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.

Clinically Occult Diffuse Pleural Mesothelioma in Patients Presenting With Spontaneous Pneumothorax

OBJECTIVES

To report histologic features of unsuspected diffuse pleural mesothelioma (DPM) in surgical specimens for pneumothorax and demonstrate how ancillary markers support a diagnosis of malignancy in this context. We explored whether pneumothorax may be a clinical manifestation of mesothelioma in situ (MIS).

METHODS

A single-institution database search identified patients who underwent surgical resection for spontaneous pneumothorax (n = 229) and/or were diagnosed with DPM (n = 88) from 2000 to 2020.

RESULTS

Spontaneous pneumothorax without clinical, radiologic, or intraoperative suspicion of mesothelioma was the initial presentation in 2 (2.3%) of 88 patients diagnosed with DPM. This represented 0.9% (2/229) of all patients undergoing surgical management of pneumothorax but accounted for a larger proportion of older patients (12.5% older than 70 years). Immunohistochemistry for BAP-1 and/or MTAP confirmed the diagnosis of DPM in 2 cases. Mesothelioma in situ was identified retrospectively by immunohistochemistry in 1 case of spontaneous pneumothorax from a 77-year-old man who developed invasive DPM 25 months later. No additional cases of MIS were identified in 19 surgical lung resections for spontaneous pneumothorax.

CONCLUSIONS

Histologic examination of bleb resections with ancillary testing for cases with ambiguous features is essential for detection of early DPM. It is uncertain whether spontaneous pneumothorax may represent a clinical manifestation of MIS.

[Updates in the pathological diagnosis of Pleural Malignant Mesothelioma in the WHO classification of thoracic tumors (5(th) edition)]

The WHO Classification of Thoracic Tumors (5(th) edition) mainly has the following changes in the chapter of pleural malignant mesothelioma. (1) The concept of mesothelioma in situ and its diagnostic method have been established for the first time; (2) The tumour grading of pleural malignant mesothelioma was added, it was divided into low grade and high grade according to the cellular atypia, mitotic activity and presence of necrosis. (3) The morphological features of pleural malignant mesothelioma was classified into architectural pattern, cellular and stromal features, the correlation between histological features and prognosis was refined, and some of the controversial cellular types have been reclassified. In this review, we introduced the changes of related pathologic diagnosis, in the WHO Classification of Thoracic Tumors (5(th) edition) and discussed its clinical significance.

The highlights of the 15th international conference of the international mesothelioma interest group - Do molecular concepts challenge the traditional approach to pathological mesothelioma diagnosis?

Pathology plays an important role in diagnosing mesothelioma since radiological and clinical findings alone cannot distinguish mesothelioma reliably from its many mimics. The long-held gold standard for pathological diagnosis requires a tissue biopsy that, in addition to mesothelial phenotype, demonstrates invasion, but this is challenged by the WHO recognition of mesothelioma in situ (MIS) and concurrent acknowledgement of all mesotheliomas as malignant. Tumor sampling and ancillary techniques are of paramount importance for diagnosis of MIS. Standardisation of these techniques, cut-off points and terminology, and an updated staging system are urgently required. These clinically relevant issues and the impact of new developments were illustrated at the pathology session of 15th meeting of the International Mesothelioma Interest Group. It was reported that combination of losses in p16 nuclear expression, with cut-off ≤ 1%, and cytoplasmic MTAP with cut-off ≥ 30% demonstrated increased specificity (96%) and high sensitivity (86%) for CDKN2A HD detection. Otherwise, the combination of p16 IHC and CDKN2A HD may improve prognosis. The potential usefulness of pleural effusions for early diagnosis was demonstrated in a retrospective study investigating pleural effusions had been diagnosed as benign prior to mesothelioma diagnosis. Alterations of BAP1 (IHC) and CDKN2A (FISH) were detectable 2 or more years prior diagnosis. Moreover, analysis of gene expression profiles in cytology samples by principal component analysis discriminated reactive hyperpasia from epitheliod mesothelioma. Early diagnosis, including cytology diagnosis, is being acyively investigated. Since no treatment recommendations exist for MIS, pathologists recognise the need for international collaborations to fully characterise this rare entity. Clear communication with the clinical teams is required to ensure optimum patient care. The data reported in this meeting are encouraging and open avenues for further work that will allow even earlier diagnosis and better characterisation of mesothelioma progression, based on changes in gene expression, including epigenetic changes.

Whole exome sequencing reveals BAP1 somatic abnormalities in mesothelioma in situ

OBJECTIVES

We have recently described the first cases of mesothelioma in situ, identified as a pure surface population of mesothelial cells that have lost BAP1 nuclear staining, in the setting of no clinically/radiologically demonstrable mesothelial tumor. These cases have a high propensity to develop invasive mesothelioma. The genetic events that lead to the development of mesothelioma in situ are unknown, nor is it known whether mesothelioma in situ cases carry somatic or germline mutations.

MATERIAL AND METHODS

Whole exome sequencing (WES) was performed on two cases of mesothelioma in situ (1 pleural, 1 peritoneal) and paired formalin fixed paraffin embedded normal tissue to characterize driver mutations and copy number alterations.

RESULTS

The analysis demonstrated somatic alterations in the BAP1 gene only. The pleural mesothelioma in situ showed copy number loss and LOH in the BAP1 locus on chromosome 3. The peritoneal mesothelioma in situ showed both a BAP1 somatic splice site mutation involving intron 5-exon 6 boundary (A126_splice) with an allelic fraction of 10%, and BAP1 copy number loss. No other driver point mutations, indels or somatic DNA copy number alterations reported to occur in invasive mesothelioma, or novel genetic alterations, were identified.

CONCLUSION

Whole exome sequencing confirms that mesothelioma in situ development is associated with BAP1 somatic mutations/deletions, and suggests that BAP1 mutation/deletion represents a very early event in the development of malignant mesothelioma. Whether BAP1 mutation/deletion alone is sufficient to lead to invasive mesothelioma or whether additional genetic alterations are required remains to be determined.

Putative Malignant Pleural Mesothelioma in situ (MPMIS) with Sequential Acquisition of Genomic Alterations on Fluorescence in situ Hybridization (FISH) Examination

Malignant pleural mesothelioma (MPM) is a rare and deadly disease. A precursor in situ lesion, malignant pleural mesothelioma in situ (MPMIS), has recently been proposed. On cytological examination, the distinction between reactive and malignant mesothelial cells is often challenging, and sometimes even impossible without ancillary methods. Fluorescence in situ hybridization (FISH) for detection of 9p21 deletion is a powerful diagnostic tool in this context, both in histological and in cytological specimens. Here, we present a case of MPM with initial presentation as a putative MPMIS with disomic chromosomal pattern and homozygous 9p21 deletion with subsequent development of an aneuploid pattern after whole genome duplication during tumor progression.