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

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.

Establishing mesothelioma patient-derived organoid models from malignant pleural effusions

OBJECTIVES

Pleural mesothelioma is a cancer arising in the cells that line the lungs and chest wall with poor survival and poor response to first-line therapy. Organoid models of cancer can faithfully recapitulate the genetic and histopathological characteristics of individualized tumors and have potential to be used for precision medicine, however methods of establishing patient-derived mesothelioma organoids have not been well established in the published literature.

MATERIALS AND METHODS

Long-term mesothelioma patient-derived organoids were established from ten malignant pleural effusion fluids. Mesothelioma patient-derived organoids were compared to the corresponding biopsy tissue specimens using immunohistochemistry labelling for select diagnostic markers and the TruSight Oncology-500 sequencing assay. Cell viability in response to the chemotherapeutic drug cisplatin was assessed.

RESULTS

We established five mesothelioma patient-derived organoid cultures from ten malignant pleural effusion fluids collected from nine individuals with pleural mesothelioma. Mesothelioma patient-derived organoids typically reflected the histopathological and genomic features of patients' matched biopsy specimens and displayed cytotoxic sensitivity to cisplatin in vitro.

CONCLUSION

This is the first study of its kind to establish long-term mesothelioma organoid cultures from malignant pleural effusions and report on their utility to test individuals' chemotherapeutic sensitivities ex vivo.

Reliably making the primary diagnosis of mesothelioma utilizing serous fluid cytology specimens: an institutional experience

INTRODUCTION

The diagnosis of mesothelioma has historically been challenging, especially on serous fluid cytology (SFC). Distinguishing between reactive and neoplastic mesothelial cells can be difficult on cytomorphology alone. However, additional ancillary tests, such as BRCA1 associated protein-1 immunohistochemistry and fluorescence in situ hybridization for cyclin-dependent kinase inhibitor 2A deletion, can provide a sensitive and highly specific method of proving malignancy.

MATERIALS AND METHODS

SFC specimens diagnosed as mesothelioma, suspicious for mesothelioma (SM), and atypical mesothelial cells (AMCs) since 2012 were identified by querying the laboratory information system. Clinical data and pathologic parameters were gathered.

RESULTS

One hundred ten cases of mesothelioma, SM, and AMC were identified. Of these, 61 cases had a definitive diagnosis of mesothelioma on SFC. Average age at SFC diagnosis was 67 years (26-87 years), with most patients being male (67%). Out of the 61 cases, 11 cases (18%) had an initial diagnosis of mesothelioma made on SFC specimens, with 5 of these 11 cases being in patients that never received a histologic diagnosis of mesothelioma. Ancillary studies were utilized in all 11 cases. An initial diagnosis of metastatic mesothelioma was made on SFC in 9 cases (15%). For 6 of these 9 cases, the SFC diagnosis was the sole diagnosis of metastatic mesothelioma without a companion histologic diagnosis. In addition, 15 cases were diagnosed as SM, with 11 of these cases following a definitive mesothelioma diagnosis. Thirty-four cases were diagnosed as AMC, with 27 cases following a definitive mesothelioma diagnosis.

CONCLUSIONS

The diagnosis of mesothelioma can be reliably made on SFC with the appropriate cytomorphology criteria and/or confirmatory ancillary testing.

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.

New developments in mesothelial pathology

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