The Separation of Benign and Malignant Mesothelial Proliferations: New Markers and How to Use Them

Pleural mesothelioma in situ : a comprehensive review

Pleural mesothelioma is a rare and aggressive cancer that affects the pleura. In recent years, there has been increasing interest and attention in detecting and diagnosing early-stage or precancerous forms of mesothelioma because of its severe prognosis and short life expectancy at the time of diagnosis. Mesothelioma in situ represents a clear opportunity to improve and innovate the diagnostic approach and the multimodality treatment of mesothelioma: the diagnosis of pleural mesothelioma at the 'in-situ phase' means early disease detection and thus paves the way to new possible curable strategies. Since 2021, when mesothelioma in situ was finally identified and described as a new histological entity, its diagnosis and management became a challenge and the subject of ongoing research; several aspects remain open and still outstanding as regards diagnostic techniques, time and probability of progression, need for and methods of follow up, aggressive and early surgery. This narrative review aims to provide a comprehensive overview of mesothelioma in situ covering its definition, risk factors, diagnostic criteria, and tricky aspects of early detection. It also highlights its clinical significance, new perspectives, and potential future indications in the context of pleural mesothelioma multidisciplinary management.

Mesothelioma: morphologic and immunohistochemical findings

This paper reviews some basic and some new concepts in the diagnosis of mesothelioma. The term "malignant mesothelioma" is no longer recommended; rather, any tumor labeled "mesothelioma" is presumed to be malignant. Clinical and radiologic information is very useful in the diagnosis of mesothelioma; in particular, nodular pleural thickening on CT is usually a marker of malignancy. The literature on markers that separate mesotheliomas from metastatic carcinomas has become very complex and frequently misleading, with many recommended markers actually demonstrating poor specificity. However, newer data show that a combination of HEG1 (clone SKM9-2) and claudin‑4 staining provides extremely high accuracy in separating epithelioid mesotheliomas from non-small-cell lung carcinomas with just two immunostains. This combination works at other sites as well, but caution should be used when high-grade serous carcinoma is in the differential, because all "mesothelioma" markers can also stain high-grade serous carcinomas. There are, unfortunately, no sensitive or specific markers for sarcomatoid mesotheliomas. A variety of immunohistochemical and fluorescence in situ hybridization (FISH) markers are useful in separating benign from malignant mesothelial proliferations; immunohistochemal staining for BAP1, MTAP (or CDKN2A FISH), and NF2/Merlin (or NF2 FISH) will enable the diagnosis of most mesotheliomas. Mesothelioma in situ is now recognized as either a single layer of bland cuboidal mesothelial cells that have lost BAP1, and sometimes MTAP, on immunohistochemical staining, or a process that is morphologically identical to a well-differentiated papillary mesothelial tumor that has lost BAP1/MTAP. Mesothelioma in situ probably always progresses to invasive mesothelioma, but this process is often quite slow.

Mesothelioma: molecular pathology and biomarkers

Diffuse mesotheliomas are characterized by recurrent genomic alterations involving tumor suppressors and epigenetic regulators such as BAP1, CDKN2A, MTAP, and NF2. Depending on the differential diagnosis as informed by histologic assessment, one can apply the appropriate immunohistochemical and/or molecular panels to reach the correct pathologic diagnosis, sometimes even in cases with limited tissues. Biomarkers aid in the diagnosis of mesothelioma in the following scenarios: 1) For a tumor that is overtly malignant, how can one distinguish mesothelioma from other tumors? 2) For a mesothelial proliferation, how can one distinguish mesothelioma from a reactive process? To distinguish mesotheliomas from carcinomas, at least two positive and two negative markers are currently recommended. To distinguish sarcomatoid mesothelioma from pleomorphic carcinoma, even more markers-and sometimes molecular testing-are needed. To distinguish mesothelioma from reactive mesothelial conditions, useful immunohistochemical biomarkers include BAP1, MTAP, and merlin, which serve as surrogates for the corresponding gene mutation status. In patients with unusual clinical history, for tumors with a peculiar microscopic appearance, and/or in cases with an equivocal immunophenotypic profile, molecular testing can help to exclude mimics and to confirm the pathologic diagnosis.

Loss of MTAP Expression by Immunohistochemistry Is a Surrogate Marker for Homozygous 9p21.3 Deletion in Urothelial Carcinoma

Homozygous deletion of the chromosomal region 9p21.3 is common in urothelial carcinoma (UC) and leads to loss of several genes, including CDKN2A and MTAP, resulting in loss of MTAP protein expression. Here, we aimed to explore the diagnostic potential of MTAP immunohistochemistry (IHC) as a surrogate marker for homozygous 9p21.3 deletion (9p21 homozygous deletion [HD]) in UC. MTAP status was determined by IHC on 27 UC tissue specimens with known 9p21.3 status as defined by fluorescence in situ hybridization in matched cytological specimens, by IHC and fluorescence in situ hybridization on a tissue microarray (TMA) containing 359 UC at different stages, and by IHC on 729 consecutive UC from routine practice. Moreover, we analyzed a longitudinal series of matched specimens from 38 patients with MTAP-negative recurrent UC. MTAP loss by IHC was found in all 17 patients with 9p21 HD and in 2/8 cases without 9p21 HD. In the TMA, MTAP loss was more common in metastases (53%) than in muscle-invasive (33%) and non-muscle-invasive UC (29%) (P = .03). In the consecutive series, 164/729 (22%) cases showed loss of MTAP expression. In 41 of these 164 cases (25%), loss of MTAP expression was heterogenous. We also discovered loss of MTAP expression in flat urothelium adjacent to MTAP-negative low-grade UC, suggesting true flat low-grade neoplasia that could not be diagnosed by morphology alone. Longitudinal analysis of recurrences showed persistent negative MTAP status over time in 37/38 (97%) patients. MTAP IHC can serve as a surrogate marker for 9p21 HD in UC and as a diagnostic tool to differentiate reactive urothelium from urothelial neoplasia. It also provides a unique opportunity to study clinicopathological associations and the heterogeneity of 9p21 HD across the whole spectrum of UC manifestations.

Molecular and Immunohistochemical Testing in Mesothelioma and Other Mesothelial Lesions

CONTEXT.—

Molecular testing has increasingly been utilized in the evaluation of mesothelioma. Diffuse mesothelioma comprises multiple distinct genetic subgroups. While most diffuse mesotheliomas lack oncogenic kinase mutations and instead harbor alterations involving tumor suppressors and chromatin regulators, a minor subset of tumors is characterized by uncommon alterations such as germline mutations, genomic near-haploidization, ALK rearrangement, ATF1 rearrangement, or EWSR1::YY1 fusion.

OBJECTIVE.—

To provide updates on the salient molecular features of diffuse mesothelioma, mesothelioma in situ, and other mesothelial lesions: well-differentiated papillary mesothelial tumor, adenomatoid tumor, peritoneal inclusion cyst, and others. We consider the diagnostic, prognostic, and predictive utility of molecular testing in mesothelial lesions.

DATA SOURCES.—

We performed a literature review of recently described genetic features, molecular approaches, and immunohistochemical tools, including BAP1, MTAP, and merlin in mesothelioma and other mesothelial lesions.

CONCLUSIONS.—

Our evolving understanding of the molecular diversity of diffuse mesothelioma and other mesothelial lesions has led to considerable changes in pathology diagnostic practice, including the application of immunohistochemical markers such as BAP1, MTAP, and merlin (NF2), which are surrogates of mutation status. In young patients and/or those without significant asbestos exposure, unusual mesothelioma genetics such as germline mutations, ALK rearrangement, and ATF1 rearrangement should be considered.

Testicular/paratesticular mesothelial tumours: Uncommon histopathologic entities in a very complex anatomical site

Mesothelial tumours of the testicular/paratesticular region are uncommon, poorly characterised and difficult-to-diagnose lesions. They encompass entirely benign proliferations (adenomatoid tumour) and malignant, very aggressive tumours (mesothelioma) whose morphological features can be overlapping, highly variable and confounding. Moreover, testicular/paratesticular mesothelial tumours comprise relatively new entities with indolent behaviour (well-differentiated papillary mesothelial tumour) as well as tumours which cannot be correctly included in any of the aforementioned categories and whose classification is still controversial. The molecular profile of such tumours represents an open issue. In fact, despite the recent discoveries about the genomic landscape of mesothelial proliferations at other sites (pleura, peritoneum), testicular/paratesticular mesothelial tumours, and namely mesotheliomas, are too rare to be extensively studied on large case series and they could arguably hide relevant differences in their molecular background when compared to the more common pleural/peritoneal counterparts.The aim of this review is to provide a guide for the pathological assessment of testicular/paratesticular mesothelial tumours. Herein, we describe the most recent updates on this topic according to the latest (year 2022) World Health Organisation Classification of Urinary and Male Genital Tumours (5th edition) and current literature. The diagnostic criteria, the main differentials and the role of ancillary techniques in the diagnosis of mesothelial testicular/paratesticular tumours are discussed.

New developments in mesothelial pathology

This review article examines some new and some problem areas in mesothelial pathology, four of which are discussed, as follows. (1) The concept of mesothelioma in situ: this lesion is defined as a single layer of bland mesothelial cells without evidence of invasion, but that have lost BAP1 and/or MTAP by immunohistochemistry. Benign reactions can exactly mimic mesothelioma in situ, but a hint to the correct diagnosis is a story of recurrent pleural effusions/ascites of unknown aetiology without radiological or direct visual evidence of tumour. (2) The nature of well-differentiated papillary mesothelial tumour (WDPMT): WDPMT has a long history of arguments regarding its behaviour, and this uncertainty can now be seen to arise, in part, from the observation that some forms of mesothelioma in situ microscopically look exactly like WDPMT. Hence, it is recommended to always run at least a BAP1 stain on any lesion that looks like WDPMT. Both flat and WDPMT-like mesothelioma in situ are strongly associated with eventual development of invasive mesothelioma, but this process is relatively slow. (3) New immunostains for separating mesothelioma from other tumours: here, it is proposed that in most cases, and particularly when the differential is epithelioid mesothelioma versus non-small cell lung cancer, one can make this separation with extremely high sensitivity and specificity using just two stains: HEG1 and claudin-4. (4) Markers for separating benign from malignant mesothelial proliferations: this topic is briefly reviewed, with an indication of which markers are generally accepted and the best utilisation and possible limitations of each marker.

Peritoneal papillary mesothelioma in situ: BAP1 mutation with indolent behavior for 15 years

Papillary mesothelioma in situ (PMIS) is a rare and enigmatic disease. Most instances manifest as lesions of the peritoneal serosa. The pathogenesis and behavior of peritoneal PMIS are still poorly understood, and separation from benign well differentiated peritoneal mesothelial tumors (WDPMT) may be challenging. We describe the 15-year long course of a PMIS in an adult male in which inactivating mutations of BAP1, encoding BRCA1 associated protein 1 (BAP1), were identified. Tumor samples were obtained on 2 occasions more than 8y apart. In both samples, the tumor cells were bland, with occasional focal infiltration into the stalks of larger papillary lesions. However, no invasion into subserosal adipose tissue was identified. In both samples the tumor cells did not express nuclear BAP1. Comprehensive genomic analysis of the initial tumor sample revealed a somatic inactivating mutation in BAP1 (predicted effect, Y223*) and a somatic variant of IRS2 (A701_V702insAA). An additional inactivating mutation in BAP1 (predicted effect, T69fs*5) was detected in the later sample. The patient did not receive any treatment and is still alive 15 years after initial presentation. Our experience supports the view that peritoneal PMIS may follow an indolent course for many years and prompts the question whether these tumors should uniformly be treated aggressively.

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.

Comparison of immunohistochemical mesothelial biomarkers in paired biopsies and effusion cytology cell blocks from pleural mesothelioma

OBJECTIVE

Traditionally, the diagnosis of pleural mesothelioma is based on histological material. Minimally invasive effusion cytology specimens are an alternative that, like biopsies, require ancillary analyses. Validation of immunohistochemical (IHC) analyses on cytology, including the surrogate markers for molecular alterations BAP1 and MTAP, is of interest.

METHODS

IHC for eight different markers was performed on 59 paired formalin-fixed, paraffin-embedded pleural biopsies and pleural effusion cell blocks with mesothelioma. Immunoreactivity in ≥10% of tumour cells was considered positive/preserved. The concordance between histological and cytological materials was assessed.

RESULTS

The overall percentage of agreement between the histological epithelioid component in 58 biopsies and paired cell blocks was 93% for calretinin, 98% for CK5, 97% for podoplanin, 90% for WT1, 86% for EMA, 100% for desmin, 91% for BAP1, and 72% for MTAP. For 11 cases with biphasic or sarcomatoid histology, the concordance between cytology and the histological sarcomatoid component was low for calretinin, CK5, and WT1 (all ≤45%). For the whole cohort, loss of both BAP1 and MTAP was seen in 40% while both markers were preserved in 11% of the biopsies for epithelioid histology. The corresponding numbers were 54% and 8%, respectively, for the paired cell blocks.

CONCLUSIONS

Generally, a high concordance for IHC staining was seen between paired biopsies and pleural effusion cell blocks from mesotheliomas, but the somewhat lower agreement for WT1, EMA, and especially MTAP calls for further investigation and local quality assurance. The lower concordance for the sarcomatoid subtype for some markers may indicate biological differences.

Well-differentiated Papillary Mesothelial Tumour of the Tunica Vaginalis Testis - A Rare Lesion, but one Pathologists Should Know About Two Patient Reports and a Review of the Literature

Besides malignant mesothelioma, benign mesothelial neoplasms do exist in the tunica vaginalis testis. However, histological criteria remain controversial, thus leading to diagnostic uncertainty and difficulty in their classification according to their biological behavior. In recent years, molecular markers have emerged that aid in the differentiation of benign and malignant mesothelial proliferations throughout the body. Here, we present two middle-aged men with well-differentiated papillary mesothelial tumors and a review of the literature. By now, more than a year after surgery, one patient showed no recurrence of disease after partial or complete orchiectomy without further treatment, for the second no information is available. In conclusion, well-differentiated papillary mesothelial tumors represent rare lesions in the tunica vaginalis testis, but one pathologists should know about to prevent unnecessary treatment and suffering of patients.

Peritoneal Mesothelioma-An Update

A remarkable amount of new information has been generated on peritoneal mesothelioma (PeM), ranging from nomenclature changes, including the removal of "malignant" when referring to this neoplasm and the use of the term "tumor" rather than "mesothelioma" to designate the neoplasm formerly known as "well-differentiated papillary mesothelioma", to the acknowledgment that PeMs can be associated with tumor predisposition syndromes or germline mutations. Although the disease is still more frequently seen in caucasian males, PeM is not uncommon in women. In addition, it can represent a diagnostic challenge when it has an uncommon presentation (ie, paraneoplastic syndrome or incidental finding) or when it has confounding histologic features. Ancillary testing, including immunohistochemical stains, in situ hybridization for CDKN2A or NF2 , and molecular studies, in selected cases, allows its correct diagnosis. The molecular landscape of PeM is still a work in progress; however, some findings, such as ALK gene rearrangements and EWSR1/FUS-ATF1 fusions, are specifically seen in PeM of young patients. The biological behavior of PeM is variable; however, cytoreductive surgery and hyperthermic intraperitoneal chemotherapy have markedly improved the survival of patients affected by this disease.

Mesotheliomas and Benign Mesothelial Tumors: Update on Pathologic and Imaging Findings

A diverse spectrum of benign entities and malignant neoplasms originate from the monotonous mesothelium that lines the serosal membranes of the pleural, pericardial, and peritoneal cavities. The mesothelium of myriad sites shows a common origin from the lateral plate mesoderm; primary mesothelial tumors thus demonstrate similar pathogenesis, imaging findings, and treatment options. Significant changes have been made in the 2021 World Health Organization (WHO) classification schemata of the pleural and pericardial tumors on the basis of recent advances in pathology and genetics. While malignant mesotheliomas are biologically aggressive malignancies that occur primarily in patients exposed to asbestos with attendant poor survival rates, well-differentiated papillary mesothelial tumors and adenomatoid tumors charter a benign clinical course with an excellent prognosis. Mesothelioma in situ is a newly characterized entity represented by recurrent unexplained pleural effusions without any identifiable mass at imaging or thoracoscopy. Immunohistochemical markers based on BAP1, MTAP, CDKN2A, and TRAF7 gene mutations help differentiate diffuse mesotheliomas from benign mesothelial proliferations and localized mesotheliomas. Cross-sectional imaging modalities, including US, CT, MRI, and fluorine 18-fluorodeoxyglucose (FDG) PET/CT, permit diagnosis and play a major role in staging and assessing surgical resectability. Imaging studies are invaluable in providing noninvasive and quantitative assessment of tumor response in patients with unresectable disease. Owing to significant overlap in patient characteristics and pathomorphology, accurate diagnosis based on advanced histopathology techniques and genetic abnormalities is imperative for optimal management and prognostication. While patients with nonepithelioid pleural mesotheliomas benefit from immunotherapy, novel targeted therapies for CDKN2A-, NF2-, and BAP1-altered mesotheliomas are under consideration. ^© RSNA, 2023 Quiz questions for this article are available through the Online Learning Center.

The cytologic diagnosis of mesothelioma: are we there yet?

INTRODUCTION

Mesothelioma is a rare but highly aggressive malignancy with poor prognosis that frequently present with recurrent effusions. Establishing the diagnosis by cytology can lead to early diagnosis and treatment and consequently improve prognosis.

MATERIALS AND METHODS

This review examines the cytological diagnosis of mesothelioma in the context of its historical and morphologic evolution and provides an update of the current reporting systems. Clues to identify the mesothelial and malignant nature of the sample are detailed as well as the supporting ancillary tests.

RESULTS

Cytologically, the samples are overwhelmingly cellular and malignancy is recognized by both architectural and cytological atypia. Numerous variably sized clusters and enlarged cells are easily identified, some with papillary architecture and collagen cores. Recognizing the mesothelial nature of the cells and supportive immunostains are essential to rule out the differential diagnosis of metastatic carcinomas and reactive mesothelium. Current ancillary tests such as homozygous deletion of CDKN2A, loss of BRCA1-associated protein, and methylthioadenosine phosphorylase expression can provide further support of malignancy.

CONCLUSIONS

At this time with the aid of current ancillary tests and in the hands of cytopathologists with adequate experience with the interpretation of effusions, the diagnosis of mesothelioma can be established with accuracy in most cases.

Usefulness of NF2 hemizygous loss detected by fluorescence in situ hybridization in diagnosing pleural mesothelioma in tissue and cytology material: A multi-institutional study

OBJECTIVES

BAP1, CDKN2A, and NF2 are the most frequently altered genes in pleural mesotheliomas (PM). Discriminating PM from benign mesothelial proliferation (BMP) is sometimes challenging; it is well established that BAP1 loss, determined by immunohistochemistry (IHC), and CDKN2A homozygous deletion (HD), determined by fluorescence in situ hybridization (FISH), are useful. However, data regarding the diagnostic utility of NF2 FISH in PM is limited. Thus, we performed a multi-institutional study examining the utility of NF2 alterations determined by FISH for diagnosing PM in combination with BAP1 loss and CDKN2A HD.

MATERIALS AND METHODS

Multi-institutional PM cases, including 106 surgical and 107 cell block samples as well as 37 tissue cases of benign mesothelial proliferation (BMP) and 31 cell block cases with reactive mesothelial cells (RMC), were collected and analyzed using IHC for BAP1 and FISH for CDKN2A and NF2.

RESULTS

In PM, NF2 FISH revealed hemizygous loss (HL) in 54.7% of tissue cases (TC) and 49.5% of cell block cases (CBC), with about 90% of HL being monosomy. CDKN2A HD or BAP1 loss were detected in 75.5%/65.4% TC or 63.6%/60% CBC, respectively. BMP or RMC showed no BAP1 loss, CDKN2A HD, or NF2 HL. For discriminating PM from BMP, a combination of BAP1 loss, CDKN2A HD, and NF2 HL yielded enhanced sensitivity of 98.1% TC/94.4% CBC. BAP1 loss, CDKN2A HD, or NF2 HL were observed in 69%, 70%, or 58% of epithelioid PM, but in 9%, 91%, or 27% of sarcomatoid PM, respectively. Histotype, histological gradings, and CDKN2A deletion status showed significant differences in overall survival, while BAP1 loss and NF2 HL did not.

CONCLUSION

NF2 HL, consisting predominantly of monosomy, can be detected by FISH in both TC and CBC of PM, and is effective for distinguishing PM from BMP, especially when combined with BAP1 loss and CDKN2A HD.

Well differentiated papillary mesothelial tumor: a new name and new problems

Well-differentiated papillary mesothelial tumor (WDPMT, formerly called well-differentiated papillary mesothelioma) is a morphologically distinctive lesion composed of expansile papillae with a myxoid core covered by a single layer of generally bland mesothelial cells. Whether some WDPMT are precursors of invasive mesothelioma is uncertain, and this question is confounded by shallow biopsies of ordinary diffuse mesotheliomas that have superficial areas resembling WDPMT as well as by misinterpretation of some cases of mesothelioma in situ. Genetic analyses on a very small number of published cases of peritoneal WDPMT have shown a variety of mutations/copy number losses that do not overlap at all with those that are found recurrently in invasive mesotheliomas. The newly described entity of mesothelioma in situ usually appears as a single layer of mesothelial cells that have lost BAP1 by immunostaining, but sometimes is papillary and produces a morphologic mimic of WDPMT. We propose that, at least in the peritoneal cavity where most WDPMT occur, there are two morphologically identical but functionally distinct lesions: one is true WDPMT, a process that is probably benign, and the other is papillary mesothelioma in situ with the configuration of WDPMT. For that reason immunostaining for BAP1, and if necessary MTAP or CDKN2A FISH, should always be performed on cases with the appearance of WDPMT. It is possible, but speculative, that the small number of reports in the literature which describe invasive mesothelioma arising from WDMPT are actually describing invasive mesothelioma arising from mesothelioma in situ that looks like WDPMT.

Peritoneal Pathology Review: Mullerian, Mucinous and Mesothelial Lesions

This review provides an overview of the pathology of selected benign and malignant lesions of the female peritoneum and their often-encountered differential diagnoses. It includes endometriosis and its related lesions, endosalpingiosis, pseudomyxoma peritonei (PMP) and related ovarian/appendiceal pathology, and malignant and benign mesothelial tumors. The current terminology associated with PMP is also discussed.

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.

Molecular and Immunohistochemical Characterization of Mesothelioma of the Tunica Vaginalis

AIMS

Malignant mesothelioma (MM) of the tunica vaginalis (TV) is a rare and aggressive tumor, and the molecular features and staining profile with contemporary immunohistochemical (IHC) biomarkers are largely unexplored. We characterize the clinicopathologic, molecular, and IHC features of MM (N=13) and mesothelial neoplasms of uncertain malignant potential (MUMP) (N=4).

METHODS AND RESULTS

Targeted next-generation sequencing was performed on 7 MMs and 2 MUMPs. IHC was performed for MTAP, BAP1 and SOX6. 13 adenomatoid tumors were also assessed with SOX6. MM were epithelioid (7/13) or biphasic (6/13). In MM, NF2 (5/7; 71%), CDKN2A (3/7; 43%), and BAP1 (2/7; 29%) were most frequently altered. Non-recurrent driver events were identified in PTCH1 and TSC1. In contrast, none of these alterations were identified in MUMPs; however, one MUMP harbored a TRAF7 missense mutation. By IHC, loss of MTAP (2/12; 17%) and BAP1 (2/9; 22%) was infrequent in MM, whereas both were retained in the MUMPs. SOX6 was positive in 9/11 (82%) MMs, and negative in all MUMPs and adenomatoid tumors.

CONCLUSIONS

Testicular MM exhibit a similar mutational profile to those of the pleura/peritoneum; however, alterations in CDKN2A and BAP1 are less common. These findings suggest that although MTAP and BAP1 IHC are specific for MM, their sensitivity in testicular MMs appears lower. In addition, rare tumors may harbor targetable alterations in driver genes (PTCH1 and TSC1) that are unusual in MMs at other anatomic sites. SOX6 is sensitive for MM; accordingly, the presence of SOX6 expression argues against a benign neoplastic process.

Usefulness of Cyclin D1/Podoplanin Dual Immunohistochemical Stain to Differentiate Malignant Mesothelioma From Reactive Mesothelial Proliferations

OBJECTIVES

To examine the potential of cyclin D1/podoplanin dual immunohistochemical stain to differentiate malignant mesothelioma from reactive mesothelial proliferations.

METHODS

Cyclin D1/podoplanin dual immunohistochemistry was performed on 34 surgical cases of reactive mesothelial proliferations, malignant mesothelioma, and nonmesothelioma malignancies.

RESULTS

All 15 reactive mesothelial proliferations demonstrated less than 50% cyclin D1 staining with variable to diffuse podoplanin staining. In 6 (60%) of 10 cases of epithelioid malignant mesothelioma, the dual stain supported the diagnosis. Less than 50% cyclin D1 staining was noted in the remaining four cases, including small biopsy specimens or cases with focal papillary architecture. The five cases of sarcomatoid/desmoplastic/biphasic mesothelioma showed more than 50% cyclin D1 staining with focal to absent podoplanin staining. Well-differentiated papillary mesothelioma appears to demonstrate less than 25% cyclin D1 staining.

CONCLUSIONS

The cyclin D1/podoplanin dual stain is reliable and may be used to aid in differentiation of benign mesothelial proliferations from malignant tumors. In addition, histologic features and other ancillary testing may support the classification of cases with an inconclusive cyclin D1/podoplanin staining.

Sarcomatoid mesothelioma originating from mesothelioma in situ: are methylthioadenosine phosphorylase loss and CDKN2A homozygous deletion poor prognostic factors for preinvasive mesothelioma?

The diagnosis of mesothelioma in situ (MIS) is challenging with conventional diagnostic approaches. Although recent advances in genomic-based assays have made it possible to diagnose MIS, the prognosis, treatment indications, and prognostic factors remain unclear. Previous reports have shown that MIS progresses to invasive mesothelioma; however, to the best of our knowledge, progression to sarcomatoid mesothelioma has not yet been reported. A 73-year-old man was diagnosed with MIS associated with methylthioadenosine phosphorylase (MTAP) loss and a CDKN2A homozygous deletion. Strikingly, pathological examination revealed that the MIS lesion had progressed to sarcomatoid mesothelioma. In analyses of previously reported cases and our case, MIS with a CDKN2A homozygous deletion or MTAP loss progressed to invasive mesothelioma earlier than that without them, indicating that a CDKN2A homozygous deletion and MTAP loss could be poor prognostic factors. Genomic analyses might be useful for predicting the prognosis of MIS and contributing to an optimal treatment.

Significance of p53 immunostaining in mesothelial proliferations and correlation with TP53 mutation status

p53 immunohistochemistry has long been proposed for the separation of benign from malignant mesothelial proliferations, with the older literature suggesting that any degree of positivity supported a diagnosis of mesothelioma. However, using modern immunohistochemistry platforms in other organ systems, notably gynecologic tumors, it has become clear that p53 staining can represent wild-type protein, and only specific staining patterns (absent, overexpression, or cytoplasmic expression) are indicative of a TP53 mutation. We applied these principles to two tissue microarrays containing 94 mesotheliomas and 66 reactive mesothelial proliferations. Seven/65 (11%) epithelioid mesotheliomas showed aberrant staining (four absent and three overexpression patterns) as did 5/29 (17%) of sarcomatoid mesotheliomas (all overexpression patterns). We sequenced the TP53 gene (exons 2-11) in five of the epithelioid and three of the sarcomatoid cases with aberrant staining as well as 12 epithelioid and eight sarcomatoid mesotheliomas with wild-type staining. All three sarcomatoid cases with aberrant staining showed mutated TP53, as did three of the epithelioid cases; in two of the epithelioid cases no mutation was detected, most likely because of large deletions not detected by this assay. In contrast, none of the 20 mesotheliomas with wild-type staining contained mutated TP53. We conclude that absent or overexpression p53 staining patterns can be used as a marker of a malignant vs. a benign mesothelial proliferation. The sensitivity of p53 staining by itself is low, but here addition of p53 to BAP1/MTAP staining increased sensitivity from 72 to 81% for epithelioid and 38 to 50% for sarcomatoid mesotheliomas.

Malignant pleural mesothelioma: an update

Malignant mesotheliomas are rare types of cancers that affect the mesothelial surfaces, usually the pleura and peritoneum. They are associated with asbestos exposure, but due to a latency period of more than 30 years and difficult diagnosis, most cases are not detected until they reach advanced stages. Treatment options for this tumor type are very limited and survival ranges from 12 to 36 months. This review discusses the molecular physiopathology, current diagnosis, and latest therapeutic options for this disease.

Application of the International System for Reporting Serous Fluid Cytopathology with Cytohistological Correlation and Risk of Malignancy Assessment

The International System for Reporting Serous Fluid Cytopathology (TIS) classifies serous effusions into five categories: non-diagnostic (ND), negative for malignancy (NFM), atypia of unknown significance (AUS), suspicious for malignancy (SFM) and malignant (MAL). The main objectives of this classification comprise the establishment of a universal code of communication between cytopathologists and clinicians and histopathologists, as well as between different laboratories worldwide, paving the way for the setting of clinical management guidelines based on the risk of malignancy assessment for each diagnostic category. We retrieved the total number of pleural and peritoneal effusion cases of our department for the three-year time period between 2018 and 2020, yielding a total of 528 and 500 cases, respectively. We then proceeded to reclassify each specimen according to TIS guidelines and calculate the risk of malignancy (ROM) for each category by searching each patients’ histology records, medical history and clinical follow-up. For pleural effusions, 3 (0.57%) cases were classified as ND, 430 (81.44%) cases as NFM, 15 (2.84%) as AUS, 15 (2.84%) as SFM and 65 (12.31%) as MAL. ROM amounted to 0%, 5.3%, 33.33%, 93.33% and 100% for each category, respectively. As far as peritoneal effusions are concerned, 6 (1.2%) were categorized as ND with ROM estimated at 16.66%, 347 (69.4%) as NFM (ROM = 9%), 13 (2.6%) as AUS (ROM = 38.46%), 12 (2.4%) as SFM (ROM = 83.33%) and 122 (24.4%) as MAL (ROM = 100%). Our results underline the utility of the current classification, both as a means of communication between doctors of different specialties and as general guidelines for the further clinical management of patients.

UHRF1 Immunohistochemical Staining Separates Benign Reactive Spindle Cell Mesothelial Proliferations From Sarcomatoid Mesotheliomas

The separation of benign from malignant mesothelial proliferations is often a difficult pathologic problem. UHRF1 (ubiquitin-like with plant homeodomain and ring finger domains-1) is a regulator of DNA methylation and an epigenetic driver of various human cancers. It has recently been reported that UHRF1 is overexpressed in mesotheliomas. We asked whether UHRF1 immunohistochemistry could be used to separate benign from malignant mesothelial proliferations. Initial studies showed that UHRF1 stained mesothelial cells but also endothelial and other non-neoplastic cells, so that accurate counting of positive mesothelial cells was difficult. Therefore, we ran dual UHRF1-AE1/AE3 stains on 2 tissue microarrays containing 40 reactive mesothelial proliferations and 61 mesotheliomas and only counted UHRF1 staining in keratin-positive cells. On average 10.3±8.6% (mean±SD; range: 0% to 36, median: 6.8%) of epithelioid mesothelioma cells stained compared with 5.3±4.8% (range: 0% to 15%, median: 4.1%) of reactive epithelial mesothelial cells. This difference was statistically significant but there was too much overlap to use diagnostically. In contrast, 37±26% (range: 2.5% to 95%, median: 31%) of cells in sarcomatoid mesotheliomas compared with 1.2±1.2% (range: 0% to 3.0%, median: 1.0%) of cells in reactive spindle cell mesothelial proliferations stained. To confirm this difference we stained whole sections of 21 sarcomatoid mesotheliomas and 19 cases of organizing pleuritis. Staining of mesothelial cells was seen in 2.1±2.4% (range: 0% to 6.8%, median: 1.0%) of organizing pleuritis cases and 44±22% (range: 14% to 90%, median: 41%) of sarcomatoid mesotheliomas. We conclude that dual UHRF1-AE1/AE3 immunohistochemistry is very useful for separating benign spindle cell mesothelial proliferations from sarcomatoid mesotheliomas.

Deep-learning based classification distinguishes sarcomatoid malignant mesotheliomas from benign spindle cell mesothelial proliferations

Sarcomatoid mesothelioma is an aggressive malignancy that can be challenging to distinguish from benign spindle cell mesothelial proliferations based on biopsy, and this distinction is crucial to patient treatment and prognosis. A novel deep learning based classifier may be able to aid pathologists in making this critical diagnostic distinction. SpindleMesoNET was trained on cases of malignant sarcomatoid mesothelioma and benign spindle cell mesothelial proliferations. Performance was assessed through cross-validation on the training set, on an independent set of challenging cases referred for expert opinion ('referral' test set), and on an externally stained set from outside institutions ('externally stained' test set). SpindleMesoNET predicted the benign or malignant status of cases with AUC's of 0.932, 0.925, and 0.989 on the cross-validation, referral and external test sets, respectively. The accuracy of SpindleMesoNET on the referral set cases (92.5%) was comparable to the average accuracy of 3 experienced pathologists on the same slide set (91.7%). We conclude that SpindleMesoNET can accurately distinguish sarcomatoid mesothelioma from benign spindle cell mesothelial proliferations. A deep learning system of this type holds potential for future use as an ancillary test in diagnostic pathology.

Evidence-based diagnostic performance of novel biomarkers for the diagnosis of malignant mesothelioma in effusion cytology

Cytology effusions are often the only material available for diagnosing malignant pleural mesothelioma (MPM). However, the cytomorphological features alone are not always diagnostic, and cytology samples preclude an assessment for pleural tissue invasion. Accordingly, immunohistochemical, soluble, and molecular biomarkers have been developed. The aim of this study is to provide quantitative evidence regarding the diagnostic performance of novel biomarkers. To that end, a systematic literature review was performed of articles dealing with a loss of BRCA1-associated protein 1 (BAP1), methylthioadenosine (MTAP), 5-hydroxymethylcitosine (5-hmC), glucose transporter 1 (GLUT1), insulin like-growth factor II messenger RNA-binding protein 3 (IMP3), enhanced zeste homologue 2 (EZH2) staining, cyclin-dependent kinase inhibitor 2A (CDKN2A) homozygous deletion (HD) testing, soluble mesothelin, and microRNA quantification in cytological samples for the diagnosis of MPM versus reactive atypical mesothelial cells. Sensitivity and specificity were extracted, and a meta-analysis was performed. The quality of the studies was assessed with Quality Assessment of Diagnostic Accuracy Studies 2, and the quality of the evidence was evaluated with the Grading of Recommendations Assessment, Development, and Evaluation approach. Seventy-one studies were included. BAP1 loss showed a sensitivity of 0.65 (confidence interval [CI], 0.59-0.71) and a specificity of 0.99 (CI, 0.93-1.00). MTAP loss and p16 HD showed 100% specificity with sensitivities of 0.47 (CI, 0.38-0.57) and 0.62 (CI, 0.53-0.71), respectively. BAP1 loss and CDKN2A HD combined showed maximal specificity and a sensitivity of 0.83 (CI, 0.78-0.89). GLUT1 and IMP3 showed sensitivities of 0.82 (CI, 0.70-0.90) and 0.65 (CI, 0.41-0.90), respectively, with comparable specificity. Mesothelin showed a sensitivity of 0.73 (CI, 0.68-0.77) and a specificity of 0.90 (CI, 0.84-0.93). In conclusion, some of the recently emerging biomarkers are close to 1.00 specificity. Their moderate sensitivity on their own, however, can be significantly improved by the use of 2 biomarkers, such as a combination of BAP1 and CDKN2A with fluorescence in situ hybridization or a combination of BAP1 and MTAP immunohistochemistry.

DNA Methylation Profiling Discriminates between Malignant Pleural Mesothelioma and Neoplastic or Reactive Histologic Mimics

The diagnosis of malignant pleural mesothelioma (MPM) is challenging because of its potential overlap with other neoplasms or even with reactive conditions. DNA methylation analysis is effective in diagnosing tumors. In the present study, this approach was tested for use in MPM diagnosis. The DNA methylation patterns of a discovery cohort and an independent-validation cohort of MPMs were compared to those of 202 cases representing malignant and benign diagnostic mimics (angiosarcoma, desmoid-type fibromatosis, epithelioid sarcoma, leiomyosarcoma, lung adenocarcinoma, lung squamous cell carcinoma, melanoma, nodular fasciitis, reactive mesothelial hyperplasia, sclerosing fibrous pleuritis, solitary fibrous tumor, and synovial sarcoma). By both unsupervised hierarchical clustering and t-distributed stochastic neighbor embedding analysis, MPM samples in the discovery cohort exhibited a DNA methylation profile different from those of other neoplastic and reactive mimics. These results were confirmed in the independent validation cohort and by in silico analysis of the MPM-The Cancer Genome Atlas data set. Copy number variation profiles were also inferred to identify molecular hallmarks of MPM, including CDKN2A and NF2 deletions. Methylation profiling was effective in the diagnosis of MPM, although caution is advised in samples with low tumor cell content.

CD146 immunohistochemical staining for the separation of benign from malignant mesothelial proliferations

The separation of benign from malignant mesothelial cells is often a challenging problem. Some studies have suggested that immunohistochemical staining of CD146 can be used to make this distinction, but there are marked differences in the reported results. Here, we assessed CD146 expression in tissue microarray specimens of 32 epithelioid reactive mesothelial hyperplasias, 17 spindle cell reactive mesothelial proliferations, 43 epithelioid mesotheliomas, and 31 sarcomatoid mesotheliomas. We found that, although the specificity of CD146 for epithelioid mesotheliomas versus reactive epithelial mesothelial proliferations was high (94%), staining intensity and extent was usually low and sensitivity was poor (23%). For sarcomatoid mesotheliomas versus reactive spindle cell mesothelial processes, both measures (33% sensitivity, 76% specificity) were inadequate. Furthermore, strong staining of endothelial cells and fibroblasts often created difficulties in interpretation. In comparison, BAP1 was lost in 21/43 (49%) epithelioid and 9/31 (29%) sarcomatoid mesotheliomas and methylthioadenosine phosphorylase (MTAP) was lost in 9/40 (23%) epithelioid and 7/29 (24%) sarcomatoid mesotheliomas from these TMAs. There was no association between CD146 staining and BAP1 or MTAP retention/loss. We conclude that CD146 staining is probably not useful for separating malignant from benign mesothelial proliferations.

Serous fluid cytopathology: Past, present, and future

Serous fluids/effusions are common cytology samples that reflect a wide range of diseases and lend themselves to a multitude of investigations that include microscopy, chemical analysis, cell count, cultures, and analysis for biomarkers and immunomarkers. In recent years, effusions have also served as a liquid biopsy that can be interrogated by molecular tests for thoranostic and prognostic markers and selection of targeted therapy. The recently published International Reporting System for Serous Fluid Cytopathology (IRSSF) provides a standardized reporting terminology with well- defined diagnostic criteria. This editorial provides a global review of the progress in the work-up of effusions and a summary of the IRSSF book and its most significant contributions. The editorial also includes a summary of the diagnostic categories including their definition and the significant relevant information.

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.

c-MET immunohistochemistry for differentiating malignant mesothelioma from benign mesothelial proliferations

The separation of benign from malignant mesothelial proliferations can be a difficult problem for the surgical pathologist. c-MET is a receptor tyrosine kinase that is overexpressed and detectable by immunohistochemistry in many malignancies, including malignant mesothelioma. Whether c-MET is also expressed in benign mesothelial reactions is unclear from the literature. To determine whether c-MET immunohistochemistry can separate benign from malignant mesothelial processes, we stained 2 tissue microarrays containing 33 reactive epithelioid mesothelial proliferations (E-RMPs), 23 reactive spindle cell mesothelial proliferations, 45 epithelioid malignant mesotheliomas (EMMs), and 26 sarcomatoid/desmoplastic mesotheliomas (SMMs) for c-MET and compared the results with immunohistochemistry for two established markers, BAP1 and methylthioadenosine phosphorylase (MTAP). Membrane staining for c-MET was evaluated using a 12-point H-score classified as negative (score = 0), trace (score = 1-3), moderate (score = 4-6), and strong (score = 8-12). Staining was seen in only 3 of 33 (all trace) E-RMPs compared with 36 of 45 (80%) EMMs (chi-square comparing reactive and malignant = 39.80, p = 1.2 × 10^-8). The H-score was >3 (moderate or strong) in 24 of 45 (53%) EMMs. Addition of BAP1 staining to the c-MET-negative/trace EMM increased sensitivity to 75% (32/42), whereas similar addition of MTAP staining increased sensitivity to 77% (33/43). No benign spindle cell proliferations showed staining compared with 10 of 26 (38%) positive SMMs, but only 4 (15%) SMMs were classified as moderate or strong. We conclude that moderate/strong c-MET staining can be used to support a diagnosis of EMM vs an epithelial reactive proliferation. c-MET is too insensitive to use for detecting SMM.