Reproducibility of Malignant Pleural Mesothelioma Histopathologic Subtyping

Highlighting the immunohistochemical differences of malignant mesothelioma subtypes via case presentations

Malignant mesothelioma (MM) is a rare tumor of mesothelial cells, with an increasing incidence both in developed and developing countries. MM has three major histological subtypes, in order of frequency, according to the World Health Organization (WHO) Classification of 2021: epithelioid, biphasic, and sarcomatoid MM. Distinction may be a challenging task for the pathologist, due to the unspecific morphology. Here, we present two cases of diffuse MM subtypes to emphasize the immunohistochemical (IHC) differences, and to facilitate diagnostic difficulties. In our first case of epithelioid mesothelioma, the neoplastic cells showed cytokeratin 5/6 (CK5/6), calretinin, and Wilms-tumor-1 (WT1) expression, while remaining negative with thyroid transcription factor-1 (TTF-1). BRCA1 associated protein-1 (BAP1) negativity was seen in the neoplastic cells' nucleus, reflecting loss of the tumor suppressor gene. In the second case of biphasic mesothelioma, expression of epithelial membrane antigen (EMA), CKAE1/AE3, and mesothelin was observed, while WT1, BerEP4, CD141, TTF1, p63, CD31, calretinin, and BAP1 expressions were not detected. Due to the absence of specific histological features, the differentiation between MM subtypes could be a challenging task. In routine diagnostic work, IHC may be the proper method in distinction. According to our results and literature data, CK5/6, mesothelin, calretinin, and Ki-67 should be applied in subclassification.

New Insights in Pleural Mesothelioma Classification Update: Diagnostic Traps and Prognostic Implications

The 2021 WHO Classification of Tumors of the Pleura has introduced significant changes in mesothelioma codification beyond the three current histological subtypes-epithelioid, sarcomatoid and biphasic. Major advances since the 2015 WHO classification include nuclear grading and the introduction of architectural patterns, cytological and stromal features for epithelioid diffuse mesothelioma. Mesothelioma in situ has been recognized as a diagnostic category. Demonstration of loss of BAP1 or MTAP by immunohistochemistry, or CDKN2A homozygous deletion by FISH, is valuable in establishing the diagnosis of epithelioid mesothelioma. Recent emerging data proved that grading and histological subtypes have prognostic implications and may be helpful to patient risk stratification and clinical management. Nevertheless, the latest mesothelioma classification increases the already non-negligible diagnostic pitfalls, especially concerning differential diagnosis of pre-invasive tumors. In this review, recent changes in histologic classification of mesothelioma and advances in molecular markers are presented and their relation to diagnostic challenges and prognostic implications is discussed.

Adenomatoid mesothelioma arising from the diaphragm: a case report and review of the literature

BACKGROUND

Adenomatoid mesothelioma is a rare subtype of malignant mesothelioma that can be confused with adenomatoid tumors, which are classified as benign. The clinical features and optimal management of adenomatoid mesothelioma have not been elucidated in the literature. In this report, we present an extremely rare case of adenomatoid mesothelioma that developed on the peritoneal surface of the diaphragm as well as a literature review of adenomatoid mesothelioma in the abdominal cavity.

CASE PRESENTATION

The patient was a 61-year-old Japanese woman who had undergone resection of a malignant peripheral nerve sheath tumor of the hand 18 years prior. She was diagnosed with clinical stage I lung adenocarcinoma on follow-up chest radiography. Simultaneously, a 20-mm enhancing nodule with slow growth on the right diaphragm was detected on contrast-enhanced computed tomography. She presented no specific clinical symptoms. At this point, the lesion was suspected to be a hypervascular tumor of borderline malignancy, such as a solitary fibrous tumor. After a left upper lobectomy for lung adenocarcinoma, she was referred to our department, and laparoscopic tumor resection was performed. Adenomatoid tumors were also considered based on the histopathological and immunohistochemical analyses, but we made the final diagnosis of adenomatoid mesothelioma using the results of the genetic profile. The patient remains alive, with no recurrence noted 6 months after surgery.

CONCLUSION

We encountered a valuable case of adenomatoid mesothelioma of peritoneal origin. There are some previously reported cases of adenomatoid mesothelioma and adenomatoid tumors that may need to be recategorized according to the current classification. It is important to accumulate and share new findings to clarify the clinicopathological characteristics and genetic status of adenomatoid mesothelioma.

Pleural mesothelioma classification-update and challenges

Mesothelial tumors are classified into benign or preinvasive tumors, and mesotheliomas. The benign or preinvasive group includes adenomatoid tumors, well-differentiated papillary mesothelial tumors, and mesothelioma in situ. Malignant tumors are mesotheliomas and can be localized or diffuse. Histological classification of invasive mesotheliomas into three major subtypes-epithelioid, sarcomatoid, and biphasic is prognostically important. It also plays a significant role in the treatment decisions of patients diagnosed with this deadly disease. Grading and subtyping of epithelioid mesotheliomas have been one of the major changes in the recent WHO classification of pleural tumors. Mesothelioma in situ has emerged as a precisely defined clinico-pathologic entity that for diagnosis requires demonstration of loss of BAP1 or MTAP by immunohistochemistry, or CDKN2A homozygous deletion by FISH. The use of these two biomarkers improves the diagnostic sensitivity of effusion specimens  and limited tissue samples and is valuable in establishing the diagnosis of epithelioid mesothelioma. In this review, recent changes in the histologic classification of pleural mesothelioma, importance of ancillary diagnostic studies, and molecular characteristics of mesotheliomas are discussed.

Molecular characterization of pleomorphic mesothelioma: a multi-institutional study

The molecular alterations of pleomorphic mesotheliomas are largely unknown. In the present study, we performed whole-exome sequencing (WES) on 24 pleomorphic mesotheliomas in order to better characterize the molecular profile of this rare histologic variant. BAP1 protein expression and CDKN2A deletion by FISH were also evaluated. Significantly mutated genes included BAP1 (35%), NF2 (13%), LATS2 (8%), TP53 (5%), and LATS1 (3%). BAP1 alterations most frequently co-occurred with deletions of chromosomes 4, 9, and 13. Other important genetic alterations in pleomorphic mesotheliomas included truncating mutations in NF2 (3 of 24; 12.5%), LATS2 (2 of 24; 8%), TP53 (1 of 24; 4%), and PBRM1 (1 of 24; 4%). Focal losses of chromosome 9p21 were most common copy number alterations (11 of 24 cases; 46%), and were assessed by WES and targeted FISH. The second most common were deletions of chromosome 4 (8 of 24; 33% pleomorphic mesotheliomas). Three cases of pleomorphic mesothelioma did not show any mutations, copy number alterations, or LOH. This first WES analysis of pleomorphic mesotheliomas did not identify novel or unique mutations. In contrast to transitional mesothelioma that was reclassified as sarcomatoid variant based on transcriptome data, pleomorphic mesotheliomas are molecularly heterogeneous and therefore their reclassification into single subtype is more difficult.

Comparison of Nuclear Grade, Necrosis, and Histologic Subtype Between Biopsy and Resection in Pleural Malignant Mesothelioma: An International Multi-Institutional Analysis

OBJECTIVES

Numerous studies on malignant mesothelioma (MM) highlight the prognostic importance of histologic subtype, nuclear grade, and necrosis. This study compares these parameters in paired biopsy and resection specimens of pleural MM.

METHODS

Histologic subtype, percentage of epithelioid morphology, nuclear grade, and the presence or absence of necrosis were compared in 429 paired biopsies and resection specimens of pleural MM from 19 institutions.

RESULTS

Histologic subtype was concordant in 81% of cases (κ = 0.58). When compared with resection specimens, epithelioid morphology at biopsy had a positive predictive value (PPV) of 78.9% and a negative predictive value (NPV) of 93.5%; sarcomatoid morphology showed high PPV (92.9%) and NPV (99.3%), and biphasic morphology PPV was 89.7% and NPV was 79.7%. Agreement of the percentage of epithelioid morphology was fair (κ = 0.27). Nuclear grade and necrosis were concordant in 75% (κ = 0.59) and 81% (κ = 0.53) of cases, respectively. Nuclear grade showed moderate (κ = 0.53) and substantial (κ = 0.67) agreement from patients with and without neoadjuvant therapy, respectively, and necrosis showed moderate (κ = 0.47 and κ = 0.60) agreement, respectively, in the same subsets of paired specimens.

CONCLUSIONS

Paired biopsy-resection specimens from pleural MM show overall moderate agreement in pathologic parameters. These findings may help guide postbiopsy management and triage of patients with MM.

Epidemiology and outcome of peritoneal and pleural mesothelioma subtypes in Norway. A 20 year nation-wide study

BACKGROUND

Mesothelioma of the pleural or peritoneal cavities is one of the deadliest cancer types. The incidence of pleural subtypes has decreased over time due to decrease in asbestos exposure, and the current treatment landscape is changing due to introduction of novel therapies. In this study we have analysed contemporary epidemiological data of mesothelioma on a national level before the advent of immunotherapy.

MATERIAL AND METHODS

Complete national data on 1509 pleural and peritoneal malignant mesothelioma from the Cancer Registry of Norway from 2000 to 2019 are presented. Age standardised incidence and median survival were calculated.

RESULTS

The age-standardised incidence of pleural mesothelioma among males has decreased from 1.7 per 100 000 in 2000-2004 to 1.1 in 2015-2019, whereas the incidence for females has been stable, lower than 0.3 per 100 000 throughout the period. Incidence of peritoneal mesotheliomas remained low, below 0.08 per 100 000. The female to male ratio among pleural mesotheliomas was 1:7 with no differences among morphological subtypes, whereas this ratio was 1:1.2 in peritoneal mesotheliomas. Median age at diagnosis for pleural mesothelioma was 73 years and 76 years for females and males respectively in the last 5-year period, and 67 years for peritoneal mesotheliomas of both sexes. Median survival among pleural mesotheliomas has been stable, with significantly worse prognosis among sarcomatoid subtype (5.4 months) compared to epithelioid subtype (15.8 months). Peritoneal mesothelioma of the epithelioid subtype, representing 38% of cases, had a median survival of 43.3 months, contrasting the non-epithelioid subtype of 5.1 months.

DISCUSSION

Mesothelioma is still a significant disease with a dismal prognosis. Improvement in treatment is warranted.

Malignant Pleural Mesothelioma Epidemiology in the United States From 2000 to 2016

Introduction

Pleural mesothelioma constitutes about 80% of all mesotheliomas. The peak incidence of malignant mesothelioma estimated using the cancer registries was in early 1990 to 2000 in the United States. The disease is primarily associated with asbestos exposure. The latency period between asbestos exposure and the development of malignant pleural mesothelioma (MPM) can range anywhere from 15 to 60 years. Asbestos exposure was peaked during the industrial revolution and World War II due to military and shipyard exposures. It is often difficult for the pathologist to distinguish different histological subtypes; due to the disease's rarity and the inadequate tissue sample obtained. There is no available data on the difference in epidemiology of different subtypes of MPM. Surveillance Epidemiology and End Results (SEER), cancer incidence data include population-based registries covering approximately 34.6% of the U.S. population. Here in our study, we analyze malignant pleural mesothelioma epidemiology in the United States, emphasizing different histological subtypes.

Methods

SEER data from 2000 to 2016 was used in our study. The primary site of cancer is selected as pleura, and malignant behavior only is selected as the filter. Data were analyzed using the SEER stat program. Overall epidemiology of MPM and epidemiology of epithelioid, fibrous, and biphasic histological subtypes were analyzed separately. We used annual percentage change (APC) to evaluate the trend in the epidemiology of MPM.

Results summary

A total of 11,857 cases of MPM were included in the primary cohort from the SEER 18 registry from 2000 to 2016. The total prevalence of MPM was highest in 2009 and was lowest in 2016. The APC in MPM incidence during this period is -2.0. After removing 5,989 cases with non-specified histology during the same period, the APC for each histological type is -0.7 for fibrous type, 1.8 for epithelioid type, and 2.9 for biphasic type. Out of 17 regional registries included in the study, the greatest statistically significant change in APC was seen in the Hawaiian registry -4.1. In contrast, the lowest statistically significant difference was seen in Seattle (Puget Sound) registry -1.7. The APC in the incidence of MPM among males during the study period was -2.4 while that of females was -0.9. The Iowa registry showed a statistically significant increase in APC of the epithelioid malignant mesothelioma with a statistically insignificant reduction in the overall MPM APC.

Conclusion

The overall incidence of MPM in the United States is declining, while the data showed an increase in the incidence of epithelioid and biphasic histological subtypes. The authors believe that these conflicting results can be attributed to improved histological diagnosis and improved biopsy techniques.

Pathology of Malignant Pleural Mesothelioma

Despite multiple diagnostic toolkits, the diagnosis of diffuse malignant pleural mesothelioma relies primarily on proper histologic assessment. The definitive diagnosis of diffuse malignant pleural mesothelioma is based on the pathologic assessment of tumor tissue, which can be obtained from core biopsy sampling, pleurectomy, or other more extensive resections, such as extrapleural pneumonectomy. Given its rarity and overlapping microscopic features with other conditions, the histologic diagnosis of diffuse malignant pleural mesothelioma is challenging. This review discusses the pathologic features and the differential diagnosis of diffuse malignant pleural mesothelioma, including select diagnostic pitfalls.

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.

Presence of pleomorphic features but not growth patterns improves prognostic stratification of epithelioid malignant pleural mesothelioma by 2-tier nuclear grade

AIMS

Nuclear grade has been recently validated as a powerful prognostic tool in epithelioid malignant pleural mesothelioma (E-MPM). In other studies histological parameters including pleomorphic features and growth patterns were also shown to exert prognostic impact. The primary aims of our study are (i) externally validate the prognostic role of pleomorphic features in E-MPM and (ii) investigate if evaluating growth pattern in addition to 2-tier nuclear grade improves prognostication.

METHODS AND RESULTS

614 consecutive cases of E-MPM from our institution over a period of 15 years were retrospectively reviewed, of which 51 showed pleomorphic features. E-MPM with pleomorphic features showed significantly worse overall survival compared to those without (5.4 versus 14.7 months). Tumours with predominantly micropapillary pattern showed the worst survival (6.2 months) followed by solid (10.5 months), microcystic (15.3 months), discohesive (16.1 months), trabecular (17.6 months) and tubulo-papillary (18.6 months). Sub-classification of growth patterns into high grade (solid, micropapillary) and low grade (all others) led to good separation of overall survival (10.5 versus 18.0 months) but did not predict survival independent of 2-tier nuclear grade. A composite score comprised of growth pattern and 2-tier nuclear grade did not improve prognostication compared with nuclear grade alone. Intra-tumoural heterogeneity in growth patterns is ubiquitous.

CONCLUSIONS

Our findings support the incorporation of E-MPM with pleomorphic features in the epithelioid subtype as a highly aggressive variant distinct from 2-tier nuclear grade. E-MPM demonstrates extensive heterogeneity in growth pattern but its evaluation does not offer additional prognostic utility to 2-tier nuclear grade.

Update on the pathologic diagnosis of malignant mesothelioma

Malignant mesothelioma is an uncommon tumor that may be difficult to diagnose. The International Mesothelioma Interest Group has been writing guidelines for pathological diagnosis that are periodically updated. The guidelines are being updated based on published literature in the last 3 years, and experience of more than 20 leading international pathologists in the field who will be co-authors. Updates were discussed by attendees of the Pulmonary Pathology Society Biennial Meeting (Dubrovnik, Croatia, June 2019). Areas with significant advancements/changes include utilization of immunohistochemistry (establishing mesothelial lineage and benign versus malignant), prognosis and nuclear grading, biphasic malignant mesothelioma, transitional pattern, malignant mesothelioma in situ, and therapeutic/molecular targets.

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.

Deep learning-based classification of mesothelioma improves prediction of patient outcome

Malignant mesothelioma (MM) is an aggressive cancer primarily diagnosed on the basis of histological criteria¹. The 2015 World Health Organization classification subdivides mesothelioma tumors into three histological types: epithelioid, biphasic and sarcomatoid MM. MM is a highly complex and heterogeneous disease, rendering its diagnosis and histological typing difficult and leading to suboptimal patient care and decisions regarding treatment modalities². Here we have developed a new approach-based on deep convolutional neural networks-called MesoNet to accurately predict the overall survival of mesothelioma patients from whole-slide digitized images, without any pathologist-provided locally annotated regions. We validated MesoNet on both an internal validation cohort from the French MESOBANK and an independent cohort from The Cancer Genome Atlas (TCGA). We also demonstrated that the model was more accurate in predicting patient survival than using current pathology practices. Furthermore, unlike classical black-box deep learning methods, MesoNet identified regions contributing to patient outcome prediction. Strikingly, we found that these regions are mainly located in the stroma and are histological features associated with inflammation, cellular diversity and vacuolization. These findings suggest that deep learning models can identify new features predictive of patient survival and potentially lead to new biomarker discoveries.

Application of a Neural Network Whole Transcriptome-Based Pan-Cancer Method for Diagnosis of Primary and Metastatic Cancers

IMPORTANCE

A molecular diagnostic method that incorporates information about the transcriptional status of all genes across multiple tissue types can strengthen confidence in cancer diagnosis.

OBJECTIVE

To determine the practical use of a whole transcriptome-based pan-cancer method in diagnosing primary and metastatic cancers and resolving complex diagnoses.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional diagnostic study assessed Supervised Cancer Origin Prediction Using Expression (SCOPE), a machine learning method using whole-transcriptome RNA sequencing data. Training was performed on publicly available primary cancer data sets, including The Cancer Genome Atlas. Testing was performed retrospectively on untreated primary cancers and treated metastases from volunteer adult patients at BC Cancer in Vancouver, British Columbia, from January 1, 2013, to March 31, 2016, and testing spanned 10 822 samples and 66 output classes representing untreated primary cancers (n = 40) and adjacent normal tissues (n = 26). SCOPE's performance was demonstrated on 211 untreated primary mesothelioma cancers and 201 treatment-resistant metastatic cancers. Finally, SCOPE was used to identify the putative site of origin in 15 cases with initial presentation as cancers with unknown primary of origin.

RESULTS

A total of 10 688 adult patient samples representing 40 untreated primary tumor types and 26 adjacent-normal tissues were used for training. Demographic data were not available for all data sets. Among the training data set, 5157 of 10 244 (50.3%) were male and the mean (SD) age was 58.9 (14.5) years. Testing was performed on 211 patients with untreated primary mesothelioma (173 [82.0%] male; mean [SD] age, 64.5 [11.3] years); 201 patients with treatment-resistant cancers (141 [70.1%] female; mean [SD] age, 55.6 [12.9] years); and 15 patients with cancers of unknown primary of origin; among the treatment-resistant cancers, 168 were metastatic, and 33 were the primary presentation. An accuracy rate of 99% was obtained for primary epithelioid mesotheliomas tested (125 of 126). The remaining 85 mesotheliomas had a mixed etiology (sarcomatoid mesotheliomas) and were correctly identified as a mixture of their primary components, with potential implications in resolving subtypes and incidences of mixed histology. SCOPE achieved an overall mean (SD) accuracy rate of 86% (11%) and F1 score of 0.79 (0.12) on the 201 treatment-resistant cancers and matched 12 of 15 of the putative diagnoses for cancers with indeterminate diagnosis from conventional pathology.

CONCLUSIONS AND RELEVANCE

These results suggest that machine learning approaches incorporating multiple tumor profiles can more accurately identify the cancerous state and discriminate it from normal cells. SCOPE uses the whole transcriptomes from normal and tumor tissues, and results of this study suggest that it performs well for rare cancer types, primary cancers, treatment-resistant metastatic cancers, and cancers of unknown primary of origin. Genes most relevant in SCOPE's decision making were examined, and several are known biological markers of respective cancers. SCOPE may be applied as an orthogonal diagnostic method in cases where the site of origin of a cancer is unknown, or when standard pathology assessment is inconclusive.