glucose transporter 1 (GLUT1) and insulin-like growth factor 2 mRNA-binding protein 3 (IMP3) are informative, but difficult to interpret when reactive proliferations aberrantly stain positive. BRCA1-associated protein 1 (BAP1) nuclear staining loss is highly specific for mesothelioma, but sensitivity is low in sarcomatoid tumors. Cyclin-dependent kinase inhibitor 2A (CDKN2A)/p16 homozygous deletion, assessed by fluorescence in situ hybridization, is more specific for mesothelioma with better sensitivity, even in the sarcomatoid variant. The surrogate marker methylthioadenosine phosphorylase (MTAP) has been found to demonstrate excellent diagnostic correlation with p16. The purpose of this review is to provide an essential appraisal of the literature regarding the diagnostic value of many of these emerging biomarkers for malignant mesothelioma in effusion cytology.

Testing for BAP1 loss and CDKN2A/p16 homozygous deletion improves the accurate diagnosis of mesothelial proliferations in effusion cytology

BACKGROUND

A number of ancillary tests have been developed that aid in the diagnosis of mesothelioma in cytology specimens. The aim of this retrospective study was to determine whether testing for BAP1 and CDKN2A/p16 status in effusion specimens preceding the tissue diagnosis of mesothelioma would improve diagnostic accuracy and allow an earlier diagnosis of malignancy.

METHODS

The study cohort included 99 matched cytology fluid specimens from 74 patients with a surgical specimen diagnosis of malignant mesothelioma (67 epithelioid, 7 biphasic, 55 pleural, and 19 peritoneal). BAP1 immunohistochemistry and p16 fluorescence in situ hybridization (FISH) were performed retrospectively.

RESULTS

BAP1 or p16 FISH testing revealed a loss in 7 of 18 (39%) samples originally categorized as benign/reactive, 20 of 33 (61%) interpretable samples categorized as atypical, and 10 of 14 (71%) cases suspicious for mesothelioma. In some cases, the diagnosis of mesothelioma could have been made up to 9 months before biopsy. Similarly, loss of BAP1 or p16 was found in 28 of 30 (93%) samples categorized as malignant, with some cases diagnosable up to 6 months before biopsy. Overall, loss of BAP1 and/or CDKN2A/p16 homozygous deletion would change the diagnostic interpretation in 37 of 60 (62%) (P = .07) effusion specimens, particularly in pleural effusions (32 of 48 samples) (P = .002). The sensitivity of morphologic interpretation alone was 30.3%; however, adding testing for BAP1 and p16 resulted in an increase in sensitivity to 68.7%. (P < .0001).

CONCLUSION

These findings suggest that routine use of BAP1 immunochemistry and p16 FISH as adjunctive tests improves the diagnostic accuracy of cytology specimens and potentially allows an earlier diagnosis of malignant mesothelioma.

Integrative approach to cytologic and molecular diagnosis of malignant pleural mesothelioma

The global incidence of malignant mesothelioma (MM) causes considerable disease burden, suffering and health care costs. Beside preventive measures and ban the use of asbestos, early diagnosis would largely improve the chance of curative treatment. Current histologic criteria, however, requiring presence of invasion in the surrounding fatty tissue fail to identify MM in sufficiently early stage. Unilateral accumulation of pleural effusion is one of the earliest clinical manifestations of MM that occurs in approximately 90% of the patients. Therapeutic thoracocenthesis is necessary to remove the fluid and to relieve patients’ symptoms. This effusion is easily accessible and offers early and minimally invasive diagnosis by combining cytology with immunologic, molecular- and biomarker analyses. Typically, the fluid is rich in malignant cells and cell groups, but incipient stages of the disease may be difficult to recognize as the malignant cells can be masked by presence of inflammatory or reactive mesothelial cells. Recurrent, hemorrhagic and cell rich effusion should always be suspicious for MM and adequately prepared and analyzed to provide necessary information for subsequent therapy. Importantly, early detection of MM by integrating cytology and molecular approaches has high sensitivity and positive predictive value and has a major impact on patient survival. Thus, a conclusive positive MM cytology should lead to treatment without delay. This review summarizes molecular and diagnostic criteria of MM diagnosis.