Mesothelioma with rhabdoid features: an ultrastructural and immunohistochemical study of 10 cases

Primary pulmonary epithelioid hemangioendothelioma metastatic to the pleura and mediastinal lymph node with a prominent rhabdoid cytomorphology showing CAMTA1::WWTR1 fusion and novel PRDM1 and TNFRSF14 mutations

Cases of epithelioid hemangioendothelioma with WWTR1::CAMTA1 fusion can show rhabdoid cytomorphology. Lack of intracytoplasmic luminal spaces, marked rhabdoid cytomorphology, and variability in the expression of vascular markers makes the diagnosis of EHE challenging. Therefore, a high level of suspicion and ancillary studies (immunohistochemistry and next generation sequencing) help reach a definitive diagnosis in these cases.

Diffuse Pleural Mesothelioma: Advances in Molecular Pathogenesis, Diagnosis, and Treatment

Diffuse pleural mesothelioma (DPM) is a highly aggressive malignant neoplasm arising from the mesothelial cells lining the pleural surfaces. While DPM is a well-recognized disease linked to asbestos exposure, recent advances have expanded our understanding of molecular pathogenesis and transformed our clinical practice. This comprehensive review explores the current concepts and emerging trends in DPM, including risk factors, pathobiology, histologic subtyping, and therapeutic management, with an emphasis on a multidisciplinary approach to this complex disease.

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.

Peritoneal Mesothelioma-An Update

A remarkable amount of new information has been generated on peritoneal mesothelioma (PeM), ranging from nomenclature changes, including the removal of "malignant" when referring to this neoplasm and the use of the term "tumor" rather than "mesothelioma" to designate the neoplasm formerly known as "well-differentiated papillary mesothelioma", to the acknowledgment that PeMs can be associated with tumor predisposition syndromes or germline mutations. Although the disease is still more frequently seen in caucasian males, PeM is not uncommon in women. In addition, it can represent a diagnostic challenge when it has an uncommon presentation (ie, paraneoplastic syndrome or incidental finding) or when it has confounding histologic features. Ancillary testing, including immunohistochemical stains, in situ hybridization for CDKN2A or NF2 , and molecular studies, in selected cases, allows its correct diagnosis. The molecular landscape of PeM is still a work in progress; however, some findings, such as ALK gene rearrangements and EWSR1/FUS-ATF1 fusions, are specifically seen in PeM of young patients. The biological behavior of PeM is variable; however, cytoreductive surgery and hyperthermic intraperitoneal chemotherapy have markedly improved the survival of patients affected by this disease.

Continuous infiltration of small peritoneal macrophages in the mouse peritoneum through CCR2-dependent and -independent routes during fibrosis and mesothelioma development induced by a multiwalled carbon nanotube, MWNT-7

Although toxicities of multiwalled carbon nanotube (MWCNT) have been found to be related with activities of macrophages phagocytosing the fibers, the exact relationship between macrophage population and pathogenesis of fibrosis and mesotheliomas induced by MWCNTs is largely unknown. CCL2-CCR2 axis, a major monocyte/macrophage infiltration route, is thought to be involved in not only acute inflammation but also the formation of tumor microenvironment. We therefore described a time-course of alteration of macrophage population in an attempt to clarify the contribution of the Ccr2 gene to mesotheliomagenesis. Wild-type (WT) C57BL/6 mice and Ccr2-knockout (KO) mice were intraperitoneally administered with MWNT-7 and were sequentially necropsied at 1, 7, 28, 90, and 245 day(s) after the injection. Peritoneal fibrosis was prominent in all MWCNT-treated mice, with a lower severity in the KO mice. No differences were observed in the incidences of neoplastic lesions of mesothelia between WT and KO mice. A flow cytometric analysis revealed that after gross disappearance of macrophages after MWCNT exposure, small peritoneal macrophages (SPMs) were exclusively refurbished by the CCR2-dependent route at day 1 (as Ly-6C+MHC class II- cells), followed by additional CCR2-independent routes (as Ly-6C-MHC class II- cells); i.e., the only route in KO mice; with a delay of 1-7 days. The SPMs derived from both routes appeared to differentiate into maturated cells as Ly-6C-MHC class II+, whose ratio increased in a time-dependent manner among the total SPM population. Additionally, most macrophages expressed M1-like features, but a small fraction of macrophages exhibited an M1/M2 mixed status in MWCNT-treated animals. Our findings demonstrate a long-persistent activation of the CCL2-CCR2 axis after MWCNT exposure and enable a better understanding of the participation and potential roles of SPMs in fibrous material-induced chronic toxicities.

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.

Mesothelioma

Mesothelioma arises from the surface serosal cells lining the pleural, peritoneal, and pericardial cavities. It has three variants including: epithelioid, sarcomatous/desmoplastic, and biphasic types. Mesothelioma cells, predominantly of the epithelioid type, can shed into effusions as sheets, clusters/ morulae, papillae, or single cells. The challenges to cytologic diagnosis of mesothelioma are two-fold: 1. distinguishing mesothelial cells from metastatic malignant (most commonly carcinoma) cells; 2. distinguishing reactive mesothelial from mesothelioma cells. Immunocytochemistry is a helpful aid to cytologic evaluation for the former. The distinction of reactive mesothelial cells from mesothelioma can be more difficult, as there is considerable overlap in their appearances in effusion specimens. Recently developed ancillary molecular and genetic tests are proving to be useful in confirming the diagnosis of malignant mesothelioma in cytology specimens.

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.

SMARCB1/INI1/BAF47- deficient pleural malignant mesothelioma with rhabdoid features

Malignant mesothelioma (MM) with rhabdoid features is an MM variant. Fifteen cases have been reported previously, all of which were combined with other types of MM. Herein, we report an autopsy case of pleural MM with monomorphic rhabdoid features. The patient was a 62-year-old male without a history of asbestos exposure. An autopsy revealed a soft, granular tumor that replaced the entire left pleura and had invaded to the diaphragm and lower lobe of the lung. The tumor cells, which had eosinophilic plump cytoplasm and eccentric nuclei, were loosely cohesive. Immunohistochemistry showed that the cells were diffusely positive for calretinin, D2-40, vimentin, CAM5.2, and AE1/AE3; and negative for WT-1, TTF-1, CK7, CEA, desmin, CD34, BCL-2, S100 protein, and p40. Neither homozygous deletion of p16 nor BAP-1 protein loss was observed. Loss of INI1/BAF47 protein, an indicator of malignant rhabdoid tumor, was observed. Therefore, MM with rhabdoid features was confirmed.

Clinicopathologic Characteristics of Malignant Mesotheliomas Arising in Patients With a History of Radiation for Hodgkin and Non-Hodgkin Lymphoma

Purpose

Studies have reported an association between pleural diffuse malignant mesothelioma (PDMM) and chest radiation for lymphoma. The clinicopathologic characteristics of malignant mesotheliomas arising in these patients have not been established.

Patients and Methods

We studied 1,618 consecutive patients diagnosed with pleural PDMM from July 1993 to February 2008 and identified patients with a history of radiation for Hodgkin and non-Hodgkin lymphoma. We evaluated the histology in the surgical resection specimens and compared clinicopathologic features with overall survival.

Results

We identified 22 patients who developed PDMM after chest radiation as part of their treatment for lymphoma (mean latency time, 21.4 years; 95% CI, 17.0 to 25.8 years). Asbestos bodies in lymphoma-associated PDMM were lower than in asbestos-associated PDMM (median count, 15 v 325 bodies, respectively; P < .001) and similar to an unexposed control group (median count, 15 v 10 bodies, respectively; P = .6). Seventeen lymphoma-associated PDMMs (77%) were epithelioid and five (23%) were biphasic. Seven PDMMs (32%) had unusual histologies (pleomorphic, myxoid, clear cell, and signet ring cell). Patients with lymphoma-associated PDMM were younger than patients with asbestos-associated PDMM (median age, 45 v 64 years, respectively; P < .001) and had a significantly longer overall survival time (median, 32.5 v 12.7 months, respectively; P = .018). In multivariate analysis, independent favorable predictors for overall survival were history of prior radiation (P = .022), female sex (P < .001), age ≤ 65 years (P = .005), cytoreductive surgery (P < .001), and epithelioid histology (P < .001).

Conclusion

Patients with lymphoma-associated PDMM are likely to have unusual histologic features, are significantly younger, and seem to have a longer overall survival compared with patients with asbestos-associated PDMM.

Primary rhabdoid adrenocortical carcinoma: a clinicopathological and immunohistochemical study of three cases

AIMS

We describe three cases of primary adrenocortical carcinoma (ACC) with prominent rhabdoid features and study their clinicopathological and immunohistochemical properties.

METHODS AND RESULTS

The patients were three females aged 45-55 years (mean 49 years), none of whom had any hormone-related symptoms. Histologically, the characteristic features were rhabdoid tumour cells with prominent eosinophilic cytoplasmic inclusions, eccentric vesicular nuclei and prominent nucleoli. Areas of more conventional ACC were present in all cases. Immunohistochemically, the rhabdoid component showed a similar staining pattern to conventional ACC, being positive for steroid receptor cofactor 1, inhibin, melan A, calretinin and synaptophysin, while negative for high molecular weight cytokeratin and Pax8. In addition, antimitochondrial antibodies showed a weak reaction and nuclear expression of integrase interactor 1 (INI1) was preserved. Clinical follow-up information for two patients demonstrated that one patient was alive 237 months after diagnosis, and one patient had died of disease 4 months after diagnosis.

CONCLUSIONS

ACC with rhabdoid features are rare tumours that seem to mirror the clinical and immunohistochemical features of conventional ACC. In addition, they appear to display similar biological behaviour. Adrenocortical carcinoma should be included in the differential diagnosis when evaluating tumours of unknown origin with rhabdoid morphology.

Mesothelioma with signet-ring cell features: report of 23 cases

Signet-ring cell mesothelioma is uncommon and only two case reports have been published on this mesothelioma variant, both of which were initially misdiagnosed as signet-ring cell carcinoma. Herein are reported 23 signet-ring cell mesotheliomas that were investigated by immunohistochemistry, 12 of which were also studied by electron microscopy. Twenty-one of the cases originated in the pleura and two in the peritoneum. For comparison purposes and in order to determine the value of these techniques in the differential diagnosis of these tumors, seven cases of signet-ring cell lung adenocarcinoma were also studied. All signet-ring cell mesotheliomas were positive for calretinin, keratin 5/6, keratin 7, and mesothelin, 93% for podoplanin, and 91% for WT1; whereas, none reacted for MOC-31, CEA, TAG-72, CD15, TTF-1, napsin A, or CDX2. Among signet-ring cell lung adenocarcinomas, 100% were positive for keratin 7, CEA, and napsin A, 86% each for TTF-1 and TAG-72, 71% for CD15, and 14% for mesothelin, while all were negative for calretinin, keratin 5/6, WT1, podoplanin, and CDX2. After analyzing the results, it is concluded that the panels of markers used in the differential diagnosis of this mesothelioma variant should include those markers that are usually expressed in mesotheliomas (eg, calretinin, keratin 5/6, WT1, and podoplanin), broad-spectrum carcinoma markers that are frequently expressed in adenocarcinomas regardless of their site of origin (eg, MOC-31 and CEA), and organ-associated markers (eg, TTF-1 and napsin A for lung), which allow the site of origin of a metastatic adenocarcinoma to be established. Electron microscopy can be very useful as it permits the identification of characteristic ultrastructural mesothelioma and adenocarcinoma markers, and it also allows a better understanding of the morphologic features seen on routine light microscopy. Pathologists should be aware of this mesothelioma subtype as it can potentially be confused with other tumors that exhibit signet-ring features.

Myoepithelial carcinoma of the breast with focal rhabdoid features

Myoepithelial carcinoma of the breast is extremely rare and only 33 cases have been reported in the English literature. Herein, we report a case of myoepithelial carcinoma of the breast with focal rhabdoid features. The patient was a 67-year-old woman, who presented with a lump of the left breast that rapidly grew to 3 cm in diameter within 3 months. Lumpectomy revealed a solid and whitish colored tumor, which was composed mainly of elongated spindle-shaped cells with mild atypia, focal necrosis, and infiltrative margin. In a small area of the lesion, ovoid tumor cells exhibited eccentric nuclei with centrally located nucleoli and plump cytoplasm including round eosinophilic inclusions, resembling a rhabdoid tumor. Immunohistochemically, both types of tumor cells exhibited a myoepithelial phenotype. MIB-1 index was 30%. The cytoplasmic inclusion of the ovoid cells exhibited immunopositivity for both vimentin and cytokeratin. From these findings, this tumor was diagnosed as a myoepithelial carcinoma with focal rhabdoid features. Although rhabdoid features have been reported in some types of malignant and benign tumors, this is the first report of such features in myoepithelial carcinoma of the breast.

Deciduoid mesothelioma: report of 21 cases with review of the literature

Deciduoid mesothelioma is a rare variant of epithelioid mesothelioma that was initially considered to occur exclusively in the peritoneum of young women who had no history of asbestos exposure and to be characterized by an aggressive clinical course, but it was later demonstrated that this tumor could also occur in the pleura of older men and women who had been exposed to asbestos. Some subsequent studies have also indicated that the clinical course is no different from that of conventional epithelioid mesothelioma. Herein are reported 21 cases of deciduoid mesothelioma that were investigated using a large panel of immunohistochemical markers, 9 of which were also studied by electron microscopy. Fifteen of the patients were male and 6 were female (mean age, 60 years). Seventeen of the cases originated in the pleura and four in the peritoneum. Histologically, all of the cases were composed of large, polygonal or ovoid cells with well-defined cell borders, dense eosinophilic cytoplasm, and single or multiple nuclei. In some cases, the cells exhibited a wide variation in their size and shape, frequent loss of cell cohesion, marked nuclear atypia, and high mitotic activity (>5 per 10 HPF); whereas, in others, the cells were more cohesive, less pleomorphic, and the mitotic activity low. As the survival of patients in the first group of cases was shorter (mean, 7 months), when compared with that of the latter (mean, 23 months), it is concluded that the differences in prognosis reported in deciduoid mesothelioma are due to the existence of a high-grade subgroup that presents highly aggressive clinical behavior. Therefore, when a high-grade deciduoid mesothelioma is present, it should be reported as it can significantly affect prognosis and treatment. The use of immunohistochemistry and electron microscopy in assisting in the differential diagnosis of deciduoid mesothelioma is also discussed.

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.

Pleomorphic mesothelioma: report of 10 cases

Mesotheliomas with pleomorphic features are rare and only a few studies on this mesothelioma variant have been published. Little information regarding the immunoprofile of these tumors and none on their electron microscopic features was included in these studies. Herein are reported 10 cases of pleomorphic mesothelioma that were investigated using a large panel of immunohistochemical markers, 4 of which were also studied by electron microscopy. All of the patients were men and seven had a history of asbestos exposure. Nine of the cases originated in the pleura and one in the peritoneum. Histologically, the tumors were characterized by being composed of large, often discohesive, cells that varied in size and shape, had dense abundant eosinophilic cytoplasm, and single or multiple irregular nuclei, which often contained one or several large nucleoli. Mitotic activity was high and atypical mitoses frequent. Immunoreactivity for pan-keratin and keratin 7 was strong in all of the cases. Expression for calretinin, WT1, podoplanin, mesothelin and keratin 5/6 was also frequent, but variable. All cases were negative for MOC-31, carcinoembryonic antigen, CD15, TAG-72 and thyroid transcription factor-1. Electron microscopy often showed the presence of abundant long, slender microvilli on the cell membrane of the neoplastic cells. These findings demonstrate that, contrary to what has been suggested by some investigators, both immunohistochemistry and electron microscopy can be very helpful in assisting in the diagnosis of pleomorphic mesotheliomas. That the seven patients who underwent extrapleural pneumonectomy had extensive lymph node metastasis and that the median survival of those patients for whom follow-up information was available was only 8.2 months indicates that mesotheliomas with pleomorphic features are associated with highly aggressive clinical behavior. Therefore, when this subtype of epithelioid mesothelioma is present, it should be reported as it can significantly affect the prognosis and treatment of the patient.

Dose-dependent mesothelioma induction by intraperitoneal administration of multi-wall carbon nanotubes in p53 heterozygous mice

Among various types of multi-wall carbon nanotubes (mwcnt) are those containing fibrous particles longer than 5 μm with an aspect ratio of more than three (i.e. dimensions similar to mesotheliomagenic asbestos). A previous study showed that micrometer-sized mwcnt (μm-mwcnt) administered intraperitoneally at a dose of 3000 μg/mouse corresponding to 1 × 10⁹ fibers per mouse induced mesotheliomas in p53 heterozygous mice. Here, we report a dose-response study; three groups of p53 heterozygous mice (n = 20) were given a single intraperitoneal injection of 300 μg/mouse of μm-mwcnt (corresponding to 1 × 10⁸ fibers), 30 μg/mouse (1 × 10⁷) or 3 μg/mouse (1 × 10⁶), respectively, and observed for up to 1 year. The cumulative incidence of mesotheliomas was 19/20, 17/20 and 5/20, respectively. The severity of peritoneal adhesion and granuloma formation were dose-dependent and minimal in the lowest dose group. However, the time of tumor onset was apparently independent of the dose. All mice in the lowest dose group that survived until the terminal kill had microscopic atypical mesothelial hyperplasia considered as a precursor lesion of mesothelioma. Right beneath was a mononuclear cell accumulation consisting of cd45- or cd3-positive lymphocytes and cd45/cd3-negative f4/80 faintly positive macrophages; some of the macrophages contained singular mwcnt in their cytoplasm. The lesions were devoid of epithelioid cell granuloma and fibrosis. These findings were in favor of the widely proposed mode of action of fiber carcinogenesis, that is, frustrated phagocytosis where the mesotheliomagenic microenvironment on the peritoneal surface is neither qualitatively altered by the density of the fibers per area nor by the formation of granulomas against agglomerates. (Cancer Sci 2012; 103: 1440–1444)

Mesotheliomas with small cell features: report of eight cases

Mesotheliomas with small cell morphology are rare and only one study of such cases has been published. As a result of their rare occurrence, some investigators have cast doubt on the existence of such a histologic variant of mesothelioma. This investigator reports a series of eight cases of epithelioid mesothelioma with small cell features, all of which originated in the pleura. Seven of the patients were men and one was a woman. Four patients had a history of asbestos exposure. Histologically, four of the mesotheliomas were epithelioid and four biphasic. The proportion of small cells seen in these cases constituted 80 to 100% of the tumor included in the biopsy material and 15 to 20% of the tumor present in the pneumonectomy specimens. Immunoreactivity for calretinin, keratin 5/6, keratin 7, pan-keratin, WT1, podoplanin, and mesothelin was seen in all cases tested for these markers. All of the cases were negative for MOC-31, Ber-EP4, CEA, CD15, TAG-72, TTF-1, chromogranin A, synaptophysin, CD99, and desmin. The mean survival of the six patients for whom this information was available was 8.2 months. It is important for pathologists to be aware that mesotheliomas can present small cell features and, because of this, they can be confused with other malignancies that can exhibit similar morphology. The value of immunohistochemistry in the differential diagnosis of these tumors is discussed.

Sarcomatoid neoplasms of the lung and pleura

Sarcomatoid neoplasms of the lung and pleura are rare tumors that present a complex differential diagnosis, making them challenging for surgical pathologists. In the lung, the main tumors are the sarcomatoid carcinomas, including pleomorphic carcinoma, spindle cell carcinoma, giant cell carcinoma, carcinosarcoma, and pulmonary blastoma. They are characterized by histologic heterogeneity; molecular data support their origin from a pluripotent stem cell that undergoes neoplastic transformation with divergent epithelial and sarcomatous differentiation. Diagnosis is difficult in small biopsy specimens and typically requires a resection specimen. Despite the presence of sarcomatoid features, these tumors are classified as lung carcinomas. Pulmonary blastomas must be distinguished from pleuropulmonary blastomas, which are a unique type of thoracic sarcoma typically occurring in young children. In the pleura, the main tumors to consider are the sarcomatoid and desmoplastic types of malignant mesothelioma, solitary fibrous tumor, and desmoid tumor. While light microscopy is sufficient to diagnose most of these tumors, immunohistochemistry can be useful in selected settings. In particular, it can aid to confirm epithelial differentiation in spindle cell carcinomas and the presence of rhabdomyosarcoma in sarcomatoid carcinomas, mesotheliomas, or pleuropulmonary blastomas. For sarcomatoid and desmoplastic mesothelioma, keratin is the most useful stain because it can highlight invasive growth and mesothelial markers are positive in only the minority of cases. Clinical and radiologic correlation is needed to separate some pleomorphic carcinomas with pleural involvement from sarcomatoid malignant mesothelioma, since these poorly differentiated tumors may not express the usual immunohistochemical markers for carcinoma or mesothelioma.

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.

Diagnosis of epithelial mesothelioma using tree-based regression analysis and a minimal panel of antibodies

AIMS

Immunohistochemistry with panels of antibodies is a standard procedure to distinguish between malignant mesothelioma and metastatic adenocarcinoma. Most studies assess only the sensitivity and specificity for single antibodies, even when the paper concludes by recommending an antibody panel. It was the aim of this study to use a novel statistical approach to identify a minimal panel of antibodies, which would make this distinction in the majority of cases.

METHODS

Two hundred consecutive cases of pleural malignancy (173 pleural mesotheliomas of epithelial type and 27 cases of secondary adenocarcinoma) were investigated using a standard panel of 12 antibodies (CAM5.2, CK5/6, calretinin, HBME-1, thrombomodulin, WT-1, EMA, CEA, CD15, B72.3, BG8, and TTF-1). Regression and classification tree-based methods were applied to select the best combination of markers. The modelling procedures used employ successive, hierarchical predictions computed for individual cases to sort them into homogeneous classes.

RESULTS

Labelling for calretinin and lack of labelling for BG8 were sufficient for definite correlation with a diagnosis of malignant mesothelioma. CD15 provided further differentiating information in some cases.

CONCLUSION

A panel of three antibodies was sufficient in most cases to diagnose, or to exclude, epithelial mesothelioma. Calretinin exhibits the strongest correlative power of the antibodies tested.

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.

Histologic variants of papillary and follicular carcinomas associated with anaplastic spindle and giant cell carcinomas of the thyroid: an analysis of rhabdoid and thyroglobulin inclusions

We describe the histologic variants of papillary and follicular carcinomas associated with 109 spindle and giant cell carcinomas (SGCC) of the thyroid and determine the incidence of rhabdoid and thyroglobulin inclusions in these tumors. In addition, we searched for rhabdoid and thyroglobulin inclusions in 120 papillary carcinomas (PC) (all 15 variants included), 23 differentiated follicular carcinomas (DFC), (6 with insular pattern), 6 poorly differentiated follicular carcinomas (PDFC) and 34 follicular adenomas (FA). The following differentiated thyroid carcinomas coexisted with SGCC: 51 (46.8%) PC, (34 conventional type, 14 tall cell variant and 3 follicular variant), 6 (5.5%) DFC, 1 follicular carcinoma with insular pattern (0.9%), and 3 oncocytic carcinomas (2.8%). Eleven SGCC (10%) and 2 (33%) PDFC showed rhabdoid features, but lacked thyroglobulin inclusions. Thyroglobulin inclusions were found in 10 FA (29%), 8 (17%) follicular variants of PC and in 7 (30.4%) DFC. There were no rhabdoid inclusions in any of these differentiated thyroid tumors. Our findings support the hypothesis that most SGCC result from dedifferentiation or anaplastic transformation although the mechanisms that underlie this transformation remain unknown. The finding that only 1 (0.9%) SGCC was associated with follicular carcinoma with insular pattern contradicts the opinion that this tumor occupies an intermediate position between differentiated and anaplastic carcinomas. Rhabdoid features are markers of PDFC and SGCC while thyroglobulin inclusions are markers of FA and differentiated thyroid carcinomas with follicular phenotype.

Primary pleural epithelioid hemangioendothelioma with rhabdoid phenotype: report and review of the literature

Epithelioid hemangioendothelioma (EHE) is a rare malignant vascular tumor described in diverse locations including lung and liver. Relative to these sites, primary EHE of the serous cavities is uncommon. EHE in the serous cavities mimics mesothelioma and adenocarcinoma clinically, radiographically, cytologically, and histologically. EHEs have plasmacytoid epithelioid cells with cytoplasmic vacuoles. In addition to these features, we noted eccentric nuclei with abundant eosinophilic cytoplasm and nuclei displaced peripherally by globular cytoplasmic inclusions imparting a "rhabdoid" phenotype. These cells were often seen surrounding a hyaline core. Rhabdoid features are not unique to a single entity, and a comprehensive immunohistochemical panel is essential. We report the occurrence of pleural EHE with rhabdoid features presenting in a pleural effusion, and review the literature of primary serosal EHEs.

The Toker Tumor

Primary neuroendocrine carcinoma of the skin is a relatively rare tumor that was first described by Cyril Toker in 1972. Since the seminal paper by Toker based on simple morphologic observations and detailed clinical correlation, our understanding of the clinical, morphological, and biological attributes of these lesions has grown exponentially with their increased awareness by pathologists and clinicians as well as with the many contributions of modern diagnostic techniques. The present review focuses principally on the various morphologic appearance that these tumors are able to adopt, the role of modern special techniques for diagnosis, and the conditions that need to be considered in their differential diagnosis.