Extrapleural pneumonectomy for sarcoma: Outcomes of adult patients at a specialized center.

BACKGROUND: Extrapleural pneumonectomy (EPP) is a complex surgical procedure involving en-bloc resection of the parietal and visceral pleura, lung, pericardium, and ipsilateral diaphragm. Small case series of pleural-based sarcoma of predominantly pediatric patients suggest EPP may be a life-prolonging surgical option. We aimed to describe the characteristics and outcomes of adults who underwent EPP at a specialized sarcoma center. METHODS: Clinicopathologic variables, surgical details, and follow-up information were extracted for patients undergoing EPP for pleural-based sarcoma between August 2017 and December 2020. Primary outcomes were event-free survival (EFS) and overall survival (OS) from the date of EPP. Secondary outcomes were disease-free interval (DFI) prior to EPP, and early and late postoperative complications. RESULTS: Eight patients were identified, seven with soft tissue sarcoma and one with bone sarcoma. Patients had either localized disease with a primary thoracic sarcoma, sarcoma recurrent to the thorax, or de novo metastatic disease. All patients underwent resection of their pleural-based sarcoma by an experienced cardiothoracic surgeon, and some patients had pre or postoperative treatment. The perioperative morbidity was comparable with previously published reports of EPP performed in mesothelioma patients. At median follow-up of 22.5 months, median EFS was 6.0 months and OS was 20.7 months. Six patients (75%) had disease recurrence; five (62.5%) died of progressive disease. Two patients (25%) had not recurred: one died of a radiation-related esophageal rupture, and one was alive with no evidence of disease at 37.0 months. Characteristics of those with the longest EFS included low-grade histology and achieving a metabolic response to preoperative chemotherapy. CONCLUSIONS: In adults with pleural-based sarcoma, EPP is rarely curative but appears to be a feasible salvage procedure when performed at specialized centers. Patient selection is critical with strong consideration given to multimodal therapy to optimize patient outcomes. In the absence of a confirmed response to neoadjuvant treatment, long term survival is poor and EPP should not be recommended.

Detailed clinical course of a patient with rapidly progressing sarcomatoid pleural mesothelioma without p16 deletion with systemic haematogenous metastasis to soft tissues.

Sarcomatoid mesothelioma is difficult to differentiate from other mesotheliomas. Here, we describe the case of a man in his early 80s with sarcomatoid mesothelioma and a history of asbestos exposure. He initially presented with right-sided chest pain and was examined. Right-sided pleural effusion was detected; therefore, he was hospitalised. Based on the observed pleural effusion and biopsy result, the presence of a malignant tumour was excluded; hence, he was diagnosed with benign asbestos pleurisy. He subsequently developed left-sided pleural effusion, masses and lung nodules, and died 9.5 months after the initial examination. A definitive diagnosis of sarcomatoid mesothelioma with rapid systemic progression was established after detailed investigations using autopsy specimens. This rare case of mesothelioma-without p16 deletion (detected using fluorescence in situ hybridisation)-presented differently from the usual sarcomatoid mesothelioma.

The Impact of 3D Nichoids and Matrix Stiffness on Primary Malignant Mesothelioma Cells.

Malignant mesothelioma is a type of cancer that affects the mesothelium. It is an aggressive and deadly form of cancer that is often caused by exposure to asbestos. At the molecular level, it is characterized by a low number of genetic mutations and high heterogeneity among patients. In this work, we analyzed the plasticity of gene expression of primary mesothelial cancer cells by comparing their properties on 2D versus 3D surfaces. First, we derived from primary human samples four independent primary cancer cells. Then, we used Nichoids, which are micro-engineered 3D substrates, as three-dimensional structures. Nichoids limit the dimension of adhering cells during expansion by counteracting cell migration between adjacent units of a substrate with their microarchitecture. Tumor cells grow effectively on Nichoids, where they show enhanced proliferation. We performed RNAseq analyses on all the samples and compared the gene expression pattern of Nichoid-grown tumor cells to that of cells grown in a 2D culture. The PCA analysis showed that 3D samples were more transcriptionally similar compared to the 2D ones. The 3D Nichoids induced a transcriptional remodeling that affected mainly genes involved in extracellular matrix assembly. Among these genes responsible for collagen formation, COL1A1 and COL5A1 exhibited elevated expression, suggesting changes in matrix stiffness. Overall, our data show that primary mesothelioma cells can be effectively expanded in Nichoids and that 3D growth affects the cells' tensegrity or the mechanical stability of their structure.

Involvement of Palliative Care in Malignant Pleural Mesothelioma Patients and Associations with Survival and End-of-Life Outcomes.

Malignant pleural mesothelioma is a rare, aggressive, and incurable cancer with a poor prognosis and high symptom burden. For these patients, little is known about the impact of palliative care consultation on outcomes such as mortality, hospital admissions, or emergency department visits. The aim of this study is to determine if referral to supportive and palliative care in patients with malignant pleural mesothelioma is associated with survival and decreased hospital admissions and emergency department visits. This is a retrospective chart review. Study participants include all malignant pleural mesothelioma patients seen at The Ottawa Hospital-an acute care tertiary center-between January 2002 and March 2019. In total, 223 patients were included in the study. The mean age at diagnosis was 72.4 years and 82.5% were male. Of the patients diagnosed between 2002 and 2010, only 11 (9.6%) were referred to palliative care. By comparison, of those diagnosed between 2011 and 2019, 49 (45.4%) were referred to palliative care. Median time from diagnosis to referral was 4.1 months. There was no significant difference in the median survival of patients referred for palliative care compared to those who did not receive palliative care (p = 0.46). We found no association between receiving palliative care and the mean number of hospital admissions (1.04 vs. 0.91) from diagnosis to death, and an increase in mean number of emergency department visits in the palliative care group (2.30 vs. 1.18). Although there was increased utilization of palliative care services, more than half of the MPM patients did not receive palliative care despite their limited survival. There was an increase in emergency department visits in the palliative care group; this may represent an increase in the symptom burden (i.e., indication bias) in those referred to palliative care.

Mesothelin antigen density influences anti-mesothelin chimeric antigen receptor T cell cytotoxicity.

BACKGROUND AIMS: Several anti-mesothelin (MSLN) chimeric antigen receptor (CAR) T cells are in phase 1/2 clinical trials to treat solid-organ malignancies. The effect of MSLN antigen density on MSLN CAR cytotoxicity against tumor cells has not been examined previously, nor are there data regarding the effect of agents that increase MSLN antigen density on anti-MSLN CAR T cell efficacy. METHODS: MSLN antigen density was measured on a panel of pancreatic cancer and mesothelioma cell lines by flow cytometry. In parallel, the cytotoxicity and specificity of two anti-MSLN CAR T cells (m912 and SS1) were compared against these cell lines using a real-time impedance-based assay. The effect of two MSLN 'sheddase' inhibitors (lanabecestat and TMI-1) that increase MSLN surface expression was also tested in combination with CAR T cells. RESULTS: SS1 CAR T cells were more cytotoxic compared with m912 CAR T cells against cell lines that expressed fewer than ∼170 000 MSLN molecules/cell. A comparison of the m912 and amatuximab (humanized SS1) antibodies identified that amatuximab could detect and bind to lower levels of MSLN on pancreatic cancer and mesothelioma cell lines, suggesting that superior antibody/scFv affinity was the reason for the SS1 CAR's superior cytotoxicity. The cytotoxicity of m912 CAR T cells was improved in the presence of sheddase inhibitors, which increased MSLN antigen density. CONCLUSIONS: These data highlight the value of assessing CAR constructs against a panel of cells expressing varying degrees of target tumor antigen as occurs in human tumors. Furthermore, the problem of low antigen density may be overcome by concomitant administration of drugs that inhibit enzymatic shedding of MSLN.

Mesothelioma of Uncertain Malignant Potential (MUMP) of the Tunica Vaginalis: Proposal for Reclassification as "Complex Mesothelial Tumor of the Tunica Vaginalis".

A well-differentiated papillary mesothelial tumor (WDPMT) and malignant mesothelioma are 2 well-recognized entities arising from the testis tunica vaginalis. Another mesothelial lesion exclusively seen at this site is mesothelioma of uncertain malignant potential (MUMP)-a lesion reminiscent of WDPMT yet demonstrating variable proportions of more complex architectural patterns that could be confused with invasion. MUMP was first described in 2010 with a total of 11 cases reported to date. Herein, we describe 19 additional patients who underwent hydrocelectomy, excision, and/or orchiectomy. Novel morphologic patterns found in addition to the 2010 series include spindle cells, keloidal-type collagen, and multicystic architecture lined by bland mesothelial cells. Clinical follow-up in 9 patients for more than 1 year (1.5 to 22.5 y, median 4.5 y) revealed no evidence of disease recurrence or metastases. Despite greater architectural complexity, MUMP has (1) bland cytology; (2) merging in with WDPMT areas; (3) low mitotic rate and Ki-67 nuclear labeling index; (4) retention of MTAP and BAP1 expression; and (5) benign clinical follow-up. If these cases were malignant mesotheliomas, one would have expected at least some of the patients to demonstrate disease recurrence/progression without adjuvant therapy within the available follow-up time, particularly with limited resection in most patients. Thus, we propose that "mesothelioma of uncertain malignant potential" be renamed as "complex mesothelial tumor of the tunica vaginalis." Using the term "complex" draws a contrast with the simple cuboidal lining and simple papillary architecture seen in WDPMT. Also, labeling the lesion as "tumor" removes the stigmata of "uncertain malignant potential" and "mesothelioma" that are alarming to patients and clinicians, and potentially could unduly lead to more extensive surgery in an attempt at "complete" resection. At the same time, not definitively labeling the lesion as benign allows recommendations for follow-up.

Comparison of Immunohistochemistry, Next-generation Sequencing and Fluorescence In Situ Hybridization for Detection of MTAP Loss in Pleural Mesothelioma.

9p21 deletions involving MTAP/CDKN2A genes are detected in diffuse pleural mesotheliomas (DPM) but are absent in benign mesothelial proliferations. Loss of MTAP expression by immunohistochemistry (IHC) is well accepted as a surrogate for 9p21 deletion to support a diagnosis of DPM. Accurate interpretation can be critical in the diagnosis of DPM, but variations in antibody performance may impact interpretation. The objectives of this study were to compare the performance of MTAP monoclonal antibodies (mAbs) EPR6893 and 1813 and to compare MTAP expression by IHC with 9p21 copy number status in DPM. Cytoplasmic expression of MTAP IHC with mAbs EPR6893 (ab126770; Abcam) and 1813 (NBP2-75730, Novus Biologicals) was evaluated in 56 DPM (47 epithelioid, 7 biphasic, and 2 sarcomatoid) profiled by targeted next-generation sequencing. 9p21 Copy number status was assessed by Fraction and Allele-Specific Copy Number Estimates from Tumor Sequencing (FACETS) analysis and also by CDKN2A fluorescence in situ hybridization in discrepant cases when material was available. MTAP mAb 1813 showed stronger immunoreactivity, more specific staining, and no equivocal interpretations compared to mAb EPR6893 which showed equivocal staining in 19 (34%) of cases due to weak or heterogenous immunoreactivity, lack of definitive internal positive control, and/or nonspecific background staining. MTAP expression with mAb 1813 showed near perfect agreement with 9p21 copy number by combined FACETS/fluorescence in situ hybridization calls (κ = 0.85; 95% CI, 0.71-0.99; P < .001). MTAP IHC with mAb 1813 was 96% sensitive, 86% specific, and 93% accurate for 9p21 homozygous deletion. The findings of this study suggest that interpretation of MTAP IHC is improved with mAb 1813 because mAb EPR6893 was often limited by equivocal interpretations. We show that MTAP IHC and molecular assays are complementary in detecting 9p21 homozygous deletion. MTAP IHC may be particularly useful for low tumor purity samples and in low-resource settings.

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.

The over diagnosis of diffuse mesothelioma: An analysis of 311 cases with recommendations for the avoidance of pitfalls.

BACKGROUND: The diagnosis of mesothelioma may be challenging. We investigated a large database of cases in order to determine the frequency with which a diagnosis of mesothelioma was made incorrectly and the most frequent causes of error. DESIGN: A database including more than 4000 consultation cases of histologically confirmed mesothelioma was examined to identify cases in which mesothelioma was diagnosed by at least one pathologist when the available information pointed towards a different diagnosis. RESULTS: There were 311 cases misdiagnosed as mesothelioma. The most common category was metastatic carcinoma to the pleura or peritoneum (129 cases: 73 lung carcinomas, 15 renal cell carcinomas). The next most common category was primary lung cancer (111 cases: 55 sarcomatoid carcinoma, 56 pseudomesotheliomatous carcinoma). The third most common category was primary malignancies arising from or near the serosal membranes (33 cases). The fourth most common category was fibrous pleurisy (38 cases). The most common errors were failure to consider important radiographic information regarding the gross distribution of tumor, lack of awareness or consideration of another malignancy, overreliance on certain immunohistochemical results, and failure to perform certain diagnostic histochemical, immunohistochemical, or ultrastructural studies. CONCLUSIONS: There are a number of diagnostic pitfalls that can lead to the over diagnosis of mesothelioma. Careful attention to clinical and radiographic information as well as performance of appropriate ancillary tests can help to prevent such misdiagnoses. Detailed examples will be presented to assist in the avoidance of these pitfalls with emphasis on the most commonly observed errors.

Single-cell transcriptomic analysis of human pleura reveals stromal heterogeneity and informs in vitro models of mesothelioma.

The pleural lining of the thorax regulates local immunity, inflammation and repair. A variety of conditions, both benign and malignant, including pleural mesothelioma, can affect this tissue. A lack of knowledge concerning the mesothelial and stromal cells comprising the pleura has hampered the development of targeted therapies. Here, we present the first comprehensive single-cell transcriptomic atlas of the human parietal pleura and demonstrate its utility in elucidating pleural biology. We confirm the presence of known universal fibroblasts and describe novel, potentially pleural-specific, fibroblast subtypes. We also present transcriptomic characterisation of multiple in vitro models of benign and malignant mesothelial cells, and characterise these through comparison with in vivo transcriptomic data. While bulk pleural transcriptomes have been reported previously, this is the first study to provide resolution at the single-cell level. We expect our pleural cell atlas will prove invaluable to those studying pleural biology and disease. It has already enabled us to shed light on the transdifferentiation of mesothelial cells, allowing us to develop a simple method for prolonging mesothelial cell differentiation in vitro.

The diagnosis of asbestosis in the 21st century: a clinicopathological correlation of 102 cases.

Asbestosis, defined as diffuse pulmonary fibrosis caused by inhalation of asbestos fibers, occurs after heavy exposures to asbestos dust over several decades. Because workplace exposures have been significantly curtailed since the banning of asbestos in insulation products, we were interested in examining the clinicopathological characteristics of cases diagnosed in the 21st century. The consultation files of one of the authors (VLR) were reviewed for cases of asbestosis diagnosed since 1/1/2001. 102 cases were identified, with a median age of 75 years (range: 45-89). There were 100 men and 2 women. The women were from Turkey and Brazil (none from the United States). Malignancies were present in 78 cases, including 38 lung cancers, 29 pleural mesotheliomas, and 8 peritoneal mesotheliomas. The grade of asbestosis was available in 88 cases (median severity of 2; scale: 1-4). Pleural plaque was present in 94% of cases. The most common exposure categories were insulators (39), shipyard workers (16), asbestos manufacturing (9), boiler workers (8) and pipefitter/welders (6). The median duration of exposure was 33 years (range: 2-49 years). Lung fiber burden analysis was performed in 34 cases, with amosite being the predominant fiber type. Results were compared with similar information for 475 cases diagnosed prior to 1/1/2001.

SPARC Is a Novel Positive Immunohistochemical Marker of Epithelioid Mesothelioma to Differentiate It From Lung Adenocarcinoma and/or Squamous Cell Carcinoma.

Epithelioid mesothelioma with a solid histologic pattern (solid epithelioid mesothelioma) is difficult to distinguish from a poorly differentiated squamous cell lung carcinoma and/or solid lung adenocarcinoma. Thus, immunohistochemical markers are essential for diagnosis; however, the sensitivity and specificity of pre-existing mesothelial markers are suboptimal, particularly for differentiation from squamous cell carcinoma. Using a cancer-dependency map, we analyzed gene expression data of pleural mesothelioma and non-small cell lung cancer cell lines (squamous cell carcinoma and adenocarcinoma) and identified secreted protein acidic and cysteine-rich (SPARC) as a promising candidate for the differential diagnosis of epithelioid mesothelioma from lung squamous cell carcinoma and/or lung adenocarcinoma. SPARC expression in mesothelioma and lung cancer cell lines was validated using reverse-transcription polymerase chain reaction, western blotting, and immunohistochemistry. Immunohistochemical staining was performed using anti-SPARC antibodies against solid epithelioid mesothelioma, solid lung adenocarcinoma, and poorly differentiated lung squamous cell carcinoma. SPARC positivity was seen in 42/45 (93.3%) of solid epithelioid mesothelioma, 2/40 (5%) solid lung adenocarcinoma, and 2/45 (4.5%) of lung squamous cell carcinomas. The sensitivity, specificity, and diagnostic accuracy for differentiating solid epithelioid mesothelioma from lung cancer (solid lung adenocarcinoma and poorly differentiated lung squamous cell carcinoma) were 93.3, 95.2, and 94.6%, respectively. In conclusion, SPARC is a novel mesothelial marker that can be used to differentiate epithelioid mesothelioma from squamous cell carcinoma and lung adenocarcinoma.

Atypical B cells (CD21-CD27-IgD-) correlate with lack of response to checkpoint inhibitor therapy in NSCLC.

INTRODUCTION: Checkpoint inhibitor (CI) therapy has revolutionized treatment for non-small cell lung cancer (NSCLC). However, a proportion of patients do not respond to CI therapy for unknown reasons. Although the current paradigm in anti-tumor immunity evolves around T cells, the presence of tertiary lymphoid structures and memory B cells has been positively correlated with response to CI therapy in NSCLC. In addition, double negative (DN) (CD27- IgD-) B cells have been shown to be abundant in NSCLC compared to healthy lung tissue and inversely correlate with the intratumoral presence of memory B cells. Nonetheless, no study has correlated DN B cells to survival in NSCLC. METHODS: In this study, we evaluated the presence and phenotype of B cells in peripheral blood with flow cytometry of patients with NSCLC and mesothelioma before receiving CI therapy and correlated these with clinical outcome. RESULTS: Non-responding patients showed decreased frequencies of B cells, yet increased frequencies of antigen-experienced CD21- DN (Atypical) B cells compared to responding patients and HC, which was confirmed in patients with mesothelioma treated with CI therapy. CONCLUSIONS: These data show that the frequency of CD21- DN B cells correlates with lack of response to CI therapy in thoracic malignancies. The mechanism by which CD21- DN B cells hamper CI therapy remains unknown. Our findings support the hypothesis that CD21- DN B cells resemble phenotypically identical exhausted B cells that are seen in chronic infection or function as antigen presenting cells that induce regulatory T cells.

Psychosocial well-being among patients with malignant pleural mesothelioma.

OBJECTIVES: The investigators conducted a psychosocial needs assessment of mesothelioma patients through self-report measures of quality of life (QOL), coping, depression, and social support. METHODS: Patients with malignant pleural mesothelioma (MPM) (N = 67) completed a battery of assessments at a single timepoint after being approached during routine medical oncology clinic appointments or by letter. RESULTS: Participants were predominately male (70.0%; n = 47) and ranged in age from 35 to 83 years old (M = 65.61, SD = 9.71). Most participants were white (88.0%; n = 59), and 10.0% (n = 7) were identified as Hispanic. The majority were married or living with a partner (93.0%; n = 62) and had some college or more education (64.0%; n = 43). Fourteen percent of participants (n = 11) endorsed significantly elevated depression symptoms. No significant demographic or clinical differences in depressed compared to nondepressed participants were observed, with a trend toward those identifying as Hispanic and those who were divorced as being more likely to be depressed. For the total sample, the most frequently endorsed coping strategies were active coping, emotional support, and acceptance. SIGNIFICANCE OF RESULTS: The present study did not identify any clear correlates of depression or QOL among patients with MPM. This research contributes to the small literature on psychosocial functioning in patients with MPM and provides putative directions for future larger studies and the development of interventions to provide appropriate support to diverse patients with MPM.

Meta-Analysis on the Combination of Chemotherapy With Programmed Death-Ligand 1 and Programmed Cell Death Protein 1 Blockade as First-Line Treatment for Unresectable Pleural Mesothelioma.

INTRODUCTION: Dual immune checkpoint blockers regimen represents a standard first-line therapy in unresectable pleural mesothelioma (PM). Novel combination strategies, including immune checkpoint blockers and antiangiogenic drugs, are currently under investigation in this setting. We aimed to assess the efficacy of the chemoimmunotherapy combination by reference to literature evidence. METHODS: A systematic review and meta-analysis of trials with first-line platinum-based chemotherapy associated with programmed death-ligand 1 and programmed cell death protein 1 agent in unresectable PM. We estimated the weighted summary proportion of disease response, along with the landmark probability of survival outcomes. RESULTS: A total of 349 patients with unresectable PM from four trials (DREAM, PrE0505, JME-001, and IND.227) were included, 79% (n = 274) with epithelioid and 21% (n = 75) with nonepithelioid histologic type. In aggregate, the objective response rate was 59.2% (95% confidence interval [CI]: 50.3%-67.9%) and disease control rate was 92.2% (95% CI: 89.2%-94.8%). Comparing epithelioid versus nonepithelioid tumors, the objective response rate was 64.5% versus 46.4%, (p < 0.001) and the disease control rate was 92.3% versus 80.0%, (p = 0.043), with an OR of 2.56 (95% CI: 1.51-4.32) for disease response and of 3.37 (95% CI: 0.99-11.47) for disease control. The aggregated estimated probability of progression-free survival was 63% (95% CI: 53%-71%) at 6 months and 25% (95% CI: 21%-31%) at 12 months, whereas the 6-, 12- and 24-month overall survival rates were 88% (95% CI: 81%-93%), 71% (95% CI: 61%-79%) and 39% (95% CI: 34%-45%), respectively. CONCLUSIONS: According to our analysis, first-line chemoimmunotherapy holds promise as a new treatment approach for PM, exhibiting encouraging survival outcomes and an enhanced response rate, including for the epithelioid subtype. Ongoing studies are necessary to establish its precise placement within the treatment algorithm.

Mesothelioma in situ of the peritoneum: report of three cases and review of the literature.

AIM: Diagnosis of mesothelioma in situ (MIS) is historically controversial and, until recently, specific features defining the entity have not been well characterized. Most reported cases of MIS occurred in the pleura; peritoneal MIS is very rare. This study investigates the morphologic features and results of ancillary testing in peritoneal MIS. METHODS: We present three patients with peritoneal MIS, as defined by a single layer of mesothelial cells with loss of nuclear BRCA-1-associated protein-1 (BAP1) immunostaining and without evidence of invasive tumour by microscopic evaluation, imaging, or direct examination of the peritoneum. Histology and immunostains were reviewed by three expert thoracic pathologists with multidisciplinary input. Next-generation sequencing (NGS) was performed in all three cases. A literature review was conducted to characterize this rare precursor lesion. RESULTS: BAP1 was lost in all three lesions, while methylthioadenosine phosphorylase (MTAP) was retained in two (not performed in the third). NGS revealed BAP1 pathogenic alterations in all three cases as well as mutations of SMO, ERCC3, TET2, and U2AF1. Progression to invasive mesothelioma occurred in one patient at 13 months postdiagnosis (case 1). One patient was diagnosed at age 24 and was later found to harbour a BAP1 germline mutation (case 3). CONCLUSION: This work describes the histologic features and clinicopathologic characteristics of peritoneal MIS in three cases, highlights BAP1 somatic and germline mutations in peritoneal MIS, and strengthens the importance of ancillary studies (including immunohistochemical and molecular studies) in the diagnosis of MIS.