Loss of expression of BAP1 is a useful adjunct, which strongly supports the diagnosis of mesothelioma in effusion cytology

Role of Immunocytochemistry in the Cytological Diagnosis of Mesothelioma

BACKGROUND

Mesothelioma is an aggressive malignancy of the serosal surfaces with very poor prognosis. It traditionally manifests in older patients and at an advanced stage which results in minimal improvement in prognosis despite the recent advances in management. Early detection would therefore significantly impact management and potentially improve survival. Mesothelioma frequently presents with recurrent effusions, posing cytology as the initial procedure in the workup. A definitive diagnosis would not only spare the patients additional diagnostic procedures but also potentially afford them an opportunity for early surgical intervention and therapy.

SUMMARY

In this article, we review the role of immunocytochemistry (ICC) in the workup of mesothelioma. The various ICC markers to confirm or rule out mesothelial lineage are reviewed. In addition, newly introduced molecular surrogates that confirm the malignant nature of the mesothelial cells and support a definitive diagnosis of mesothelioma are discussed. We also briefly discuss the theranostic implications of such markers and potential impact of such recent advances on the cytological diagnosis and reporting of mesothelioma.

KEY MESSAGES

The cytological diagnosis of mesothelioma no longer requires the extensive expertise in morphological analysis and can be offered based on supporting ICC that confirms the mesothelial lineage and malignant nature of the cells.

Cytologic diagnosis and differential diagnosis of histiocytic signet ring cells in effusion specimens

Objective

Benign histiocytic proliferation in effusion specimens can be found in a variety of diseases such as rheumatoid arthritis, systemic lupus erythematosus, microorganism infections, trauma, reactive eosinophilic pleuritis, and others. In addition, nodular histiocytic/mesothelial hyperplasia is another well-recognized rare cause. The previous studies have shown that proliferative histiocytes have raisinoid nuclei and abundant eosinophilic granular cytoplasm and can be confused with malignant lesions, especially metastatic carcinomas. In this study, we evaluated the cytomorphology of benign histiocytes, discussed the diagnosis and differential diagnosis, and the clinical significance of histiocytic signet ring cells in effusion cytology.

Material and Methods

Seven hundred and fifty-five benign effusion cases (433 pleural effusions and 322 abdominal fluids) were found over 1 year. Among benign cases, 35 cases (28 pleural effusions and seven abdominal fluids) were included with findings of dominantly histiocytic signet ring cell morphology as well as immunohistochemical (IHC) stains. The clinical findings were also correlated.

Results

In contrast to the well-documented cytomorphology of raisinoid nuclei and eosinophilic cytoplasm of proliferative histiocytes in previous studies, we find that these cells predominately presented as signet ring cell morphology with clear cytoplasm. The most characteristic findings of benign histiocytes in pleural effusions are: (1) cells are arranged in sheets and/or scattered individual cells, but no two- or three-dimensional cell clusters; (2) cells are intermediate in size and with normal N/C ratio; (3) cells have eccentric located nuclei and abundant clear cytoplasm, giving signet ring cell appearance; (4) nuclei have fine granular chromatin pattern, no hyperchromia or coarse chromatin pattern, no nuclear atypia; and (5) immunohistochemical (IHC) stains demonstrate a strongly positivity for macrophage-histiocyte lineage marker CD68, but negativity for epithelial markers and mesothelial markers. Clinically, these patients do not demonstrate nodularity or lesions in the mesothelial lining of serous cavities.

Conclusion

Our study provides a detailed characterization of benign histiocytic signet ring cells in effusion cytology. The differential diagnosis of histiocytic signet ring cells is broad. The most important differential diagnoses are metastatic adenocarcinoma and epithelioid signet ring cell mesothelioma. The accurate diagnosis is critical for the appropriate clinical management of the patient. Cytopathologists should be aware of the diagnostic pitfalls of benign histiocytic signet ring cells in effusion samples in daily practice.

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.

The World Health Organization Reporting System for Lung Cytopathology-A Review of the First Edition

The World Health Organization Reporting System for Lung Cytopathology is the first international system that was developed to standardize the reporting of lung cytopathology specimens across all settings of cytopathology practice. The system is composed of five diagnostic categories, which apply to all lung cytopathology specimen types. Each category contains cytomorphologic criteria, an estimated risk of malignancy, and clinical management recommendations. International uniformity in the reporting of lung cytopathology will refine the communication between cytopathologists and clinicians and ultimately improve patient care. Furthermore, standardizing the cytomorphologic criteria for each lesion will improve reproducibility among cytopathologists and highlight areas in lung cytopathology that require further research. The system also provides best practice recommendations for the selection of ancillary tests to aid in the diagnosis of each lesion, or group of lesions, keeping in mind that resources will vary across different practice settings. The goal of this review is to summarize the cytomorphologic criteria, potential diagnostic pitfalls, ancillary testing, estimated risk of malignancy, and clinical management recommendations for each diagnostic category.

Reliability of assessing morphologic features with prognostic significance in cytology specimens of epithelioid diffuse pleural mesothelioma and implications for cytopathology reporting

BACKGROUND

The World Health Organization incorporates morphologic features with prognostic significance in the 2021 classification of epithelioid diffuse pleural mesothelioma (E-DPM). Although cytology specimens are often the first and occasionally the only specimen available for patients with DPM, these features have not yet been investigated in cytology.

METHODS

Nuclear atypia, pleomorphic features, necrosis, and architectural patterns were retrospectively assessed in 35 paired cytology and concurrent/consecutive surgical pathology specimens of E-DPM. Agreement between pairs was determined via unweighted κ scores. Discordant cases were re-reviewed to determine the reasons for disagreement.

RESULTS

Interpretation of nuclear atypia in cytology was concordant with histology in all cases (κ = 1.000; p < .001). The presence of pleomorphic features and necrosis was concordant in 97.1% (κ = 0.842; p < .001) and 85.7% (κ = 0.481; p = .001) of paired cases, respectively. Assessment of architectural patterns in cytology showed only slight agreement with histology (κ = 0.127; p = .037). In cytology cases (n = 23) with cell block material available, assessment of nuclear atypia and the presence of pleomorphic features showed perfect agreement (κ = 1.000; p < .001, each), the presence of necrosis showed moderate agreement (κ = 0.465; p = .008), and assessment of architectural patterns showed slight agreement (κ = 0.162; p = .15) in paired specimens. Most disagreements were due to sampling differences between cytology and histology specimens.

CONCLUSIONS

Although complete nuclear grading of E-DPM is not possible given the unreliability of mitotic counts in cytology, assessment of nuclear atypia in cytology specimens is shown to be reliable. Identification of pleomorphic features and necrosis is also reliable despite occasional sampling issues. Assessment of architectural patterns is more limited in cytology.

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.

A novel prognostic nomogram for predicting survival in diffuse pleural mesothelioma

Recent advances in the management of diffuse pleural mesothelioma (DPM) have increased interest in prognostication and risk stratification on the basis that maximum benefit of combination immunotherapy appears to be seen in patients who otherwise would have the worst prognosis. Various grading schemes have been proposed, including the recently published Mesothelioma Weighted Grading Scheme (MWGS). However, predictive modelling using deep learning algorithms is increasingly regarded as the gold standard in prognostication. We therefore sought to develop and validate a prognostic nomogram for DPM. Data from 369 consecutive patients with DPM were used as independent training and validation cohorts to develop a prognostic tool that included the following variables: age, sex, histological type, nuclear atypia, mitotic count, necrosis, and BAP1 immunohistochemistry. Patients were stratified into four risk groups to assess model discrimination and calibration. To assess discrimination, the area-under-the-curve (AUC) of a receiver-operator-curve (ROC), concordance-index (C-index), and dissimilarity index (D-index) were calculated. Based on the 5-year ROC analysis, the AUC for our model was 0.75. Our model had a C-index of 0.67 (95% CI 0.53-0.79) and a D-index of 2.40 (95% CI 1.69-3.43). Our prognostic nomogram for DPM is the first of its kind, incorporates well established prognostic markers, and demonstrates excellent predictive capability. As these factors are routinely assessed in most pathology laboratories, it is hoped that this model will help inform prognostication and difficult management decisions, such as patient selection for novel therapies. This nomogram is now freely available online at: https://nomograms.shinyapps.io/Meso_Cox_ML/.

BAP1 loss induces mitotic defects in mesothelioma cells through BRCA1-dependent and independent mechanisms

The tumour suppressor BRCA1-associated protein 1 (BAP1) is the most frequently mutated cancer gene in mesothelioma. Here we report novel functions for BAP1 in mitotic progression highlighting the relationship between BAP1 and control of genome stability in mesothelioma cells with therapeutic implications. Depletion of BAP1 protein induced proteasome-mediated degradation of BRCA1 in mesothelioma cells while loss of BAP1 correlated with BRCA1 loss in mesothelioma patient tumour samples. BAP1 loss also led to mitotic defects that phenocopied the loss of BRCA1 including spindle assembly checkpoint failure, centrosome amplification and chromosome segregation errors. However, loss of BAP1 also led to additional mitotic changes that were not observed upon BRCA1 loss, including an increase in spindle length and enhanced growth of astral microtubules. Intriguingly, these consequences could be explained by loss of expression of the KIF18A and KIF18B kinesin motors that occurred upon depletion of BAP1 but not BRCA1, as spindle and astral microtubule defects were rescued by re-expression of KIF18A and KIF18B, respectively. We therefore propose that BAP1 inactivation causes mitotic defects through BRCA1-dependent and independent mechanisms revealing novel routes by which mesothelioma cells lacking BAP1 may acquire genome instability and exhibit altered responses to microtubule-targeted agents.

Adenomatoid tumors of ovary mimicking malignancy: report of 2 cases and literature review

BACKGROUND

Adenomatoid tumors (ATs) are benign tumors originating from the mesothelium. ATs of the ovary are rare, and can easily be confused with malignancy due to the histomorphological diversity. Thus, it is difficult in histopathological and differential diagnosis, especially during intraoperative frozen pathological diagnosis, which directly affects the resection scope of surgery.

CASE PRESENTATION

In this study, we reported two patients (58 and 41 year old) with ovarian ATs. AT of patient 1 occurred in both ovaries at different time points and she had been diagnosed with Hashimoto's thyroiditis. AT of patient 2 occurred in right ovary. Intraoperative frozen pathological diagnosis was performed in both cases and laparoscopic salpingo-oophorectomy was undergone on the lesion side according to benign freezing diagnostic result. Ovarian ATs, the final diagnoses of the 2 cases were concluded after histological, extensive immunohistochemical (IHC), histochemical, and fluorescence in situ hybridization analyses.

CONCLUSIONS

Our results show that ovarian ATs may not be related to BAP1 or CDKN2A/p16 mutations. In addition, the case 1 suggests that ATs may be associated with immune dysregulation. When encountering such similar lessions, we recommend that a series of immunohistochemical, histochemical and molecular biological techniques should be used for diagnosis and differential diagnosis to avoid misdiagnosis. Improving understanding of the rare ovarian ATs which mimic malignancy is necessary to prevent overresection.

Incidence and Risk Factors of Chest Wall Metastasis at Biopsy Sites in Patients with Malignant Pleural Mesothelioma

To investigate the incidence and risk factors of chest wall metastasis (CWM) at biopsy sites in patients with malignant pleural mesothelioma (MPM). This retrospective cohort study was conducted in 262 consecutive MPM patients who underwent multimodal treatment in which including neoadjuvant chemotherapy (NAC) and curative-intent surgery, from August 2009 to March 2021. CWM was evaluated radiologically (r-CWM) and pathologically (p-CWM). We also investigated the risk factors of p-CWM and the consistency between r-CWM and p-CWM. Of 262 patients, 25 patients were excluded from analysis due to missing data or impossibility of evaluation. Of the eligible 237 patients, pleural biopsy was performed via video-assisted thoracoscopic surgery in 197 (83.1%) and medical thoracoscopy in 40 (16.9%). Pleurodesis was performed after pleural biopsy in 74 patients (31.2%). All patients received NAC followed by curative-intent surgery. Radiological examination showed r-CWM in 43 patients (18.1%), while pathological examination showed p-CWM in 135 patients (57.0%). The incidence of p-CWM was significantly higher in the patients who received pleurodesis after pleural biopsy (77.0% vs. 47.9%, <0.001). Multivariate logistic regression analysis for p-CWM revealed that pleurodesis is an independent risk factor of p-CWM (adjusted hazard ratio, 3.46; 95% confidence interval, 1.84−6.52, <0.001). CWM at the biopsy site was pathologically proven in more than half of the patients (57.0%) who received NAC followed by curative-intent surgery, which was higher than the numbers diagnosed by radiological examinations (p-CWM: 57.0% vs. r-CWM: 18.1%). Pleurodesis after pleural biopsy is an independent risk factor of p-CWM.

Diagnostic capacity of BAP1 and MTAP in cytology from effusions and biopsy in mesothelioma

INTRODUCTION

Serous effusion is often the first sign of mesothelioma. Diagnosis based on cytologic material from the effusions remains controversial and complementary biopsy is usually required. However, obtaining representative tissue sample may be challenging, while obtaining cytologic material is a minimally invasive procedure, providing potential for an earlier diagnosis. Loss of BRCA1-associated protein (BAP1), combined with loss of methylthionadenosine phosphorylase (MTAP) detected by immunohistochemistry, have shown to be reliable markers in the diagnosis of mesothelioma on histologic sections. Here we evaluate the value of these biomarkers in cytologic specimens.

MATERIALS AND METHODS

The BAP1 and MTAP expression in specimens of 162 mesothelioma patients (156 pleural, 6 peritoneal)-71 cytologic, 91 histologic (44 epithelioid, 31 biphasic, 16 sarcomatoid)-and 20 patients with reactive mesothelial proliferations were investigated.

RESULTS

The loss of BAP1 and/or MTAP was highly sensitive and specific in differentiating mesothelioma from reactive mesothelial proliferations, with no significant difference between pleural effusions and biopsies, specificity of 100% in both and a sensitivity of 78.9% and 80.2%, respectively (P = 0.3). There was a 100% concordance of the expression of BAP1 and MTAP in cytologic and corresponding histopathologic samples. Loss of BAP1 and/or MTAP in histologic sections discriminated sarcomatoid, biphasic, and epithelioid mesothelioma from reactive mesothelial proliferations with a sensitivity of 81.2%, 83.9%, and 77.3% respectively.

CONCLUSION

Loss of expression of BAP1 and/or MTAP differentiated mesothelioma from reactive mesothelial proliferations with excellent specificity and high sensitivity in cytologic samples, comparable to histopathologic sections.

Evaluation of DNA/RNA quality from cell block of malignant mesothelioma and lung adenocarcinoma

Malignant mesothelioma (MM) is a rare and highly lethal tumor that arises from mesothelial tissue on the surface of the chest and abdominal cavity. Cytological examination of body fluids, including pleural fluid and ascites, is essential for the differentiation of malignant mesothelioma from other carcinomas, such as lung and gastrointestinal carcinomas and metastatic tumors. To evaluate the effectiveness of cell block preparation procedures, which are used for immunocytochemical staining and genetic panel analysis of tumor-specific gene mutations, we used various fixatives. We also evaluated the effects of immunostaining, and the quality of nucleic acids for genetic analysis.

METHODS

Cell blocks were prepared using the malignant mesothelioma cell lines MESO4 and H226 and non-small cell lung carcinoma cell line HCC78. The cells were fixed using 10% neutral buffered formalin and four different fixatives for liquid cytology. Fixed cells were formed into cell clusters using sodium alginate or centrifugation, and paraffin-embedded cell blocks were prepared.

RESULTS

Cell blocks were morphologically evaluated by hematoxylin and eosin and immunocytological staining, and the nucleic acid quality was evaluated by DNA/RNA extraction, qPCR, and next-generation sequence analysis. D2-40 and WT1 staining differed depending on the fixation solution and the cell cluster formation method; however, the degree of nucleic acid degradation was not impaired by any method.

CONCLUSION

Although the morphological evaluation of cytology specimens is affected by the method of cell block preparation, it is still useful for nucleic acid extraction and gene panel analysis, as long as there are sufficient amounts of tumor cells.

Somatic Mutation of BAP1 Can Lead to Expression Loss in Non-Small Cell Lung Carcinoma: Next Generation Sequencing and IHC Analysis in A Large Single Institute Cohort

Introduction. As a tumor suppressor, germline and somatic inactivation of BRCA1 associated protein 1 gene (BAP1) is a common finding in mesothelioma, melanocytic tumors, clear cell renal cell carcinoma and several other epithelial, mesenchymal and neural tumors. Incidence of BAP1 genetic alterations and subsequent expression loss has not been well established in non-small cell lung carcinoma (NSCLC) by large-scale studies. Design. After IRB approval, a total of 356 NSCLC cases of our institution between July 2016 and June 2020 were reviewed. The study cohort consisted of 214 (60%) adenocarcinomas, 89 (25%) squamous cell carcinomas, and 53 (15%) diagnosed as "non-small cell lung carcinoma" without specified subtype. All tumors underwent comprehensive target cancer gene next generation sequencing (Oncomine Assay). The protein expression status of BAP1 was subsequently evaluated by immunohistochemistry. Results. BAP1 somatic mutations were detected in 8 NSCLC tumors (incidence: 2.2%). Tumors harboring BAP1 mutations were all diagnosed at advanced stage and carried at least one additional genetic alteration. Immunohistochemically, four tumors showed complete loss of BAP1 protein expression, including two adenocarcinomas which harbored different missense BAP1 mutations and another two with bioinformatically predicated deleterious frameshifting mutations. Conclusion. Compared with known BAP1 loss associated other malignancies, such as mesothelioma, inactivation of BAP1 by somatic mutation is a rare occurrence in NSCLC. BAP1 mutations and loss of expression in NSCLC are accompanied by other complex genetic alternations, suggesting BAP1 mutation maybe a late event NSCLC carcinogenesis.

A Critical Assessment of Current Grading Schemes for Diffuse Pleural Mesothelioma With a Proposal for a Novel Mesothelioma Weighted Grading Scheme (MWGS)

Although there is early support for schemes based on nuclear grade, necrosis and mitotic rate, there is currently no widely implemented grading system for diffuse pleural mesothelioma (DPM). We investigated current systems and propose a novel Mesothelioma Weighted Grading Scheme (MWGS). The MWGS assigns weighted scores from 0 to 10 based on age (≤74, >74 yrs: 0,1); histologic type (epithelioid, biphasic, sarcomatoid: 0,1,2); necrosis (absent, present: 0,2); mitotic count per 2 mm2 (≤1, 2 to 4, ≥5: 0,1,2); nuclear atypia (mild, moderate, severe: 0,1,2); and BRCA1-associated protein 1 (BAP1) expression (lost, retained: 0,1). A score of 0 to 3 is low grade, 4 to 6 intermediate grade, and 7 to 10 high grade. In 369 consecutive DPMs, median survival was 17.1, 10.1, and 4.1 months for low, intermediate, and high grades (P<0.0001). A progressive increase in score correlated with worsening overall survival (P<0.0001). Interobserver concordance was substantial (κ=0.588), with assessment of nuclear grade being the most subjective parameter (κ=0.195). We compared the MWGS to the 2-tiered system discussed in the World Health Organization (WHO) fifth edition. The WHO system predicted median survival in epithelioid (median 18.0 vs. 11.3 mo, P=0.003) and biphasic (16.2 vs. 4.2 mo, P=0.002), but not sarcomatoid DPM (5.4 vs. 4.7 mo, P=0.407). Interestingly, the WHO grading system was prognostic in cases with BAP1 loss (median survival 18.7 vs. 10.4 mo, P<0.0001), but not retained BAP1 expression (8.9 vs. 6.2 mo, P=0.061). In conclusion, the WHO scheme has merit in epithelioid/biphasic and BAP1-deficient DPM, however, the MWGS can be used for risk stratification of all DPMs, regardless of histologic subtype and BAP1 status.

State-of-the-art cytology of pleural fluid, focusing on the diagnosis of mesothelioma

Mesothelioma has always been a challenging diagnosis to render in body cavity cytology samples. This review is a timely update on pleural fluid cytology and ancillary studies that should be considered in the diagnosis of mesothelial proliferations, specifically mesotheliomas. Information about new diagnostic approaches and ancillary studies in mesothelioma was obtained from the peer-reviewed literature and the authors' experiences. Although the morphological diagnosis of mesothelioma is fraught with numerous challenges given the overlap with other diagnostic entities, there are a variety of immunohistochemical and fluorescence in situ hybridization studies available to help in determining mesothelial origin and in distinguishing malignant proliferations from the more common benign or reactive mesothelial proliferations. Although ancillary studies can be helpful, there are important pitfalls to be aware of when interpreting these cases, and this review highlights some of the challenges that require caution.

Diagnostic utility of BAP1 for malignant pleural mesothelioma in pleural fluid specimens with atypical morphology

OBJECTIVE

To assess the utility of BRCA1-associated protein 1 (BAP1) immunohistochemistry (IHC) for the diagnosis of malignant pleural mesothelioma (MPM) in fluid samples with atypical cytology.

METHODS

Pleural fluid samples with an atypical mesothelial proliferation (diagnostic categories: 'atypical' and 'suspicious') received between January 2015 and March 2018 at a tertiary referral centre were identified. Results of routine IHC testing were recorded for each case. BAP1 by IHC was performed and a final diagnosis sought from subsequent pathology specimens, medical records, or consensus clinical diagnosis.

RESULTS

Of 50 cases identified, 41 were reported as atypical and 9 as suspicious. Seven (14%) demonstrated loss of BAP1 staining, 40 retained BAP1 staining, 1 had heterogeneous staining, and 2 had insufficient cells for analysis. All seven cases with BAP1 loss were diagnosed with MPM on follow-up. Of those with retained BAP1, 52.5% (21) were subsequently diagnosed with MPM, while 40% (16) had non-MPM diagnoses after a median follow-up of 24 months. Three cases were not further investigated based on patient and clinician decision. The case with heterogeneous staining was diagnosed as mesothelioma by clinical consensus.

CONCLUSIONS

BAP1 IHC loss is highly specific for malignancy and has value as a rule-in test. Even in a tertiary centre with clinical interest in the cytological diagnosis of MPM this investigation was able to increase diagnostic accuracy beyond routine IHC studies. Cytological criteria remain valuable, as retained BAP1 in an atypical or suspicious mesothelial proliferation cannot exclude malignancy.

Primary Ovarian Mesothelioma: A Case Series with Electron Microscopy Examination and Review of the Literature

Primary ovarian mesothelioma is a rare, aggressive neoplastic disease with a poor prognosis. At onset, the tumor is only rarely limited to the ovaries and usually already widespread in the peritoneum. The rarity of this entity and the difficulties differentiating it from either ovarian carcinoma or peritoneal mesothelioma may lead to frequent misdiagnoses and may raise some concerns about its histogenesis. Thus, reporting such rare cases is fundamental to gain greater awareness of this neoplasm and try to answer unsolved questions. Herein, we described four cases of histological diagnoses of ovarian mesothelioma extrapolated by the regional mesothelioma register of Apulia (southern Italy). In all cases, a detailed medical history was collected according to national mesothelioma register guidelines. A broad panel of antibodies was used for immunohistochemistry to confirm the diagnoses. Moreover, ovarian tissue samples were also examined by transmission and scanning electron microscopy, detecting asbestos fibers and talc crystals in two cases. Because of the few cases described, we reviewed the English literature in the Medline database, focusing on articles about ovarian mesothelioma "misclassification", "misdiagnosis", "diagnostic challenge" or "diagnostic pitfall" and on unsolved questions about its histogenesis and possible risk factors.

Immunochemistry in the work-up of mesothelioma and its differential diagnosis and mimickers

The differential diagnosis in cellular effusions with cytological atypia often includes malignant mesothelioma (MM), reactive mesothelial proliferation, and malignancies of metastatic origin, particularly carcinomas. The International Reporting System for Serous Fluid recently established guidelines for reporting MM. In conjunction with the cytomorphologic evaluation, the role of immunochemistry (IC) was emphasized as a very useful tool in the workup of serous fluids, especially with the availability of novel markers. Utilizing a panel of markers, IC allows the characterization of the cells, whether mesothelial or not, and when mesothelial origin is established, IC can frequently assist in delineating its benign or malignant nature. IC can also confirm metastatic disease, allowing the identification of the primary origin in most cases. This review summarizes the current status of IC and its role in the diagnosis of MM and its differential diagnosis in serous fluids.

Fluorescence in situ hybridization detection of chromosome 22 monosomy in pleural effusion cytology for the diagnosis of mesothelioma

BACKGROUND

Malignant pleural mesothelioma (MPM) is characterized by mutations in several genes, including cyclin-dependent kinase-inhibitor 2A/p16 in the 9p21 locus, BRCA1-associated protein 1 (BAP1), and neurofibromatosis type 2 (NF2) in the 22q12 locus. Recent studies indicate that fluorescence in situ hybridization (FISH) detects hemizygous loss of NF2 in tissue specimens of MPM. The authors investigated whether NF2 FISH, either alone or in combination with other diagnostic assays (9p21 FISH, methylthioadenosine phosphorylase [MTAP] immunohistochemistry [IHC], and BAP1 IHC), effectively distinguishes MPM cells from reactive mesothelial cells (RMCs) in cell blocks prepared from pleural effusions.

METHODS

FISH assays were used to examine the deletion status of NF2 and 9p21, and IHC was used to determine the expression of MTAP and BAP1 in cell blocks from 54 cases with MPM and 18 cases with RMCs.

RESULTS

Hemizygous NF2 loss (chromosome 22 monosomy or hemizygous deletion) showed 51.9% sensitivity (48.1% for chromosome 22 monosomy and 3.7% for hemizygous deletion) and 100% specificity in differentiating MPM cells from RMCs. Combinations of NF2 FISH, 9p21 FISH, and BAP1 IHC assays yielded greater sensitivity (98.1%) than any assay alone (9p21 FISH, 61.1%; MTAP IHC, 52.8%; or BAP1 IHC, 60.4%). The level of hemizygous NF2 loss in cell blocks positively correlated with that in corresponding tissues. Furthermore, to overcome cytologic specimen-specific challenges, FISH combined with cytokeratin AE1/AE3 immunofluorescence was necessary in 25.9% of MPM cases for FISH assessment of predominantly scattered MPM cells.

CONCLUSIONS

NF2 FISH alone or in combination with other diagnostic assays effectively differentiates MPM cells from RMCs in cell blocks prepared from pleural effusions.

Pleural mesothelioma classification update

The 2015 WHO classification of pleural mesotheliomas includes three major histologic subtypes-epithelioid, sarcomatoid, and biphasic. Recent genomic data has supported the need for a more granular and clinically valid classification beyond the three current subtypes. Because of tumor rarity and overlapping histologic features with other tumor types, diagnostic immunohistochemical work up is essential component in establishing the final diagnosis of mesothelioma. The use of BAP1 and CDKN2A/MTAP improves the diagnostic sensitivity of effusion specimens and are valuable in establishing the diagnosis of epithelioid mesothelioma. The major change in the forthcoming WHO classification is the inclusion of mesothelioma in situ as a diagnostic category. In this review, we discuss recently proposed changes in the histologic classification of pleural mesothelioma, differential diagnosis, and importance of ancillary diagnostic studies.

Large-scale analysis of BAP1 expression reveals novel associations with clinical and molecular features of malignant pleural mesothelioma

BRCA1-associated protein-1 (BAP1) expression is commonly lost in several tumors including malignant pleural mesothelioma (MPM). Presence or absence of immunohistochemical BAP1 nuclear staining in tumor cells is currently used for differential diagnosis of MPM. In this study, a large cohort of 596 MPM tumors with available clinical data was analyzed to examine associations of BAP1 staining pattern with clinical and molecular features that may reflect the impact of BAP1 mutation on MPM biology. Cases were classified according to the BAP1 staining pattern of tumor cells. Exome and RNA-sequencing data were available for subsets of cases. Levels of mRNA encoding claudin 15 (CLDN15) and vimentin (VIM) were determined using RT-qPCR on 483 cases to estimate the relative proportions of epithelial-like and mesenchymal-like components in each tumor. Four BAP1 staining patterns were observed: single-pattern nuclear staining (36%), single-pattern cytoplasmic staining (25%), single-pattern absent staining (12%), and combinations of these staining patterns (27%). This study confirmed prior reports that nuclear BAP1 is more frequently associated with wild-type BAP1 and sarcomatoid histology. However, no associations between BAP1 staining pattern(s) and mutations in specific protein domains and/or mutation type were observed. BAP1 staining patterns were significantly associated (p < 0.001) with BAP1 gene expression, MPM histologic subtypes, molecular clusters, and markers of epithelial-to-mesenchymal transition. Frequent observation of combinations of BAP1 staining patterns in MPM tumors indicated intra-tumoral heterogeneity of BAP1 status. Cytoplasmic BAP1 staining was identified as a putative indicator of favorable prognosis in non-epithelioid MPM. In conclusion, novel significant associations among different BAP1 staining patterns and subgroups of MPM tumors were observed, suggesting that the role of BAP1 in tumor progression may be more complex than its presumed tumor suppressor function. Cytoplasmic staining was identified as a putative indicator of favorable prognosis in non-epithelioid MPM, potentially addressing a critical need in clinical decision-making in this disease. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

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

The separation of benign from malignant mesothelial proliferations is an important clinical but often a difficult morphologic problem. Over the last roughly 10 years a variety of new markers that aid in this separation have been published and some older recommended markers reconsidered. Unlike previous, and largely unusable, empiric immunohistochemical (IHC) stains, these new markers, some using IHC and some using fluourescent in situ hybridization (FISH), are largely based on documented genomic abnormalities in malignant mesotheliomas. However, no marker works in all situations; rather, markers need to be chosen by the morphology of the process in question (epithelial vs. spindled) and the body cavity of interest (pleural vs. peritoneal). It is also important to be familiar with the exact pattern, for example nuclear versus cytoplasmic loss, that indicates a positive test. Furthermore, no single marker is 100% sensitive even with the optimal morphology/location, so that combinations of markers are essential. This review covers the various new markers in the literature, highlights their advantages and limitations, and suggests morphology/site specific combinations that can produce sensitivities in the 80% to 90% (and perhaps higher) range. At present only BRCA-1 related protein-1 and methylthioadenosine phosphorylase IHC, and cyclin-dependent kinase inhibitor 2A (p16) FISH have sufficient publications and reproducibility of results to be considered as established markers. 5-Hydroxymethyl cytosine, enhancer of zeste homolog 2, cyclin D1, and programmed death-ligand 1 IHC, and NF2 FISH are all potentially useful but need further study. The newly described entity of malignant mesothelioma in situ sits at the interface of benign and malignant mesothelial process; criteria for this diagnosis are reviewed.

Malignant mesothelioma in situ: diagnostic and clinical considerations

In situ stages of malignancy have been characterised in various neoplasms. Mesothelioma in situ (MIS) has been a controversial diagnosis, lacking clear diagnostic criteria and understanding as to whether it is truly a premalignant lesion in the progression of malignant mesothelioma (MM). Originally understood as a concept and defined as atypical mesothelial proliferation in the presence of invasion, it has now been suggested that loss of nuclear labelling for BRCA1-associated protein-1 (BAP1) in flat, non-invasive mesothelial lesions can define MIS. This study aimed to characterise BAP1 expression in a cohort of 19 patients diagnosed with MIS (either pure MIS, n=3, or MIS-predominant invasive MM, n=16) and to compare survival between MIS, MIS-predominant MM and MM (n=114) in order to gain insight into the characteristics of MIS. We defined pure MIS as any architectural pattern of surface mesothelial cells with loss of BAP1 in the absence of invasion, but in specimens with superficial stromal invasion we also accepted the original definition of cytologically and architecturally atypical mesothelial proliferation, in the absence of inflammatory features, with or without loss of BAP1. We observed that MIS associated with minimal invasion was associated with significantly improved survival compared to MM (8 months vs 22 months). This suggests that MIS is indeed a precursor to MM and that these cases represent earlier stage disease. Loss of BAP1 was present in 60% of mesotheliomas with invasion, so not all early cases can be detected by BAP1 loss, but our study provides evidence that BAP1 loss may be an early molecular alteration in MM pathogenesis in patients that have loss of BAP1. We confirm that BAP1 loss can be useful for diagnosis of pure MIS in surgical specimens, permitting earlier diagnosis. However, identification of a predominant MIS component with minimal invasion has prognostic and conceptual implications. Whilst no approved therapy is available for MIS, close follow up of patients with BAP1 mutation in mesothelial cells and/or diagnosis of MIS is required to monitor for disease progression and potentially investigate earlier treatment interventions.

Biological Mechanisms and Clinical Significance of BAP1 Mutations in Human Cancer

Among more than 200 BAP1-mutant families affected by the "BAP1 cancer syndrome," nearly all individuals inheriting a BAP1 mutant allele developed one or more malignancies during their lifetime, mostly uveal and cutaneous melanoma, mesothelioma, and clear-cell renal cell carcinoma. These cancer types are also those that, when they occur sporadically, are more likely to carry somatic biallelic BAP1 mutations. Mechanistic studies revealed that the tumor suppressor function of BAP1 is linked to its dual activity in the nucleus, where it is implicated in a variety of processes including DNA repair and transcription, and in the cytoplasm, where it regulates cell death and mitochondrial metabolism. BAP1 activity in tumor suppression is cell type- and context-dependent. BAP1 has emerged as a critical tumor suppressor across multiple cancer types, predisposing to tumor development when mutated in the germline as well as somatically. Moreover, BAP1 has emerged as a key regulator of gene-environment interaction.This article is highlighted in the In This Issue feature, p. 1079.

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.

Differentiating Between Malignant Mesothelioma and Other Pleural Lesions Using Fourier Transform Infrared Spectroscopy

Histopathology, despite being the gold standard as a diagnostic tool, does not always provide a correct diagnosis for different pleural lesions. Although great progress was made in this field, the problem to differentiate between reactive and malignant pleural lesions still stimulates the search for additional diagnostic tools. Our research using vibrational spectroscopy and principal component analysis (PCA) statistical modeling represents a potentially useful tool to approach the problem. The objective method this paper explores is based on the correlation between different types of pleural lesions and their vibrational spectra. Obtained tissue spectra recorded by infrared spectroscopy allowed us to categorize spectra in different groups using a created PCA statistical model. The PCA model was built using tissues of known pathology as the model group. The validation samples were then used to confirm the functionality of our PCA model. Student's t-test was also used for comparing samples in paired groups. The PCA model was able to clearly differentiate the spectra of mesothelioma, metastasis and reactive changes (inflammation), and place them in discrete groups. Thus, we showed that Fourier transform infrared spectroscopy combined with PCA can differentiate pleural lesions with high sensitivity and specificity. This new approach could contribute in objectively differentiating specific pleural lesions, thus helping pathologists to better diagnose difficult pleural samples but also could shed additional light into the biology of malignant pleural mesothelioma.

Genomic-based ancillary assays offer improved diagnostic yield of effusion cytology with potential challenges in malignant pleural mesothelioma

BRCA1-associated protein 1 (BAP1) or methylthioadenosine phosphorylase (MTAP) immunohistochemistry (IHC) or 9p21 fluorescence in situ hybridization (FISH) are useful for the diagnosis of malignant pleural mesothelioma (MPM). However, the effect of these assays on the diagnostic yield of effusion cytology in MPM cases with suspicious cytomorphology or the diagnostic challenges in BAP1 or MTAP IHC have not been fully elucidated. Two cohorts of cytologic preparations obtained from pleural effusions were examined: MPM cases in cohort 1 were used to evaluate whether BAP1 or MTAP IHC or 9p21 FISH increase the diagnostic yield of effusion cytology; cohort 2 included cases suspicious for MPM, to which BAP1 or MTAP IHC was applied to clarify the challenges in the clinical assessment of these assays. In cohort 1 (n = 28), either assay elevated 62.5% of class II or III cases to class V. In cohort 2 (n = 139), 21.7% of BAP1 immunocytochemistry in smears and 10.6% of BAP1 IHC and 9.4% of MTAP IHC in cell blocks, were identified to be challenging. The application of genomic-based assays increased the diagnostic yield of effusion cytology in the diagnosis of MPM. However, diagnostic challenges limit the application of these assays in some cases.

HEG1, BAP1, and MTAP are useful in cytologic diagnosis of malignant mesothelioma with effusion

BACKGROUND

The specificity and sensitivity of HEG1 for malignant mesothelioma (MM) is high. The use of BAP1/MTAP immunohistochemistry (IHC) is recommended to separate benign and malignant mesothelial proliferations. We determined how ancillary techniques can be used for the cytological diagnosis of MM with effusion.

METHODS

Cell blocks from effusions from cases with MM, reactive mesothelial cells (RMCs), and carcinomas were analyzed by IHC with HEG1, BAP1, and MTAP and with homozygous deletion (HD) of CDKN2A by fluorescence in situ hybridization. Staining scores were calculated for IHC by adding the number of categories for the staining intensity and the staining extension.

RESULTS

HEG1 was positive in all (41/41) MMs, but negative in carcinomas, except for ovarian carcinomas. Overall 76.9% (20/26) of RMCs and 28.6% (6/21) of ovarian carcinomas expressed HEG1. BAP1 loss was found in 71.1% of MMs, but none was found in RMCs. MTAP loss was found in 76.2% of MMs, but none was found in RMCs. 73.9% of MMs harbored HD of CDKN2A. There was concordance between loss of MTAP and HD of CDKN2A in 95% of MMs.

CONCLUSION

HEG1 is a good marker for mesothelial differentiation in effusion cytology. HD of CDKN2A is frequently observed in cell blocks from effusions of MMs, and MTAP IHC may act as a surrogate for HD of CDKN2A. Cell block analysis is recommended for effusions of unknown origins with the following methods: IHC with HEG1 and claudin 4 to validate the mesothelial origin, followed by BAP1 and MTAP IHC to confirm malignancy.

An Algorithmic Immunohistochemical Approach to Define Tumor Type and Assign Site of Origin

Immunohistochemistry represents an indispensable complement to an epidemiology and morphology-driven approach to tumor diagnosis and site of origin assignment. This review reflects the state of my current practice, based on 15-years' experience in Pathology and a deep-dive into the literature, always striving to be better equipped to answer the age old questions, "What is it, and where is it from?" The tables and figures in this manuscript are the ones I "pull up on the computer" when I am teaching at the microscope and turn to myself when I am (frequently) stuck. This field is so exciting because I firmly believe that, through the application of next-generation immunohistochemistry, we can provide better answers than ever before. Specific topics covered in this review include (1) broad tumor classification and associated screening markers; (2) the role of cancer epidemiology in determining pretest probability; (3) broad-spectrum epithelial markers; (4) noncanonical expression of broad tumor class screening markers; (5) a morphologic pattern-based approach to poorly to undifferentiated malignant neoplasms; (6) a morphologic and immunohistochemical approach to define 4 main carcinoma types; (7) CK7/CK20 coordinate expression; (8) added value of semiquantitative immunohistochemical stain assessment; algorithmic immunohistochemical approaches to (9) "garden variety" adenocarcinomas presenting in the liver, (10) large polygonal cell adenocarcinomas, (11) the distinction of primary surface ovarian epithelial tumors with mucinous features from metastasis, (12) tumors presenting at alternative anatomic sites, (13) squamous cell carcinoma versus urothelial carcinoma, and neuroendocrine neoplasms, including (14) the distinction of pheochromocytoma/paraganglioma from well-differentiated neuroendocrine tumor, site of origin assignment in (15) well-differentiated neuroendocrine tumor and (16) poorly differentiated neuroendocrine carcinoma, and (17) the distinction of well-differentiated neuroendocrine tumor G3 from poorly differentiated neuroendocrine carcinoma; it concludes with (18) a discussion of diagnostic considerations in the broad-spectrum keratin/CD45/S-100-"triple-negative" neoplasm.

Application of immunohistochemistry in diagnosis and management of malignant mesothelioma

Immunohistochemistry plays an indispensable role in accurate diagnosis of malignant mesothelioma, particularly in morphologically challenging cases and in biopsy and cytology specimens, where tumor architecture is difficult or impossible to evaluate. Application of a targeted panel of mesothelial- and epithelial-specific markers permits correct identification of tumor lineage in the vast majority of cases. An immunopanel including two mesothelial markers (calretinin, CK5/6, WT-1, or D2-40) and two epithelial markers (MOC-31 and claudin-4) offers good sensitivity and specificity, with adjustments as appropriate for the differential diagnosis. Once mesothelial lineage is established, malignancy-specific studies can help verify a diagnosis of malignant mesothelioma. BAP1 loss, CDKN2A homozygous deletion, and MTAP loss are highly specific markers of malignancy in a mesothelial lesion, and they attain acceptable diagnostic sensitivity when applied as a diagnostic panel. Novel markers of malignancy, such as 5-hmC loss and increased EZH2 expression, are promising, but have not yet achieved widespread clinical adoption. Some diagnostic markers also have prognostic significance, and PD-L1 immunohistochemistry may predict tumor response to immunotherapy. Application and interpretation of these immnuomarkers should always be guided by clinical history, radiographic findings, and above all histomorphology.

Interobserver variation in the assessment of the sarcomatoid and transitional components in biphasic mesotheliomas

The percentage of sarcomatoid component has an impact on prognosis in patients with biphasic malignant pleural mesothelioma. Recent study showed that the transitional pattern similar to sarcomatoid component of malignant mesothelioma has negative prognostic significance. Practice guidelines recommend quantification of sarcomatoid component despite poor diagnostic reproducibility of biphasic mesothelioma among thoracic pathologists. The aim of this study was to determine the interobserver agreement in the quantification of sarcomatoid component, and in the diagnosis of a transitional component in the biphasic malignant mesothelioma. Thirteen experts in thoracic pathology reviewed the representative H&E and cytokeratin whole-slide images of the 54 biphasic mesotheliomas, without knowledge of BAP1 or p16 deletion status, and completed the survey of 25 questions. The overall interobserver agreement in the assessment of the percentage of the sarcomatoid component in 25% increments was good (wK = 0.62). Excellent agreement was present in 14 of 54 cases (26%), and 3 cases were unanimously scored. Excellent agreement was reached for the cases with 0-24% and > 75% of the sarcomatoid component.The most commonly used criteria for the diagnosis of sarcomatoid component were malignant spindle cells, frank sarcomatoid features and high N/C ratio. The overall interobserver agreement for transitional pattern was fair (wK = 0.40). Unanimous opinion about the absence of transitional pattern was observed in only one case. At least 70% agreement regarding the presence of transitional pattern was observed in 12 cases, with the rest of the cases showing a wide range of disagreement. Morphologic characteristics that favor transitional pattern over non-transitional include sheet-like growth of cohesive, plump, elongated epithelioid cells with well-defined cell borders and a tendency to transition into spindle cells. Our study defined precise morphologic criteria that may be used in the differential diagnosis between transitional pattern and other mesothelioma subtypes including sarcomatoid and epithelioid.

Primary Malignant Pericardial Mesothelioma: A Clinical Case Series Illustrating the Necessity of a Multidisciplinary Approach

Primary malignant pericardial mesothelioma is a rare cardiac neoplasm. The authors evaluated risk factors, clinical presentation, and outcomes by reviewing all biopsy-confirmed cases at one institution. The use of multimodality imaging, detailed hemodynamic assessment for the presence of an effusive-constrictive profile, and cytology evaluation can support the diagnosis. (Level of Difficulty: Advanced.)

Signet ring cell mesothelioma; A diagnostic challenge

Signet ring cell mesothelioma is a rare variant of epithelioid mesothelioma with limited cases published. It has a male predilection and most commonly occurs on pleura; it can also arise in the peritoneal cavity. The signet ring cell morphology can pose a challenge leading to a potential diagnostic error. A variety of benign and malignant diseases, including reactive histiocytic hyperplasia, adenocarcinoma, melanoma, and lymphoma with signet ring cell morphology should be considered in the differential diagnosis. In signet ring cell mesothelioma work up, mucin stains are of limited value. Even though immunohistochemistry is routinely used in mesothelioma diagnosis, there is no sole specific mesothelial marker. Hence, a panel of mesothelial and epithelial markers are used; these should be interpreted with caution especially in this variant. Electron microscopy and genetic testing can be very helpful in distinguishing signet ring cell mesothelioma from its mimickers.

The Role of Ancillary Techniques in Pulmonary Cytopathology

Ancillary techniques play an essential role in pulmonary cytopathology. Immunoperoxidase and special stains are by far the most common ancillary techniques used in cytopathology; however, the role of molecular diagnosis is growing, especially in the fields of pulmonary oncology and infectious disease. In this article, we review the uses of ancillary techniques in lung tumor diagnosis, lung tumor classification, predictive marker determination, primary versus metastasis differential diagnosis, and infectious organism detection.

Well-differentiated papillary mesothelioma of the peritoneum is genetically defined by mutually exclusive mutations in TRAF7 and CDC42

Well-differentiated papillary mesothelioma is an uncommon mesothelial neoplasm that most frequently arises in the peritoneal cavity of women of reproductive age. Whereas malignant mesothelioma is an aggressive tumor associated with poor outcome, well-differentiated papillary mesothelioma typically exhibits indolent behavior. However, histologically differentiating between these two entities can be challenging, necessitating the development of distinguishing biomarkers. While the genetic alterations that drive malignant mesothelioma have recently been determined, the molecular pathogenesis of well-differentiated papillary mesothelioma is unknown. Here we performed genomic profiling on a cohort of ten well-differentiated papillary mesothelioma of the peritoneum. We identified that all tumors harbored somatic missense mutations in either the TRAF7 or CDC42 genes, and lacked alterations involving BAP1, NF2, CDKN2A, DDX3X, SETD2, and ALK that are frequent in malignant mesothelioma. We recently identified that another mesothelial neoplasm, adenomatoid tumor of the genital tract, is genetically defined by somatic missense mutations in the TRAF7 gene, indicating a shared molecular pathogenesis between well-differentiated papillary mesothelioma and adenomatoid tumors. To the best of our knowledge, well-differentiated papillary mesothelioma is the first human tumor type found to harbor recurrent mutations in the CDC42 gene, which encodes a Rho family GTPase. Immunohistochemistry demonstrated intact BAP1 expression in all cases of well-differentiated papillary mesothelioma, indicating that this is a reliable marker for distinguishing well-differentiated papillary mesothelioma from malignant mesotheliomas that frequently display loss of expression. Additionally, all well-differentiated papillary mesothelioma demonstrated robust expression of L1 cell adhesion molecule (L1CAM), a marker of NF-kB pathway activation, similar to that observed in adenomatoid tumors. In contrast, we have previously shown that L1CAM staining is not observed in normal mesothelial cells and malignant mesotheliomas of the peritoneum. Together, these studies demonstrate that well-differentiated papillary mesothelioma is genetically defined by mutually exclusive mutations in TRAF7 and CDC42 that molecularly distinguish this entity from malignant mesothelioma.

Loss of Nuclear BAP1 Expression Is Associated with High WHO/ISUP Grade in Clear Cell Renal Cell Carcinoma

Background

BRCA1-associated protein 1 (BAP1) mutations are frequently reported in clear cell renal cell carcinoma (ccRCC); however, very few studies have evaluated the role of these mutations in other renal cell carcinoma (RCC) subtypes. Therefore, we analyzed BAP1 protein expression using immunohistochemistry in several RCC subtypes and assessed its relationship with clinicopathological characteristics of patients.

Methods

BAP1 expression was immunohistochemically evaluated in tissue microarray blocks constructed from 371 samples of RCC collected from two medical institutions. BAP1 expression was evaluated based on the extent of nuclear staining in tumor cells, and no expression or expression in < 10% of tumor cells was defined as negative.

Results

Loss of BAP1 expression was observed in ccRCC (56/300, 18.7%), chromophobe RCC (6/26, 23.1%), and clear cell papillary RCC (1/4, 25%), while we failed to detect BAP1 expression loss in papillary RCC, acquired cystic disease-associated RCC, or collecting duct carcinoma. In ccRCC, loss of BAP1 expression was significantly associated with high World Health Organization (WHO)/International Society of Urological Pathology (ISUP) grade (p = .002); however, no significant correlation was observed between loss of BAP1 expression and survival in ccRCC. Loss of BAP1 expression showed no association with prognostic factors in chromophobe RCC.

Conclusions

Loss of BAP1 nuclear expression was observed in both ccRCC and chromophobe RCC. In addition, BAP1 expression loss was associated with poor prognostic factors such as high WHO/ISUP grade in ccRCC.

Nuclear grading, BAP1, mesothelin and PD-L1 expression in malignant pleural mesothelioma: prognostic implications

For malignant pleural mesothelioma (MPM), histopathological subtype is one of the most important prognostic factors. Several immunohistochemical stains whose expressions have possible therapeutic implications have been identified in MPM such as BAP1, mesothelin and PD-L1. The aim of our work was to evaluate the clinical significance and prognostic implications of BAP1, mesothelin and PD-L1 expression in 117 patients with a diagnosis of MPM who were diagnosed in our institution between 2002 and 2017. We also correlated this immunohistochemical profile to a recently described nuclear grading and to histopathological subtype. Mesothelin expression, BAP1 loss and PD-L1 expression were associated with histopathological subtype (p < 0.0001), BAP1 loss was more frequent in epithelioid subtype whereas PD-L1 expression was more frequent in non-epithelioid subtype. For epithelioid MPM, BAP1 expression was associated with overall survival (p = 0.034), with a longer survival when BAP1 expression is lost. Necrosis and nuclear grading are associated with overall survival (p = 0.0048 and <0.0001, respectively), with longer survival when necrosis was absent and for grade I. For non-epithelioid MPM, overall survival was not related to clinical, histopathological or immunohistochemical expression of BAP1, mesothelin or PD-L1. In multivariate analysis, grade I for nuclear grading was an independent prognostic factor associated with overall survival (p < 0.0001). In epithelioid and non-epithelioid MPM, we analysed overall survival in subgroups with combined mesothelin, BAP1 and PD-L1 expression. In epithelioid MPM, BAP1 retained/mesothelin negativity/PD-L1 > 1%, and BAP1 retained/mesothelin positivity/PD-L1 > 1% profiles, are associated with shorter overall survival. In non-epithelioid MPM, BAP1 loss/mesothelin negativity/PD-L1 > 1% is associated with shorter overall survival. Our work confirms that nuclear grading in epithelioid MPM is a strong and independent prognosis factor. Moreover, this study on several promising immunohistochemical stains whose expressions have possible therapeutic implications identifies subgroups with a poor prognosis.

The diagnostic role of BAP1 in serous effusions

The aim of this study was to analyze the diagnostic role of BAP1 in effusion cytology. Effusions (n = 258), consisting of 53 malignant mesotheliomas and 205 other cancers, the majority carcinomas (62 breast, 60 ovarian, 31 lung, 51 carcinomas of other origin, 1 melanoma), were analyzed for BAP1 expression using immunohistochemistry. BAP1 was lost in 46 (87%) mesotheliomas compared with 4 (2%) of 205 other cancers (P < .001), resulting in sensitivity and specificity of 87% and 98%, respectively. There was no significant difference between peritoneal (n = 14) and pleural (n = 39) mesotheliomas. The 4 carcinomas with loss of BAP1 included 1 ovarian, 1 breast, 1 uterine cervical, and 1 gastric carcinoma. The present study supports the role of BAP1 as a highly sensitive and specific marker for malignant mesothelioma in serous effusions and argues for inclusion of this test in all specimens in which this diagnosis is considered.

Malignant peritoneal mesothelioma in a woman with bilateral ovarian serous borderline tumour: Potential interactions between the two diseases

We report a case of a 59-year-old woman with peritoneal malignant mesothelioma and no previous exposure to asbestos with a diagnosis of bilateral ovarian serous borderline tumour with peritoneal implants one year before. We discuss the histopathological and immunohistochemical findings to explain possible and potential interactions between the two diseases. To our knowledge, the association of both serous borderline ovarian tumour and malignant peritoneal mesothelioma has never been described before in the same woman and in such a tight temporal connection. This finding raises numerous issues about the origin of the two tumours and further biomolecular studies are needed to fully understand the carcinogenetic process. From a clinical point of view, this case report can be useful to gynaecologists because it leads to recommend a careful examination of the peritoneal cavity during a surgical resection of borderline serous tumour. Moreover, it may suggest performing a close follow-up associated with a careful surveillance of the patient, especially in the case of micropapillary pattern, to oncologists. A complete clinical approach could help to detect sooner possible relapses or other metachronous malignancies.

•

Serous borderline ovarian tumour usually has an indolent course.

•

Malignant peritoneal mesothelioma is an aggressive malignancy.

•

The concurrence of both diseases has never been described before in the same woman.

•

We discuss the possible and potential interactions between the two diseases.

Positive nuclear BAP1 immunostaining helps differentiate non-small cell lung carcinomas from malignant mesothelioma

The differential diagnosis between pleural malignant mesothelioma (MM) and lung cancer is often challenging. Immunohistochemical (IHC) stains used to distinguish these malignancies include markers that are most often positive in MM and less frequently positive in carcinomas, and vice versa. However, in about 10-20% of the cases, the IHC results can be confusing and inconclusive, and novel markers are sought to increase the diagnostic accuracy.We stained 45 non-small cell lung cancer samples (32 adenocarcinomas and 13 squamous cell carcinomas) with a monoclonal antibody for BRCA1-associated protein 1 (BAP1) and also with an IHC panel we routinely use to help differentiate MM from carcinomas, which include, calretinin, Wilms Tumor 1, cytokeratin 5, podoplanin D2-40, pankeratin CAM5.2, thyroid transcription factor 1, Napsin-A, and p63. Nuclear BAP1 expression was also analyzed in 35 MM biopsies. All 45 non-small cell lung cancer biopsies stained positive for nuclear BAP1, whereas 22/35 (63%) MM biopsies lacked nuclear BAP1 staining, consistent with previous data. Lack of BAP1 nuclear staining was associated with MM (two-tailed Fisher's Exact Test, P = 5.4 x 10-11). Focal BAP1 staining was observed in a subset of samples, suggesting polyclonality. Diagnostic accuracy of other classical IHC markers was in agreement with previous studies. Our study indicated that absence of nuclear BAP1 stain helps differentiate MM from lung carcinomas. We suggest that BAP1 staining should be added to the IHC panel that is currently used to distinguish these malignancies.