Lymphohistiocytoid mesothelioma: a rare lymphomatoid variant of predominantly sarcomatoid mesothelioma

Characterizing the distribution of alterations in mesothelioma and their correlation to morphology

OBJECTIVES

Mesothelioma is a lethal disease that arises from the serosal lining of organ cavities. Several recurrent alterations have been observed in pleural and peritoneal -mesotheliomas, including in BAP1, NF2, and CDKN2A. Although specific histopathologic parameters have been correlated with prognosis, it is not as well known whether genetic alterations correlate with histologic findings.

METHODS

We reviewed 131 mesotheliomas that had undergone next-generation sequencing (NGS) at our institutions after pathologic diagnosis. There were 109 epithelioid mesotheliomas, 18 biphasic mesotheliomas, and 4 sarcomatoid mesotheliomas. All our biphasic and sarcomatoid cases arose in the pleura. Of the epithelioid mesotheliomas, 73 were from the pleura and 36 were from the peritoneum. On average, patients were 66 years of age (range, 26-90 years) and predominantly male (92 men, 39 women).

RESULTS

The most common alterations identified were in BAP1, CDKN2A, NF2, and TP53. Twelve mesotheliomas did not show a pathogenic alteration on NGS. For epithelioid mesotheliomas in the pleura, the presence of an alteration in BAP1 correlated with low nuclear grade (P = .04), but no correlation was found in the peritoneum (P = .62). Similarly, there was no correlation between the amount of solid architecture in epithelioid mesotheliomas and any alterations in the pleura (P = .55) or peritoneum (P = .13). For biphasic mesotheliomas, cases with either no alteration detected or with an alteration in BAP1 were more likely to be epithelioid predominant (>50% of the tumor, P = .0001), and biphasic mesotheliomas with other alterations detected and no alteration in BAP1 were more likely to be sarcomatoid predominant (>50% of the tumor, P = .0001).

CONCLUSIONS

This study demonstrates a significant association between morphologic features associated with a better prognosis and an alteration in BAP1.

The 2021 World Health Organization Classification of Tumors of the Pleura: Advances since the 2015 Classification

Significant changes in the 2021 World Health Organization (WHO) Classification of Tumors of the Pleura and Pericardium since the 2015 WHO Classification include: 1) Pleural and pericardial tumors have been combined in one chapter whereas in the 2015 WHO, pericardial tumors were classified with cardiac tumors; 2) Well-differentiated papillary mesothelioma (WDPM) has been renamed well-differentiated papillary mesothelial tumor (WDPMT) given growing evidence that these tumors exhibit relatively indolent behavior; 3) Localized and diffuse mesothelioma no longer include the term "malignant" as a prefix; 4) Mesothelioma in situ (MIS) has been added to the 2021 classification since these lesions can now be recognized by loss of BAP1 and/or MTAP by immunohistochemistry and/or CDKN2A homozygous deletion by FISH; 5) The three main histologic subtypes (i.e. epithelioid, biphasic and sarcomatoid) remain the same but architectural patterns, cytologic and stromal features are more formally incorporated into the 2021 classification based on their prognostic significance; 6) Nuclear grading for epithelioid diffuse mesothelioma is introduced and it is recommended to record this and other histologically prognostic features in pathology reports; 7) BAP1, EZH2 and MTAP immunohistochemistry have been shown to be useful in separating benign mesothelial proliferations from mesothelioma; 8) Biphasic mesothelioma can be diagnosed in small biopsies showing both epithelioid and sarcomatoid components even if the amount of one component is less than 10%; and 9) The most frequently altered genes in diffuse pleural mesothelioma include BAP 1, CDKN2A, NF2, TP53, SETD2 and SETB1.

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.

Clinical significance of histologic subtyping of malignant pleural mesothelioma

Malignant mesothelioma (MM) is uncommon, but very aggressive tumor arising from the mesothelial cells of pleura, pericardium, peritoneum, and tunica vaginalis. Despite multimodality treatments 5-year survival is only 5% after the diagnosis. Histology and TNM staging system are still the best prognostic factors. Furthermore, histologic subtype of MM determines the clinical management of the patients. According to the 2015 WHO classification, MM is divided into diffuse, localized and well differentiated papillary mesothelioma. Major histologic subtypes of diffuse MM, namely epithelioid, biphasic and sarcomatoid, have different prognosis. However, in the last decade it has become evident that more detailed subclassification and histologic/cytological characterization of MM have prognostic and perhaps predictive implications. In this review, major histologic subtypes and cytological features of MM are presented and their relation with prognosis and predictive biomarkers is discussed.

Guidelines for Pathologic Diagnosis of Malignant Mesothelioma 2017 Update of the Consensus Statement From the International Mesothelioma Interest Group

CONTEXT

- Malignant mesothelioma (MM) is an uncommon tumor that can be difficult to diagnose.

OBJECTIVE

- To provide updated, practical guidelines for the pathologic diagnosis of MM.

DATA SOURCES

- Pathologists involved in the International Mesothelioma Interest Group and others with an interest and expertise in the field contributed to this update. Reference material included up-to-date, peer-reviewed publications and textbooks.

CONCLUSIONS

- There was discussion and consensus opinion regarding guidelines for (1) distinguishing benign from malignant mesothelial proliferations (both epithelioid and spindle cell lesions), (2) cytologic diagnosis of MM, (3) recognition of the key histologic features of pleural and peritoneal MM, (4) use of histochemical and immunohistochemical stains in the diagnosis and differential diagnosis of MM, (5) differentiating epithelioid MM from various carcinomas (lung, breast, ovarian, and colonic adenocarcinomas, and squamous cell and renal cell carcinomas), (6) diagnosis of sarcomatoid MM, (7) use of molecular markers in the diagnosis of MM, (8) electron microscopy in the diagnosis of MM, and (9) some caveats and pitfalls in the diagnosis of MM. Immunohistochemical panels are integral to the diagnosis of MM, but the exact makeup of panels employed is dependent on the differential diagnosis and on the antibodies available in a given laboratory. Depending on the morphology, immunohistochemical panels should contain both positive and negative markers for mesothelial differentiation and for lesions considered in the differential diagnosis. Immunohistochemical markers should have either sensitivity or specificity greater than 80% for the lesions in question. Interpretation of positivity generally should take into account the localization of the stain (eg, nuclear versus cytoplasmic) and the percentage of cells staining (>10% is suggested for cytoplasmic and membranous markers). Selected molecular markers are now being used to distinguish benign from malignant mesothelial proliferations. These guidelines are meant to be a practical diagnostic reference for the pathologist; however, some new pathologic predictors of prognosis and response to therapy are also included.

Challenges and controversies in the diagnosis of malignant mesothelioma: Part 2. Malignant mesothelioma subtypes, pleural synovial sarcoma, molecular and prognostic aspects of mesothelioma, BAP1, aquaporin-1 and microRNA

Pleural malignant mesothelioma (MM) includes several unusual and even rare but distinctive histological subtypes, in addition to the usual subdivision into epithelioid, biphasic and sarcomatoid MM. Criteria for discrimination between fibrous pleuritis versus desmoplastic mesothelioma include evidence of neoplastic invasion for diagnosis of desmoplastic MM, but this histological assessment is complicated by the recently-described 'fake fat phenomenon' in cases of fibrous pleuritis. The distinction between biphasic and monophasic synovial sarcoma of the pleura versus biphasic and sarcomatoid MM can be problematical and is most cogently based upon molecular detection of the t(X;18) translocation, whereas a clear diagnosis of MM for a pleural tumour histologically resembling synovial sarcoma is favoured by a negative result for this translocation and, probably, microRNA evidence supportive of a diagnosis of MM. Aquaporin-1 (AQP1) is a molecule involved in the growth of MM cells, and yet is a factor reported to correlate with improved survival rates for MM with an epithelioid component, in comparison to AQP1-poor MM, as assessed from AQP1 expression by epithelioid MM cells only (apart from co-expression by stromal endothelial cells in addition to the tumour cells). Recent reports have also focused upon germline mutations in the BRCA1-associated protein 1 (BAP1), not only in cases of familial mesothelioma, but also BAP1 deletion in sporadic MM. Prognostic factors for MM include not only the histological subtypes, but other independent variables that include (among others), AQP1 expression by mesothelioma cells, the clinical status of the patient, the serum neutrophil:lymphocyte ratio and blood thrombocytosis.

Intracavitary therapeutics for pleural malignancies

Pleural malignancies are ideal for novel therapeutic approaches because they are invariably fatal. Intrapleural (IP) chemotherapy has only marginal benefit in pleural malignancies, but may prove efficacious with hyperthermic chemotherapy administered in combination with maximal tumor debulking. IP immunotherapies may be most effective in those patients with early-stage pleural malignancy, and may prove superior to standard pleurodesis methods in control of effusion and prolongation of survival. Immunogene therapy may be unable to successfully treat bulky tumors on its own, but success may be achieved with combination approaches that combine debulking surgery and chemotherapy with IP genetic immunotherapy.

Guidelines for pathologic diagnosis of malignant mesothelioma: 2012 update of the consensus statement from the International Mesothelioma Interest Group

CONTEXT

Malignant mesothelioma (MM) is an uncommon tumor that can be difficult to diagnose.

OBJECTIVE

To provide updated practical guidelines for the pathologic diagnosis of MM.

DATA SOURCES

Pathologists involved in the International Mesothelioma Interest Group and others with an interest in the field contributed to this update. Reference material includes peer-reviewed publications and textbooks.

CONCLUSIONS

There was consensus opinion regarding (1) distinction of benign from malignant mesothelial proliferations (both epithelioid and spindle cell lesions), (2) cytologic diagnosis of MM, (3) key histologic features of pleural and peritoneal MM, (4) use of histochemical and immunohistochemical stains in the diagnosis and differential diagnosis of MM, (5) differentiation of epithelioid MM from various carcinomas (lung, breast, ovarian, and colonic adenocarcinomas, and squamous cell and renal cell carcinomas), (6) diagnosis of sarcomatoid mesothelioma, (7) use of molecular markers in the diagnosis of MM, (8) electron microscopy in the diagnosis of MM, and (9) some caveats and pitfalls in the diagnosis of MM. Immunohistochemical panels are integral to the diagnosis of MM, but the exact makeup of panels used is dependent on the differential diagnosis and on the antibodies available in a given laboratory. Immunohistochemical panels should contain both positive and negative markers. It is recommended that immunohistochemical markers have either sensitivity or specificity greater than 80% for the lesions in question. Interpretation of positivity generally should take into account the localization of the stain (eg, nuclear versus cytoplasmic) and the percentage of cells staining (>10% is suggested for cytoplasmic membranous markers). These guidelines are meant to be a practical reference for the pathologist.

Diffuse Malignant Mesothelioma

Diffuse malignant mesothelioma (DMM) is an uncommon cancer with great clinical significance because it currently remains an incurable disease, and most patients die within months after diagnosis. Although DMM incidence is leveling off or decreasing in developed countries because of the strict control of asbestos use, it is increasing in countries without adequate asbestos control. In some settings, benign, reactive mesothelial hyperplasias and organizing pleuritis can be difficult to differentiate from DMM and vice versa, and the variety of DMM's histopathologic features generates an extensive list of differential diagnoses with other malignancies, particularly, metastatic malignancies, which are more frequent in the pleura than are primary malignancies. These two issues are the topic of discussion in this review, along with a brief presentation of a case of DMM that presented in a 66-year-old man with recurrent, right pleural effusions, and in whom, diagnosis of DMM had not been suspected clinically, radiographically, surgically, grossly, or initially, on frozen section. It was not until focal invasion into the skeletal muscle was discovered on permanent sections that a diagnosis of DMM could be established.

Lymphoproliferative conditions of the serosa

CONTEXT

A diverse and complex variety of lymphoproliferative diseases may involve the serosa, with widely differing clinical outcomes encompassing a spectrum of benign and malignant conditions.

OBJECTIVE

To review lymphoproliferative disease involving the serosa and to provide a practical approach to the evaluation of lymphoid and plasma cell infiltrates in the serosa, together with a review of various tumors and tumorlike conditions that may mimic lymphoproliferative disease.

DATA SOURCES

Analysis of published literature.

CONCLUSIONS

All forms of hematologic malignancy may involve the various serosal sites, although this is usually observed as secondary involvement in persons with known lymph nodal, marrow-based, or extranodal disease. Primary pericardial, pleural, and peritoneal lymphomas are rare; many nonneoplastic conditions may mimic lymphoma and a variety of nonhematolymphoid tumors may simulate hematologic malignancies. An understanding of the role of ancillary tests, together with an appreciation of their limitations, will prevent misdiagnosis.

Lymphohistiocytoid mesothelioma of the pleura: a case report with cytological findings

Lymphohistiocytoid malignant mesothelioma is an infrequent variant of sarcomatoid mesothelioma representing approximately 0.5-3.3% of malignant mesotheliomas. It has been related to asbestos exposure. The tumor is characterized by a diffuse large histiocyte-like cells proliferation mixed with an inflammatory infiltrate of lymphocytes and plasma cells. Its cytological diagnosis is difficult. We present a case of a 67-year-old female with lymphohistiocytoid mesothelioma involving the left pleura. The cytological, histological, and immunohistochemical features are discussed.

Lymphohistiocytoid mesothelioma of the pleura

Lymphohistiocytoid mesothelioma (LHM), reported to be a rare variant of sarcomatoid mesothelioma, is challenging to differentiate from non-Hodgkin's lymphoma due to marked lymphocytic infiltration. To aid accurate recognition of LHM, we examined immunohistochemical, in situ hybridization (ISH) of Epstein-Barr virus RNA (EBER-1) mRNA, fluorescence ISH (FISH) for homozygous deletion of 9p21, and asbestos analysis in four cases (three men and 1 woman). Three patients died, while Case 4 was still alive 19 months after extrapleural pneumonectomy. Histologically, these cases were characterized by heavy lymphocytic infiltration. All neoplastic cells were positive for calretinin, AE1/AE3, and epithelial membrane antigen, but negative for CEA. EBER1 factor was negative. FISH analysis demonstrated homozygous deletion of the 9p21 locus in three of the four cases. In Case 1: (i) autopsy findings showed mesothelioma primarily located in the right parietal pleura, but metastasized into the left lung and abdominal organs; (ii) the histological findings at autopsy indicated sarcomatoid mesothelioma; and (iii) we found asbestos bodies and fibers in extracts from lung tissue (Cases 1 & 4) using digestion with bleaching fluid. LHM, an infrequent variant of sarcomatoid mesothelioma, displayed homozygous deletion of the 9p21 locus (three of four cases), and has a relatively favorable prognosis for the sarcomatoid type.

Sarcomatoid mesothelioma: a clinical-pathologic correlation of 326 cases

Sarcomatoid mesothelioma is the least common, but most aggressive of the three major histological types of mesotheliomas. This study comprises 326 cases of sarcomatoid mesotheliomas among 2000 consecutive malignant mesothelioma cases received in consultation (16%). Patients included 312 men (96%) and 14 women (4%), with a median age of 70 years (range 41-94 years). Most tumors were pleural (319; 98%), and 7 were peritoneal (2%). Some desmoplastic features were identified in 110 cases (34%), and 70 (21%) were classified as desmoplastic. Rare subtypes included two cases with a lymphohistiocytoid pattern (<1%) and eight heterologous mesotheliomas (2%). Labeling for cytokeratins (CKs) was observed in 261/280 cases (93%), and for calretinin and vimentin in 31 and 91%, respectively. Pleural plaques were present in 79% of cases for which information was available, and asbestosis was diagnosed in 34/127 cases (27%). Median survival was 3.5 months. Fiber analysis was performed in 61 cases. The median asbestos body count was 1640/g wet lung tissue (by light microscopy). Amosite fibers were the most commonly identified fibers using energy-dispersive X-ray analysis and were significantly higher in the sarcomatoid cases, as were uncoated fibers using scanning electron microscopy. This study represents the largest series of sarcomatoid and desmoplastic malignant mesotheliomas to date and confirms the diagnostic usefulness of CK immunohistochemistry. The relationship with asbestos exposure--particularly amosite--and an association with pleural plaques and less often asbestosis is confirmed.

Guidelines for pathologic diagnosis of malignant mesothelioma: a consensus statement from the International Mesothelioma Interest Group

CONTEXT

Malignant mesothelioma (MM) is an uncommon tumor that can be difficult to diagnose.

OBJECTIVE

To develop practical guidelines for the pathologic diagnosis of MM.

DATA SOURCES

A pathology panel was convened at the International Mesothelioma Interest Group biennial meeting (October 2006). Pathologists with an interest in the field also contributed after the meeting.

CONCLUSIONS

There was consensus opinion regarding (1) distinguishing benign from malignant mesothelial proliferations (both epithelioid and spindle cell lesions), (2) cytologic diagnosis of MM, (3) key histologic features of pleural and peritoneal MM, (4) use of histochemical and immunohistochemical stains in the diagnosis and differential diagnosis of MM, (5) differentiating epithelioid MM from various carcinomas (lung, breast, ovarian, and colonic adenocarcinomas and squamous cell and renal cell carcinomas), (6) diagnosis of sarcomatoid mesothelioma, (7) use of molecular markers in the differential diagnosis of MM, (8) electron microscopy in the diagnosis of MM, and (9) some caveats and pitfalls in the diagnosis of MM. Immunohistochemical panels are integral to the diagnosis of MM, but the exact makeup of panels used is dependent on the differential diagnosis and on the antibodies available in a given laboratory. Immunohistochemical panels should contain both positive and negative markers. The International Mesothelioma Interest Group recommends that markers have either sensitivity or specificity greater than 80% for the lesions in question. Interpretation of positivity generally should take into account the localization of the stain (eg, nuclear versus cytoplasmic) and the percentage of cells staining (>10% is suggested for cytoplasmic membranous markers). These guidelines are meant to be a practical reference for the pathologist.

Problems in mesothelioma diagnosis

Many centres are now seeing increasing numbers of patients with malignant mesothelioma. This presents pathologists involved in making the diagnosis with a number of problems, which can be divided into those encountered in making the distinction between mesothelioma and benign changes and those experienced in separating mesotheliomas from other types of epithelial and connective tissue tumours. Immunohistochemistry plays a major role in helping to make the diagnosis, but it should be interpreted with due regard to the clinical setting and radiological features, and with a knowledge of the wide morphological variations seen in mesothelioma. This review identifies some of these problems and addresses the uses and limitations of immunohistochemistry in different situations. It includes a discussion of some of the less common variants of mesothelioma and other pleural-based tumours that enter into the differential diagnosis.

Malignant mesothelioma with heterologous elements: clinicopathological correlation of 27 cases and literature review

Only a small number of malignant mesotheliomas with heterologous elements have been described. There are currently no criteria for diagnosis and little data regarding prognosis. We suggest that the term heterologous mesothelioma should be reserved for tumours that show malignant heterologous elements, notably osteosarcomatous, chondrosarcomatous, or rhabdomyoblastic elements but have immunohistochemical and clinical characteristics of mesothelioma. We identified 27 such cases and characterized the clinical and pathological characteristics of these tumours. In our series, 89% originated in the pleura, and 11% from the peritoneal cavity. The median age at diagnosis was 68 years, ranging from 27 to 85 years. Of these cases, 93% occurred in males and 7% in women. Of the 27 mesothelioma cases 16 (59%) were sarcomatoid, 10 (37%) were biphasic, and one was reported as epithelioid; 40% (11 cases) showed osteosarcomatous elements only, 19% showed areas of rhabdomyosarcoma only, 19% contained areas of chondrosarcoma only, and 22% exhibited osteochondromatous elements. Immunohistochemical labelling for cytokeratins was present in the majority of cases. Exposure to asbestos was identified in all the 17 cases for which an exposure history was available (63%). Median survival was 6 months after diagnosis, similar to the survival seen in sarcomatoid mesotheliomas. The differential diagnosis includes primary and secondary pleural sarcomas, including osteosarcomas and chondrosarcomas. Immunohistochemical labelling for cytokeratins is helpful in the distinction, but lack of labelling for cytokeratins in a spindle cell/sarcomatoid tumour does not exclude the diagnosis of mesothelioma, irrespective of the presence of heterologous elements. We suggest that if the anatomical distribution conforms to that of mesothelioma, a diagnosis of heterologous mesothelioma should be made in preference to a diagnosis of primary pleural osteosarcoma or chondrosarcoma, regardless of cytokeratin positivity, as for conventional non-heterologous sarcomatoid mesothelioma.

Localized malignant mesothelioma in the middle mediastinum: report of a case

Localized malignant mesothelioma in the mediastinum is rare and most known cases have been reported as "localized malignant pericardial mesothelioma." We report a case of a middle mediastinal tumor, which we were able to resect completely. Histopathological examination of the tumor confirmed that it was a malignant epithelial lymphohistiocytoid mesothelioma. We assumed that the tumor was derived from the pericardium. Local recurrence was detected 1 year after resection, and the patient died of the disease about 2 years later.

Lymphohistiocytoid Variant of Malignant Mesothelioma of the Pleura: A Series of 22 Cases

The lymphohistiocytoid variant of diffuse malignant mesothelioma is rare with very few cases described in the literature. It is characterized by mesothelial cells with a histiocytelike appearance and an associated dense lymphoid infiltrate. We studied clinicopathologic features and immunohistochemical patterns of a series of 22 cases. The histiocytelike cells had a mesothelial immunophenotype: AE1/AE3 (100%), calretinin (100%), CK5/6 (46%), and EMA (52%). The prominent lymphoid component showed a cytotoxic T-cell immunophenotype. Prognosis was similar to that of a large series of epithelioid diffuse malignant mesotheliomas. Formely, it was classified within the sarcomatoid type. We suggest that it should be reclassified as an epithelioid variant because of its similar behavioural characteristics. There was no evidence of Epstein-Barr virus-related infection.

My approach to the diagnosis of mesothelial lesions

Mesothelial lesions pose considerable diagnostic challenges not only because benign tumours, reactive proliferations and malignant mesothelioma can mimic one another, but also because the morphological patterns displayed by malignant mesothelioma can simulate a variety of epithelial and non-epithelial malignancies. Immunohistochemical markers can aid in distinguishing epithelioid malignant mesothelioma from metastatic adenocarcinoma, but because no single marker reliably separates all cases, a panel of stains is recommended. Immunohistochemical studies are of more limited value in sarcomatoid malignant mesothelioma, and other features often play an essential role. The separation of reactive mesothelial proliferations from malignant mesothelioma on small biopsy can be quite difficult, as distinguishing features, such as stromal invasion, often cannot be adequately assessed. In adequately sampled lesions, however, the distinction between malignant mesothelioma, benign mesothelial proliferations and other tumours can be achieved in most cases by using a carefully integrated approach that incorporates clinical and radiographic data, immunohistochemical studies and, in selected cases, histochemical and ultrastructural techniques.

Lymphohistiocytoid mesothelioma: a clinical, immunohistochemical and ultrastructural study of four cases and literature review

Lymphohistiocytoid mesothelioma (LHM) is a term proposed to designate a variant of mesothelioma that assumes a histiocytoid morphology and contains intense lymphocytic infiltrates. Reports on this variant are few, and its clinicopathologic and ultrastructural characteristics are still to be defined. The authors have studied 4 such cases that were identified among 120 mesotheliomas in the electron microscopy (EM) files of a single institution between 1982 and 2002. Histologically, all 4 lesions were composed of diffuse proliferations of cells with a histiocytoid appearance, admixed with an intense lymphocytic infiltrate. All 4 were associated with an unequivocal sarcomatoid component. Immunohistochemical (IHC) studies showed that the majority of histiocytoid cells were positive for CD68 and vimentin in all 4 cases, suggesting fibrohistiocytic differentiation. Immunoreactivity for calretinin and AE1:AE3 was only very focally identified in 3 of 4 cases. All cases were negative for CEA (M), Leu-M1, and B72.3. The lymphocytic component in all 4 cases was almost entirely composed of CD3- and CD8-positive, small, mature T cells, with only a minor component of CD20-positive cells and occasional eosinophils. Although all cases contained numerous CD68 positive atypical cells, co-expression of CD68 and either calretinin or keratin by individual cells was difficult to demonstrate by immunohistochemistry. Ultrastructurally, 3 of the 4 cases demonstrated very focal mesothelial differentiation as evidenced by long and slender surface microvilli, including the case with negative immunoreactivity for calretinin and cytokeratin. Review of the literature yielded 6 additional LHM cases. Analysis of all 10 cases showed a male predominance (8:2) with a mean age of 58 years (31-73 years). All 10 cases involved the pleura. Three of 10 patients had known asbestos exposure. Six of 10 patients died of disease at 2-20 months after the diagnosis (mean, 6.9 months). The findings suggest that LHM is a distinct morphological variant of sarcomatoid mesothelioma for which mesothelial differentiation is difficult to document. Many of the cells composing these tumors exhibit fibrohistiocytic differentiation. The unusual morphological pattern of LHM makes a combined modality approach, including IHC, EM, and a knowledge of the clinical/radiologic findings, essential in achieving a correct diagnosis.

Pathology of mesothelioma

In this article, progress in distinguishing mesothelioma from mesothelial hyperplasia and fibrosing pleuritis was discussed. Advances in the immunohistochemical characterization of mesothelioma, in the recognition of entities that mimic mesothelioma, and in their distinction from mesothelioma were reviewed. Cytogenetic and molecular genetic contributions to the diagnosis of pleural synovial sarcoma were briefly summarized. The diagnosis of epithelial type mesothelioma can be established in most cases. Several mesotheliomas, however, especially rare subtypes, and sarcomatoid, desmoplastic, and poorly differentiated mesotheliomas continue to present diagnostic challenges.

‘Pseudomesotheliomatous’ carcinomas of the pleura: a 10-year analysis of cases from the Environmental Lung Disease Research Group, Cardiff

AIMS

To undertake a clinicopathological study of diffuse serosal neoplasms of epithelial histogenesis which clinically and pathologically mimic malignant pleural mesothelioma.

METHODS AND RESULTS

Over a 10-year (1990-2000) study period 53 carcinomas mimicking diffuse pleural mesothelioma ('pseudomesotheliomatous' carcinoma) were identified. The study group comprised 50 men and three females, age range 33-77 (median 68) years. In 46 (87%) cases there was a history of smoking and in 40 (76%) cases a history of asbestos exposure. Histologically the pleural 'pseudomesotheliomatous' carcinomas could be divided into two broad groups: primary pulmonary carcinomas with florid pleurotropic growth (n = 47), of which 34 (70%) were adenocarcinomas; and diffuse carcinomatous involvement of the pleura by metastatic tumour (n = 6). This latter group comprised two transitional cell carcinomas of bladder, one renal (clear) cell carcinoma, one ductal pancreatic adenocarcinoma, one prostatic adenocarcinoma and one squamous cell carcinoma of parotid gland origin. Follow-up data were available in 35 cases. Regardless of tumour type, survival was poor (median 8 months) and comparable to diffuse pleural mesothelioma.

CONCLUSIONS

Pleural 'pseudomesotheliomatous' carcinomas are uncommon (comprising 6% of referrals), pathologically heterogeneous tumours with poor prognosis. Tissue diagnosis should be obtained in all cases of suspected diffuse pleural neoplasia. By light microscopy and immunophenotype many of the tumours mimicked malignant mesothelioma. In particular, an awareness that all neoplasms exhibiting squamous differentiation may express cytokeratin 5/6 and thrombomodulin is important to prevent misinterpretation. In this respect, calretinin is regarded as the most specific and sensitive mesothelial marker. Misdiagnosis may have medico-legal implications in asbestos-related compensation claims.

Primary thymic epithelial tumours of the pleura mimicking malignant mesothelioma

AIMS

To illustrate the macroscopic, light microscopic and immunophenotypic similarities that exist between primary pleural thymic epithelial tumours and diffuse malignant mesothelioma. To investigate the expression of the mesothelial markers, cytokeratin (CK) 5/6, calretinin and thrombomodulin in a series of mediastinal thymic epithelial tumours.

METHODS AND RESULTS

Over a 10-year period, 64 diffuse pleural tumours of non-mesothelial histogenesis were identified in the files of referrals to the South Wales regional thoracic centre (Llandough Hospital, Cardiff). Of these, five pleural tumours were diagnosed as primary pleural thymic epithelial neoplasms. From the files of the Mesopath group, Caen, three additional cases of thymic epithelial tumours with pleural involvement were identified. The study group comprised eight cases (four males, four females) with median age at presentation of 56 years (range 19-75 years). In one case there was a history of asbestos exposure. Macroscopically, seven tumours formed diffuse pleural masses. No mediastinal abnormality or intraparenchymal lesions were seen in five cases. By light microscopy, seven thymic epithelial neoplasms showed a lobulated architecture, one appeared extensively cystic. The tumours were of varied morphological subtypes: one medullary (WHO Type A), two mixed (WHO Type AB), three predominantly cortical (WHO Type B1) and two cortical (WHO Type B1). The subtypes morphologically mimicked sarcomatoid, biphasic, lymphohistiocytoid variant and epithelioid mesothelioma. The pleural thymic epithelial tumours showed immunoreactivity with broad spectrum cytokeratin AE1/AE3 (8/8; 100%), CK5/6 (8/8; 100%), and 1/8 (13%) expressed thrombomodulin. Calretinin showed variable nuclear and cytoplasmic expression in all cases, but equivocally in the thymic epithelial cell component. In 7/8 (88%) the thymic epithelial cells exhibited focal aberrant expression of CD20. Epithelial membrane antigen (EMA) showed focal expression in the perivascular and organoid areas in 6/8 (75%) cases. Carcinoembryonic antigen (CEA) and CD34 were uniformly negative. In 4/8 (50%) cases the lymphoid cell component was of immature phenotype expressing CD99, terminal deoxynucleotidyl transferase (TdT) and lymphoid precursors had a high proliferation fraction with Ki67. In the series of 20 primary mediastinal thymic epithelial tumours tested, mesothelial marker expression revealed CK5/6 (20/20), thrombomodulin (3/20; 15%) and calretinin (0/20; 0%). Varying amounts of calretinin-positive stromal cells were present.

CONCLUSION

Primary pleural thymic epithelial tumours are rare but may mimic malignant mesothelioma by forming diffuse serosal-based masses. In addition, both tumours may show morphological diversity (with epithelial, spindled and mixed components present). An awareness that thymic epithelial tumours may variably express the mesothelial markers CK5/6, calretinin and thrombomodulin prevents misdiagnosis. In the distinction from malignant mesothelioma a lobulated architecture and organoid features favour a thymic epithelial neoplasm. The presence of aberrant CD20 expression in a cytokeratin-positive epithelial neoplasm and/or the presence of an immature lymphoid population (by demonstration of CD1a, CD2, CD99 and TdT) indicates a thymic epithelial neoplasm. In contrast, nuclear calretinin expression favours malignant mesothelioma.

Localised spontaneous regression in mesothelioma -- possible immunological mechanism

Malignant mesothelioma (MM) is an aggressive tumor usually associated with asbestos exposure. Although it can remain stable for prolonged periods, it has not been described to spontaneously regress. MM tumors are thought to be immunogenic based both on animal studies and on the good responses in some humans treated with immunotherapy. Here we present a case of pleural MM in which a transient spontaneous regression was associated with tumor tissue infiltration with mononuclear cells and serological evidence of anti-MM reactivity. The patient's tumor eventually progressed and with this progression there was evidence of loss of serological reactivity to some, but not all, of her MM antigens. The patient survived for 20 months and, in contrast to her initial biopsy, no significant lymphoid infiltrate was detected in her MM tissue at post mortem examination.

Lymphohistiocytoid mesothelioma. An often misdiagnosed variant of sarcomatoid malignant mesothelioma

Three cases of lympho-histiocytoid mesothelioma, a rare variant of pleural sarcomatoid malignant mesothelioma, are described. Histologically, the neoplasms were characterized by a diffuse discohesive proliferation of atypical histiocytoid cells intermixed with a marked lymphocytic and lesser plasmacytic infiltrate. One case initially was misdiagnosed as a ganglioneuroma, a second case was misinterpreted as malignant lymphoma, and a third case was sent in consultation with the differential diagnosis of inflammatory pseudotumor vs mesothelioma. Immunohistochemical studies showed strong and generalized expression of cytokeratins and vimentin by the neoplastic histiocytoid cells in all 3 cases. Two cases were positive for calretinin, one of which also was positive for HBME-1, thrombomodulin, and LeuM1. None of the cases stained with the epithelial glycoprotein markers carcinoembryonic antigen, B72.3, and Ber-EP4, or the blood group antigen, BG-8. The immunophenotype of the lymphoplasmacytic infiltrate revealed predominantly reactive, mature T cells, with fewer polytypic plasma cells, histiocytes, and B cells. In lymphohistiocytoid mesothelioma, as in the usual examples of sarcomatoid mesothelioma, the demonstration of cytokeratin expression by the neoplastic cells is the most useful diagnostic finding that allows exclusion of other neoplasms with which this entity may be confused.

Advances in the treatment of malignant pleural mesothelioma

Malignant pleural mesothelioma is a neoplasm that is commonly fatal and for which there are no widely accepted curative approaches. Mesothelioma is unresponsive to most chemotherapy and radiotherapy regimens, and it typically recurs even after the most aggressive attempts at surgical resection. Multimodality approaches have been of some benefit in prolonging survival of very highly selected subgroups of patients, but they have had a relatively small impact on the majority of the patients diagnosed with this disease. As the incidence of pleural mesothelioma peaks in the United States and Europe over the next 10 to 20 years, new therapeutic measures will be necessary. This review will discuss the roles of chemotherapy, radiotherapy, surgery, and combined modality approaches in the treatment of pleural mesothelioma, as well as scientific advances made in the past decade that have led to the development of experimental techniques, such as photodynamic therapy, immunotherapy, and gene therapy, that are currently undergoing human clinical trials. These promising new avenues may modify the therapeutic nihilism that is rampant among clinicians dealing with mesothelioma.

Pleural mesothelioma: an approach to diagnostic problems

In the 1960s, a close relationship between heavy exposures to crocidolite asbestos and mesothelioma was established. The debate on the diagnosis of mesothelioma became complicated because of the possibility of litigation. Well differentiated mesothelioma cells are mucicarmine negative but alcian blue and periodic acid-Schiff (PAS) positive, which are removed by hyaluronidase and diastase digestion. By electron microscopy (EM), they show bush-like elongated, slender, and branching microvilli. By immunohistochemistry they express both keratin and vimentin but not carcinoembryonic antigenicity (CEA), B72.3, Ber-EP4, and Leu-M1. In poorly differentiated mesotheliomas, chromosomal and molecular biological alterations are common and complex but these alterations also overlap with that of poorly differentiated tumours of the lung and other organs. A poorly differentiated pleural tumour is most likely metastatic and needs good team work to locate the primary site. The diagnosis of a mesothelioma and asbestosis should be established separately. Future studies will be focused less on the phenotypic differences but more on the broad molecular and multi-phasic mechanisms of carcinogenesis, irrespective of the aetiological agents, in poorly differentiated tumours.

Diagnostic criteria for human diffuse malignant mesothelioma

Diffuse malignant mesothelioma is a rare tumor in the general population, yet is unique in that it is caused almost exclusively by exposure to asbestos with long-term latency (15 years and over). Pathologists are required to provide a reliable diagnosis of the tumor for clinicians who are responsible for the treatment of affected patients. Pathological diagnosis of diffuse malignant mesothelioma is not always easy; however, it has improved over the last few decades. Currently, comprehensive analysis, including gross appearance, histology, histochemistry, immunocytochemistry and electron microscopy is recommended as the best approach to an accurate diagnosis of diffuse malignant mesothelioma.

The histology and immunohistochemistry of small cell mesothelioma

The object of this study was to describe the histology and immunohistochemistry of 13 small cell mesotheliomas, concentrating on reliable distinctions between them and small cell carcinoma. All 13 tumours showed regions of more typical mesothelioma if multiple blocks were examined. No tumours showed the streams, ribbons, rosettes, or haematoxyphilic blood vessels that are typical of small cell carcinoma. Mitotic figures were relatively scarce and the nuclei had a particularly characteristic open appearance with prominent nucleoli and delicate chromatin. Nuclear moulding was not seen. No tumour produced neutral mucin. Immunohistochemical positivity for neuron-specific enolase (NSE) was found in 11/13, cytokeratin in 9/13 and Leu-7 in 4/13 but none was positive for chromogranin A, carcino-embryonic antigen (CEA) or leucocyte common antigen (LCA). We conclude that the accurate diagnosis of small cell mesothelioma is possible, provided that the clinical presentation is known, the tumour is adequately sampled and the microscopy carefully assessed. In small biopsy specimens, where the diagnosis is less straightforward, immunohistochemistry for CEA, and perhaps LCA and chromogranin A may be helpful. NSE and Leu-7 positivity is common in these tumours and might be misleading if interpreted as reliable evidence of neuroendocrine differentiation.

Malignant mesothelioma in the jewelry industry

We conducted a clinical, environmental, pathologic, and mineral lung burden investigation of a 61-year-old man with malignant mesothelioma. For 35 years, up until three weeks prior to pneumonectomy, the patient made asbestos soldering forms at a costume jewelry production facility. Only chrysotile asbestos was used at the plant during the last decade of the patient's employment, and recent environmental sampling of the work-place identified no other asbestos fiber type. Anticipating that the patient would add to the very small number of cases of mesothelioma attributable solely to chrysotile, we found instead that the patient's lung tissue contained large numbers of both coated and uncoated amosite asbestos fibers but, surprisingly, no chrysotile. We subsequently learned that a distributor of both chrysotile and amosite supplied the company during the first 25 years the patient was fabricating soldering forms. The findings underscore the futility of estimating environmental exposure to chrysotile on the basis of fiber counts in lung tissue. Although we previously described non-neoplastic asbestos-related disease among patients engaged in similar work, this case, to the best of our knowledge, represents the first report of mesothelioma in the commercial jewelry industry. As such, it prompted us to initiate a public health campaign to replace asbestos soldering forms in this industry with readily available, safer alternatives.

Malignant mesothelioma of peritoneum

A 57-year-old man with malaise, ascites, and abdominal pain was found to have a peritoneum studied with numerous, small nodular tumor masses. Light microscopy revealed an anaplastic malignant tumor of uncertain differentiation. Mucin stains were negative. Electron microscopy revealed pleomorphic tumor cells with diffusely distributed cytoplasmic tonofilaments and well-developed true desmosomes. No long, thin, branching microvilli were present, yet tumor cells were strongly positive for both callus keratin (polyclonal) and monoclonal cytokeratin (AE1/3) in a diffuse cytoplasmic distribution (a pattern corresponding to the diffuse cytoplasmic tonofilaments). Tumor cells were negative for Leu-M1 and carcinoembryonic antigen. The findings were most consistent with malignant mesothelioma, and additional questioning, after tissue diagnosis, revealed a work history of asbestos exposure.