Malignant Mesothelioma In Situ: Clinical and Pathologic Implications

Pleural mesothelioma in situ : a comprehensive review

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

[Benign mesothelial tumors]

Benign mesothelial tumors are rarer than malignant mesotheliomas and are often found in the peritoneum as incidental findings in women. They include the adenomatoid tumor (AT), the well-differentiated mesothelial tumor (WDPMT), the mesothelioma in situ (MIS), and the solid papillary mesothelial tumor (SPMT). ATs are always benign and predominantly manifest in the genital tract. WDPMTs can develop multifocally and are prone to recurrence, particularly in the case of incomplete resection. Only MISs are considered a confirmed precursor lesion of malignant mesothelioma according to the currently valid World Health Organization (WHO) classifications. As with malignant mesothelioma, alterations of BAP1, MTAP, and p16 are detectable for MIS in contrast to the other three tumors. SPMTs cannot be clearly assigned to the other mesothelial tumors and have so far only been described in the peritoneum in women with a benign course.

The International Association for the Study of Lung Cancer Mesothelioma Staging Project: Proposals for Revisions of the "T" Descriptors in the Forthcoming Ninth Edition of the TNM Classification for Pleural Mesothelioma

INTRODUCTION

The primary tumor (T) component in the eighth edition of pleural mesothelioma (PM) staging system is based on pleural involvement and extent of invasion. Quantitative assessment of pleural tumor has been found to be prognostic. We explored quantitative and qualitative metrics to develop recommendations for T descriptors in the upcoming ninth edition of the PM staging system.

METHODS

The International Association for the Study of Lung Cancer prospectively collected data on patients with PM. Sum of maximum pleural thickness (Psum) was recorded. Optimal combinations of Psum and eighth edition cT descriptors were assessed using recursive binary splitting algorithm, with bootstrap resampling to correct for the adaptive nature of the splitting algorithm, and validated in the eighth edition data. Overall survival (OS) was calculated by the Kaplan-Meier method and differences in OS assessed by the log-rank test.

RESULTS

Of 7338 patients submitted, 3598 were eligible for cT analysis and 1790 had Psum measurements. Recursive partitioning identified optimal cutpoints of Psum at 12 and 30 mm, which, in combination with extent of invasion, yielded four prognostic groups for OS. Fmax greater than 5 mm indicated poor prognosis. cT4 category (based on invasion) revealed similar performance to eighth edition. Three eighth edition descriptors were eliminated based on low predictive accuracy. Eighth edition pT descriptors remained valid in ninth edition analyses.

CONCLUSION

Given reproducible prognostication by Psum, size criteria will be incorporated into cT1 to T3 categories in the ninth edition. Current cT4 category and all pT descriptors will be maintained, with reclassification of fissural invasion as pT2.

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.

Mesothelioma in the pleura, pericardium and peritoneum: Recommendations from the International Collaboration on Cancer Reporting (ICCR)

AIMS

Mesothelioma is a rare malignancy of the serosal membranes that is commonly related to exposure to asbestos. Despite extensive research and clinical trials, prognosis to date remains poor. Consistent, comprehensive and reproducible pathology reporting form the basis of all future interventions for an individual patient, but also ensures that meaningful data are collected to identify predictive and prognostic markers.

METHODS AND RESULTS

This article details the International Collaboration on Cancer Reporting (ICCR) process and the development of the international consensus mesothelioma reporting data set. It describes the 'core' and 'non-core' elements to be included in pathology reports for mesothelioma of all sites, inclusive of clinical, macroscopic, microscopic and ancillary testing considerations. An international expert panel consisting of pathologists and a medical oncologist produced a set of data items for biopsy and resection specimens based on a critical review and discussion of current evidence, and in light of the changes in the 2021 WHO Classification of Tumours. The commentary focuses particularly upon new entities such as mesothelioma in situ and provides background on relevant and essential ancillary testing as well as implementation of the new requirement for tumour grading.

CONCLUSION

We recommend widespread and consistent implementation of this data set, which will facilitate accurate reporting and enhance the consistency of data collection, improve the comparison of epidemiological data, support retrospective research and ultimately help to improve clinical outcomes. To this end, all data sets are freely available worldwide on the ICCR website (www.iccr-cancer.org/data-sets).

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.

The Role of Germline Mutations in Thoracic Malignancies: Between Myth and Reality

Considering the established contribution of environmental factors to the development of thoracic malignancies, the inherited susceptibility of these tumors has rarely been explored. However, the recent introduction of next-generation sequencing-based tumor molecular profiling in the real-word setting enabled us to deeply characterize the genomic background of patients with lung cancer with or without smoking-related history, increasing the likelihood of detecting germline mutations with potential prevention and treatment implications. Pathogenic germline variants have been detected in 2% to 3% of patients with NSCLC undergoing next-generation sequencing analysis, whereas the proportion of germline mutations associated with the development of pleural mesothelioma widely varies across different studies, ranging between 5% and 10%. This review provides an updated summary of emerging evidence about germline mutations in thoracic malignancies, focusing on pathogenetic mechanisms, clinical features, therapeutic implications, and screening recommendations for high-risk individuals.

The cytological features of effusions with mesothelioma in situ: A report of 9 cases

INTRODUCTION

The diagnosis of mesothelioma in situ (MIS) is now accepted by the WHO as a pre-invasive neoplastic mesothelial proliferation and considered a diagnosis based on histologic evaluation only. Although the definition of MIS includes recurrent effusions, little is known about the cytologic features of such effusions. Since mesothelioma is usually diagnosed at an advanced stage and has a poor prognosis, early detection of a neoplastic mesothelial population in such effusions can potentially have a positive impact on the management of such a dire disease.

MATERIALS AND METHODS

We reviewed a total of 18 pleural effusions from nine patients with recurrent effusions. Of these, five patients had follow-up biopsies diagnosed as MIS and the remaining four cases had negative radiology and malignant cytology proven by molecular markers (BAP1, MTAP or CDKN2A deletion) and at least 1 year follow-up with no overt mass identified by radiology.

RESULTS

Initial effusions may mimic reactive mesothelial hyperplasia or exhibit atypia. As effusions recur, the cellularity and atypia increase and the mesothelial proliferation becomes morphologically indistinguishable from mesothelioma. Molecular alterations diagnostic of mesothelioma can be detected in these effusions, even in the initial-benign/reactive appearing ones. The cellularity and atypia detected in such effusions surpassed those noted on the biopsies, raising questions regarding the cause of such discrepancy.

CONCLUSION

The diagnosis of MIS can be suspected based on malignant effusion cytology supported by molecular alterations. We propose that the proliferation of neoplastic mesothelial clones represent a clinically silent "liquid phase MIS stage" corresponding to in situ stage in other organs.

Clinical and molecular validation of BAP1, MTAP, P53, and Merlin immunohistochemistry in diagnosis of pleural mesothelioma

BAP1 and MTAP immunostains play an important role in diagnosis of mesothelioma, but additional markers are needed to increase sensitivity. We analyzed 84 pleural mesotheliomas (51 epithelioid, 27 biphasic, 6 sarcomatoid) by a hybrid-capture next-generation sequencing (NGS) panel including complete coverage of coding and splicing regions for BAP1, CDKN2A/MTAP, NF2, and TP53 and correlated molecular findings with diagnostic immunostains for BAP1, MTAP, Merlin, and p53, respectively. Fifty-seven reactive mesothelial proliferations served as benign comparators. Loss of BAP1, MTAP, and Merlin protein expression were, respectively, 54%, 46%, and 52% sensitive and 100% specific for mesothelioma. Two-marker immunopanels of BAP1 + MTAP, BAP1 + Merlin, and MTAP + Merlin were 79%, 85%, and 71% sensitive for mesothelioma, while a three-marker immunopanel of BAP1 + MTAP + Merlin was 90% sensitive. Diffuse (mutant-pattern) p53 immunostaining was seen in only 6 (7%) tumors but represented the only immunohistochemical abnormality in 2 cases. Null-pattern p53 was not specific for malignancy. An immunopanel of BAP1 + MTAP + Merlin + p53 was 93% sensitive for mesothelioma, and panel NGS detected a pathogenic alteration in BAP1, MTAP, NF2, and/or TP53 in 95%. Together, 83 (99%) of 84 tumors showed a diagnostic alteration by either immunohistochemistry or panel NGS. Adding Merlin to the standard BAP1 + MTAP immunopanel increases sensitivity for mesothelioma without sacrificing specificity. p53 immunohistochemistry and panel NGS with complete coverage of BAP1, CDKN2A/MTAP, TP53, and NF2 may be useful in diagnostically challenging cases.

Sneaky Diagnosis of Pleural Malignant Mesothelioma in Thoracic Surgery: All That Glitters Is Not Gold

Malignant Pleural Mesothelioma (MPM) is a highly aggressive disease whose diagnosis could be challenging and confusing. It could occur with atypical presentations on every examined level. Here, we present three unconventional cases of the complex diagnostic process of MPM that we have experienced during routine practice: a patient with reactive mesothelial hyperplasia mimicking MPM, an unexpected presentation of MPM with persistent unilateral hydropneumothorax, a rare case of MPM in situ. Then, we review the relevant literature on each of these topics. Definitive biomarkers to confidently distinguish MPM from other pleural affections are still demanded. Patients presenting with persistent hydropneumothorax must always be investigated for MPM. MPM in situ is now a reality, and this raises questions about its management.

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.