Immunohistochemistry in the distinction between malignant mesothelioma and pulmonary adenocarcinoma: a critical evaluation of new antibodies

TTF-1 is a highly sensitive but not fully specific marker for pulmonary and thyroidal cancer: a tissue microarray study evaluating more than 17,000 tumors from 152 different tumor entities

Thyroid transcription factor 1 (TTF-1) immunohistochemistry (IHC) is routinely used for the distinction of primary pulmonary adenocarcinomas. However, TTF-1 can also occur in other malignancies. A tissue microarray containing 17,772 samples from 152 different tumor types was analyzed. Napsin-A, CK20, SATB2, FABP1, and Villin-1 IHC data were available from previous studies. TTF-1 staining was seen in 82 of 152 tumor categories including thyroidal cancers (19-100%), adenocarcinomas (94%), neuroendocrine tumors (67%) of the lung, small cell neuroendocrine carcinomas (71-80%), mesenchymal tumors (up to 42%), and thymomas (39%). Comparative analysis of TTF-1 and Napsin-A revealed a sensitivity/specificity of 94%/86% (TTF-1), 87%/98% (Napsin-A), and 85%/99.1% (TTF-1 and Napsin-A) for the distinction of pulmonary adenocarcinomas. Combined analysis of TTF-1 and enteric markers revealed a positivity for TTF-1 and at least one enteric marker in 22% of pulmonary adenocarcinomas but also a TTF-1 positivity in 6% of colorectal, 2% of pancreatic, and 3% of gastric adenocarcinomas. TTF-1 is a marker of high sensitivity but insufficient specificity for pulmonary adenocarcinomas. A small fraction of TTF-1-positive gastrointestinal adenocarcinomas represents a pitfall mimicking enteric-type pulmonary adenocarcinoma. Combined analysis of TTF-1 and Napsin-A improves the specificity of pulmonary adenocarcinoma diagnosis.

Utility of Immunohistochemistry in the Diagnosis of Pleuropulmonary and Mediastinal Cancers: A Review and Update

CONTEXT.—

Immunohistochemistry has become a valuable ancillary tool for the accurate classification of pleuropulmonary and mediastinal neoplasms necessary for therapeutic decisions and predicting prognostic outcome. Diagnostic accuracy has significantly improved because of the continuous discoveries of tumor-associated biomarkers and the development of effective immunohistochemical panels.

OBJECTIVE.—

To increase the accuracy of diagnosis and classify pleuropulmonary neoplasms through immunohistochemistry.

DATA SOURCES.—

Literature review and the author's research data and personal practice experience.

CONCLUSIONS.—

This review article highlights that appropriately selecting immunohistochemical panels enables pathologists to effectively diagnose most primary pleuropulmonary neoplasms and differentiate primary lung tumors from a variety of metastatic tumors to the lung. Knowing the utilities and pitfalls of each tumor-associated biomarker is essential to avoid potential diagnostic errors.

Utility of BerEp4/calretinin and desmin/epithelial membrane antigen (EMA) dual immunocytochemical staining in effusion cytology

BACKGROUND

Pleural mesothelioma (PM) is typically diagnosed late during the disease. Earlier detection can increase the chance of effective therapy. Recurrent pleural effusions are the earliest symptoms displaying an array of cytomorphological changes from reactive atypia to malignancy. Diagnosis is possible on effusion cytology by applying molecular and immunocytochemical markers, the main difficulty being when to suspect PM and to differentiate PM from metastatic adenocarcinoma and reactive mesothelial proliferations.

METHODS

We evaluated the diagnostic performance of two immunocytochemical dual stains (BerEp4/Calretinin and Desmin/Epithelial Membrane Antigen (EMA)) on 149 ethanol-fixed cytospin preparation as an initial step to solve the mentioned diagnostic difficulty. The immunocytochemical reactivity pattern was evaluated by two independent investigators. The final diagnosis corresponded to PM (n = 20), metastatic adenocarcinoma (n = 83), and mesotheliosis (n = 46) in these cases.

RESULTS

Calretinin had 99% specificity and 98% sensitivity for indicating a mesothelial phenotype, while BerEp4 distinguished the adenocarcinoma cases with 98% specificity and 99% sensitivity. EMA displayed 96% specificity and 99% sensitivity in malignant cases, while Desmin without EMA present showed 99% specificity and 96% sensitivity for indicating benign mesothelial proliferation.

CONCLUSIONS

Interpretation of the four immunoreactions is improved when performed as dual stains. The dual staining is a useful tool in the initial handling of atypical effusions and guides the subsequent choice of antibody panels for more detailed subclassification of malignant effusions.

Association of thyroid transcription factor-1 with the efficacy of immune-checkpoint inhibitors in patients with advanced lung adenocarcinoma

Background

We aimed to identify the relationship between thyroid transcription factor‐1 (TTF‐1) expression of lung adenocarcinoma and the efficacy of immune‐checkpoint inhibitor (ICI) therapy.

Methods

This retrospective multicenter study comprised patients with advanced lung adenocarcinoma treated with ICI monotherapy. We collected clinical medical records including data on TTF‐1 expression and analyzed the relationship between TTF‐1 expression and programmed death‐ligand 1 tumor proportion score (PD‐L1 TPS), objective response rate (ORR), progression‐free survival (PFS), and overall survival (OS).

Results

In total, 108 patients with lung adenocarcinoma were analyzed. The rate of TPS ≥1% and ≥50% in patients with positive TTF‐1 expression was significantly higher than that in patients with negative TTF‐1 expression (88% vs. 60%, p < 0.001; 65% vs. 24%, p < 0.001). The ORR was significantly higher in TTF‐1 positive patients than in TTF‐1‐negative patients (38% vs. 8%, p = 0.003). Among patients with TPS ≥50% and 1%–49%, the ORR in TTF‐1 positive and negative patients was 48% (26/54) versus 17% (1/6) (p = 0.21), and 32% (6/19) versus 11% (1/9) (p = 0.37), respectively. The ORR for patients with TPS <1% was 0% in both the TTF‐1 negative and positive cases. The median PFS and OS was significantly longer in TTF‐1‐positive patients than in TTF‐1‐negative patients (5.4 vs. 1.6 months, p < 0.001; 18.2 vs. 8.0 months, p = 0.041). Multivariate analysis revealed that TTF‐1‐negative status was an independent unfavorable prognostic factor for PFS.

Conclusion

Patients with TTF‐1‐positive status receiving ICI monotherapy showed better outcomes than those with TTF‐1‐negative lung adenocarcinoma.

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.

Immunochemistry in the work-up of mesothelioma and its differential diagnosis and mimickers

The differential diagnosis in cellular effusions with cytological atypia often includes malignant mesothelioma (MM), reactive mesothelial proliferation, and malignancies of metastatic origin, particularly carcinomas. The International Reporting System for Serous Fluid recently established guidelines for reporting MM. In conjunction with the cytomorphologic evaluation, the role of immunochemistry (IC) was emphasized as a very useful tool in the workup of serous fluids, especially with the availability of novel markers. Utilizing a panel of markers, IC allows the characterization of the cells, whether mesothelial or not, and when mesothelial origin is established, IC can frequently assist in delineating its benign or malignant nature. IC can also confirm metastatic disease, allowing the identification of the primary origin in most cases. This review summarizes the current status of IC and its role in the diagnosis of MM and its differential diagnosis in serous fluids.

Donor Cell Fate in Particulated Juvenile Allograft Cartilage for the Repair of Articular Cartilage Defects

BACKGROUND

Particulated juvenile allograft cartilage (PJAC) has demonstrated good clinical efficacy in repairing articular cartilage defects, but the related repair mechanism after transplant and the biological characteristics of the transplanted cells are still unclear.

PURPOSE

To study the efficacy of PJAC in repairing full-thickness cartilage defects and the specific fate of donor cells to provide experimental evidence for its clinical application.

STUDY DESIGN

Controlled laboratory study.

METHODS

Twenty female Guizhou minipigs were randomly divided into an experimental group and a control group. An 8-mm cylindrical full-thickness cartilage defect was created in the femoral trochlea of 1 knee in all minipigs. The experimental group received transplant of PJAC from 5 male juvenile Guizhou minipigs (PJAC group; n = 10) and the control group received autologous cartilage chips (ACC group; n = 10). Follow-up assessments were conducted at 1 month and 3 months to track the transplanted cells by the male-specific sex-determining region Y-linked (SRY) gene; tissue sections were hybridized in situ, and O'Driscoll histological scoring was performed according to hematoxylin and eosin staining, safranin O and fast green staining, and toluidine blue O staining, as well as immunohistochemical evaluation of aggrecan and Sry-type HMG-box 9 (SOX9).

RESULTS

All 20 Guizhou minipigs were followed; no infection or incision healing disorder occurred after the operation. By SRY in situ hybridization, the SRY signal of the transplanted cells was positive in the repaired tissue of the defect, and the SRY positive signal could still be detected in repaired tissue at 3 months postoperatively. The average number of positive cells was 68.6 ± 11.91 at 1 month and 32.6 ± 3.03 at 3 months (confocal microscope: ×400), and the difference was statistically significant. The O'Driscoll histological scores were 14 ± 0.71 in the ACC group and 9.8 ± 0.84 in the PJAC group at 1 month, and 18 ± 1.20 in the ACC group and 17.4 ± 1.14 in the PJAC group at 3 months. The scores were statistically significant between the ACC group and PJAC group at 1 month. The positive rates of SOX9 in the PJAC and ACC groups at 1 month were 67.6% ± 3.78% and 63.4% ± 5.30%, respectively, and the difference was not statistically significant (P > .05). The positive rates of SOX9 in the PJAC and ACC groups at 3 months were 68.8% ± 2.69% and 17.1% ± 1.26%, respectively, and the difference was statistically significant (P < .05). The positive rates of aggrecan in the PJAC and ACC groups at 1 month were 40.5% ± 2.78% and 42.4% ± 0.54% respectively, and the difference was not statistically significant (P > .05). The positive rates of aggrecan in the PJAC and ACC groups at 3 months were 40.8% ± 1.50% and 30.1% ± 2.44%, respectively, and the difference was not statistically significant (P > .05).

CONCLUSION

An animal model was established with Guizhou minipigs, and the cartilage defect was repaired with PJAC from male minipigs. The SRY gene positive signal could be detected from the repaired tissue by in situ hybridization, indicating that the transplanted cells survived at least 3 months. The key genes of cartilage formation, SOX9 and aggrecan, were expressed at 1 month and 3 months, and SOX9 expression was stronger in the PJAC group than the ACC group at 3 months.

CLINICAL RELEVANCE

This study suggests that it is feasible to study the biological characteristics of transplanted cells in the cartilage region by the sex-determining gene.

Asbestos and ovarian cancer: examining the historical evidence

Asbestos recently returned to the spotlight when Johnson & Johnson halted sales of baby powder due to lawsuits claiming that the talc in baby powder may have been contaminated with asbestos, which has been linked to the risk of ovarian cancer development. Although talc and asbestos have some structural similarities, only asbestos is considered causally associated with ovarian cancer by the WHO's International Agency for Research on Cancer. While it is useful to understand the types and properties of asbestos and its oncologic biology, the history of its association with ovarian cancer is largely based on retrospective observational studies in women working in high asbestos exposure environments. In reviewing the literature, it is critical to understand the distinction between associative risk and causality, and to examine the strength of association in the context of how the diagnosis of ovarian cancer is made and how the disease should be distinguished from a similar appearing but unrelated neoplasm, malignant mesothelioma. Based on contextual misinterpretation of these factors, it is imperative to question the International Agency for Research on Cancer's assertion that asbestos has a clear causal inference to ovarian cancer. This has important clinical implications in the way patients are conceivably counseled and provides motivation to continue research to improve the understanding of the association between asbestos and ovarian cancer.

Differential Diagnosis of Epithelioid Malignant Mesothelioma With Lung and Breast Pleural Metastasis: A Systematic Review Compared With a Standardized Panel of Antibodies-A New Proposal That May Influence Pathologic Practice

CONTEXT.—

Pleural mesothelioma is a rare cancer with an often-challenging diagnosis because of its potential to be a great mimicker of many other tumors. Among them, primary lung and breast cancers are the 2 main causes of pleural metastasis. The development and application of targeted therapeutic agents have made it even more important to achieve an accurate diagnosis. In this setting, international guidelines have recommended the use of 2 positive and 2 negative immunohistochemical biomarkers.

OBJECTIVES.—

To define the most highly specific and sensitive minimum set of antibodies for routine practice to use for the separation of epithelioid malignant mesothelioma from lung and breast metastasis and to determine the most relevant expression cutoff.

DESIGN.—

To provide information at different levels of expression of 16 mesothelial and epithelial biomarkers, we performed a systematic review of articles published between 1979 and 2017, and we compared those data to results from the Mesothelioma Telepathology Network (MESOPATH) of the standardized panel used in routine practice database since 1998.

RESULTS.—

Our results indicate that the following panel of markers-calretinin (poly)/thyroid transcription factor 1 (TTF-1; clone 8G7G3/1) and calretinin (poly)/estrogen receptor-α (ER-α; clone EP1)-should be recommended; ultimately, based on the MESOPATH database, we highlight their relevance which are the most sensitive and specific panel useful to the differential diagnosis at 10% cutoff.

CONCLUSIONS.—

Highlighted by their relevance in the large cohort reported, we recommend 2 useful panels to the differential diagnosis at 10% cutoff.

Application of immunohistochemistry in diagnosis and management of malignant mesothelioma

Immunohistochemistry plays an indispensable role in accurate diagnosis of malignant mesothelioma, particularly in morphologically challenging cases and in biopsy and cytology specimens, where tumor architecture is difficult or impossible to evaluate. Application of a targeted panel of mesothelial- and epithelial-specific markers permits correct identification of tumor lineage in the vast majority of cases. An immunopanel including two mesothelial markers (calretinin, CK5/6, WT-1, or D2-40) and two epithelial markers (MOC-31 and claudin-4) offers good sensitivity and specificity, with adjustments as appropriate for the differential diagnosis. Once mesothelial lineage is established, malignancy-specific studies can help verify a diagnosis of malignant mesothelioma. BAP1 loss, CDKN2A homozygous deletion, and MTAP loss are highly specific markers of malignancy in a mesothelial lesion, and they attain acceptable diagnostic sensitivity when applied as a diagnostic panel. Novel markers of malignancy, such as 5-hmC loss and increased EZH2 expression, are promising, but have not yet achieved widespread clinical adoption. Some diagnostic markers also have prognostic significance, and PD-L1 immunohistochemistry may predict tumor response to immunotherapy. Application and interpretation of these immnuomarkers should always be guided by clinical history, radiographic findings, and above all histomorphology.

Evaluation of gene expression levels in the diagnosis of lung adenocarcinoma and malignant pleural mesothelioma

Background

This study aims to evaluate gene expression levels in the diagnosis of lung adenocarcinoma and malignant pleural mesothelioma both which have a distinct treatment and prognosis.

Methods

Between January 2012 and January 2014, 12 newly diagnosed patients with a lung adenocarcinoma, 12 patients with malignant pleural mesothelioma, and eight healthy individuals as the control group were included. After treatment of the fresh samples of lung adenocarcinoma stored at -80°C for ribonucleic acid isolation, and paraffin-embedded tissues of patients with malignant pleural mesothelioma were deparaffinized, complementary deoxyribonucleic acid synthesis and expression of 84 genes associated with deoxyribonucleic acid repair were analyzed via real-time polymerase chain reaction assay. According to the expression of tumor cells, expression of each fold change was calculated.

Results

The BRCA1, BRCA2, CDK7, MLH3, MSH4, NEIL3, SMUG1, UNG, XRCC2, and XRCC4 genes showed more than five-fold higher expression in the patients with lung adenocarcinomas, compared to the control group. The patients with malignant pleural mesothelioma showed a five-fold higher expression in the APEX2, BRCA1, BRCA2, CDK7, MLH1, MLH3, MSH3, MSH4, NEIL3, PARP2, PARP3, PMS1, RAD50, RAD51, RAD51B, RAD51D, RAD52, RPA3, SMUG1, UNG, XPA, XRCC2, and XRCC4 genes, compared to the control group. Comparing malignant pleural mesothelioma with lung adenocarcinoma cases, we found that CDK7, MLH1, TREX1, PRKDC, XPA, PMS1, UNG, and RPA3 genes were overexpressed.

Conclusion

Our study results showed differences between expression profiles of deoxyribonucleic acid repair genes in lung adenocarcinoma and malignant pleural mesothelioma cells. Based on our study results, we suggest that TREX1, PRKDC, and PMS1 genes may play a key role in the differential diagnosis of these two entities.

Assessment of Biomarkers for Clinical Diagnosis of Papillary Thyroid Carcinoma with Distant Metastasis

Early diagnosis and treatment of thyroid cancers are critical for better prognosis and better survival rates. The purpose of this study was to identify potential diagnostic markers for papillary thyroid carcinomas with distant metastasis. Fifty-eight papillary thyroid tumor specimens (27 papillary thyroid carcinomas with distant metastasis and 31 without metastasis) were examined, and protein expression of pituitary tumor-transforming gene (PTTG), E-cadherin, p27kip1, vascular endothelial growth factor (VEGF)-C, metalloproteinase (MMP) 2, MMP9, chemokine receptor CXCR4, and basic fibroblast growth factor (bFGF) in these tumors was assessed by immunohistochemistry. The clinicopathological variables with diagnostic significance were determined by multivariate analysis, and their diagnostic values were evaluated by ROC curve analysis. PTTG, VEGF-C, MMP2, MMP9, CXCR4, and bFGF were overexpressed in metastatic papillary thyroid carcinomas, whereas p27kip1 expression was elevated only in carcinomas lacking metastasis. Multiple-factor binary ordinal logistic regression analysis revealed that PTTG, VEGF-C, MMP2, and bFGF were independently related to biological metastatic behavior in thyroid tumors, suggesting their potential use as biomarkers. ROC curve analysis showed that among these four proteins, VEGF-C and bFGF were the best diagnostic biomarkers. A VEGF-C and bFGF cluster was the most useful factor for the differential diagnosis between metastatic and non-metastatic papillary thyroid cancers. Thus, the combined use of VEGF-C and bFGF as biomarkers may improve the diagnostic accuracy of papillary thyroid carcinoma and may be useful in multimodal screening programs for the clinical diagnosis of papillary thyroid carcinoma and early detection of papillary thyroid carcinoma with distant metastasis.

Role of N-cadherin in proliferation, migration, and invasion of germ cell tumours

Germ cell tumors (GCTs) are the most common malignancies in young men. Most patients with GCT can be cured with cisplatin-based combination chemotherapy, even in metastatic disease. In case of therapy resistance, prognosis is usually poor. We investigated the potential of N-cadherin inhibition as a therapeutic strategy. We analyzed the GCT cell lines NCCIT, NTERA-2, TCam-2, and the cisplatin-resistant sublines NCCIT-R and NTERA-2R. Effects of a blocking antibody or siRNA against N-cadherin on proliferation, migration, and invasion were investigated. Mouse xenografts of GCT cell lines were analyzed by immunohistochemistry for N-cadherin expression. All investigated GCT cell lines were found to express N-cadherin protein in vitro and in vivo. Downregulation of N-cadherin in vitro leads to a significant inhibition of proliferation, migration, and invasion. N-cadherin-downregulation leads to a significantly higher level of pERK. N-cadherin-inhibition resulted in significantly higher rates of apoptotic cells in caspase-3 staining. Expression of N-cadherin is preserved in cisplatin-resistant GCT cells, pointing to an important physiological role in cell survival. N-cadherin-downregulation results in a significant decrease of proliferation, migration, and invasion and stimulates apoptosis in cisplatin-naive and resistant GCT cell lines. Therefore, targeting N-cadherin may be a promising therapeutic approach, particularly in cisplatin-resistant, therapy refractory and metastatic GCT.

Usefulness of Thyroid Transcription Factor-1 Immunohistochemical Staining in the Differential Diagnosis of Primary Pulmonary Tumors of Dogs

In a previous study, we characterized the reactivity of monoclonal antibody (MoAb) 8G7G3/1 to thyroid transcription factor-1 (TTF-1) in canine thyroid tumors. In this study, we have examined the reactivity of this antibody in 120 canine pulmonary tumors, including 78 primary epithelial tumors. Tissues had been fixed in formalin and routinely processed for histopathology. Nuclear staining for TTF-1 was detected in 64.2% of primary pulmonary epithelial tumors. The most common TTF-1-reactive tumor types were bronchioloalveolar carcinomas and bronchogenic carcinomas. Staining was diffuse, heterogeneous, or patchy. Nonpulmonary, metastatic epithelial tumors, except two of two thyroid carcinomas, did not react with antibody 8G7G3/1. Mesotheliomas and other mesenchymal tumors were also negative for this marker. A reduction or loss of reactivity was apparent in pulmonary epithelial tumors archived in paraffin blocks for 7-8 years. There was slight reduction in the number of positive cells or the intensity of the reaction in control tissues fixed longer than 1 week. On the basis of our limited studies and the human literature, it appears that MoAb 8G7G3/1 to TTF-1 is a highly specific (with the exception of thyroid tumors) and moderately sensitive marker for canine pulmonary epithelial tumors.

Malignant Mesothelioma Mimicking Invasive Mammary Carcinoma in a Male Breast

Malignant mesothelioma is an uncommon tumor with strong association with asbestos exposure. Few cases of malignant pleural mesothelioma metastatic to the female breast have been reported. Herein, we presented, for the first time, a case of locally infiltrating malignant pleural mesothelioma forming a mass in the breast of a male as the first pathologically confirmed manifestation of the disease. Breast ultrasound revealed an irregular mass in the right breast which involves the pectoralis muscle. Breast core biopsy revealed a proliferation of neoplastic epithelioid cells mimicking an infiltrating pleomorphic lobular carcinoma. IHC studies showed the cells to be positive for calretinin, CK5/6, WT1, and CK7. The cells were negative for MOC-31, BerEp4, ER, and PR. A final diagnosis of malignant mesothelioma, epithelioid type, was rendered. This case demonstrates the importance of considering a broad differential diagnosis in the setting of atypical presentation with application of a panel of IHC markers.

Value of podoplanin as an immunohistochemical marker in tumor diagnosis: a review and update

Podoplanin is a type I integral membrane glycoprotein that, because it is expressed in lymphatic endothelium, but not in vascular blood vessel endothelial cells, is commonly used in the identification of lymphatic endothelial differentiation in vascular endothelial neoplasms and lymphatic invasion by tumor. Because podoplanin is also expressed in mesothelial cells and fetal gonocytes, it has proved to be a useful marker for assisting in the differential diagnosis of mesotheliomas and germ cell tumors, particularly seminomas/dysgerminomas. Podoplanin expression has also been reported in a wide variety of other neoplasms, including hemangioblastomas, meningiomas, cartilaginous tumors, and follicular dendritic cell neoplasms. This article reviews the information that is currently available on the application of podoplanin immunostaining in diagnostic pathology.

Value of calretinin immunostaining in diagnostic pathology: a review and update

Calretinin is a member of the EF-hand family of calcium-binding proteins. Because its expression is highly restricted to mesotheliomas, calretinin is, at present, the most commonly used positive mesothelioma marker that is most often recommended to be included in the various immunohistochemical panels used to assist in the differential diagnosis of these tumors. Calretinin expression has also been reported to be commonly expressed in a wide variety of other neoplasms, including sex cord-stromal tumors, adrenal cortical neoplasms, olfactory neuroblastomas, Schwann cell tumors, cardiac myxomas, and ameloblastomas. This article reviews the information that is currently available on calretinin expression in tumors and on its application as an immunohistochemical marker in diagnostic pathology.

Utility of Immunohistochemistry in the Diagnosis of Pleuropulmonary and Mediastinal Cancers: A Review and Update

Context

Immunohistochemistry has become an indispensable ancillary tool for the accurate classification of pleuropulmonary and mediastinal neoplasms necessary for therapeutic decisions and predicting prognostic outcome in the era of personalized medicine. Diagnostic accuracy has significantly improved because of the continuous discoveries of tumor-associated biomarkers and the development of effective immunohistochemical panels.

Objective

To increase the accuracy of diagnosis and classify pleuropulmonary neoplasms through immunohistochemistry.

Data Sources

Literature review, authors' research data, and personal practice experience.

Conclusions

This review article has shown that appropriately selecting immunohistochemical panels enables pathologists to effectively diagnose most primary pleuropulmonary neoplasms and differentiate primary lung tumors from a variety of metastatic tumors to the lung. The discovery of new mutation-specific antibodies identifying a subset of specific gene-arranged lung tumors provides a promising alternative and cost-effective approach to molecular testing. Knowing the utilities and pitfalls of each tumor-associated biomarker is essential to avoiding potential diagnostic errors.

Pleural mesothelioma: an institutional experience of 66 cases

Background

Malignant mesothelioma of the pleura is an aggressive tumor known to be associated with asbestos. Histological diagnosis of mesothelioma is challenging and is usually aided by immunohistochemical markers.

Methods

During an 18-year period (1995-2012), 66 patients with pleural mesothelioma were diagnosed at the Samsung Medical Center in Seoul. We reviewed hematoxylin and eosin and immunohistochemical slides of pleural mesothelioma and evaluated their pathological and clinical features.

Results

The male-to-female ratio was 1.75:1, and age of patients ranged from 28 to 80 years with an average age of 56.84 years. Twenty-two out of 66 patients underwent curative pneumonectomy. Follow-up data was available in 60 patients (90.9%), and 50 of them (83.3%) died from the disease. The average overall survival was 15.39 months. Histologically, the epithelioid type was the most common, followed by the sarcomatoid and the biphasic types. Epidemiologic information was not available in most cases, and only one patient was confirmed to have a history of asbestos exposure.

Conclusions

Malignant mesothelioma of the pleura is a fatal tumor, and the therapeutic benefit of pneumonectomy remains unproven. The combination of calretinin, Wilms tumor 1, HMBE-1, and thyroid transcription factor-1 may provide high diagnostic accuracy in diagnosing mesothelioma.

Molecular classification of malignant pleural mesothelioma: identification of a poor prognosis subgroup linked to the epithelial-to-mesenchymal transition

PURPOSE

Despite research efforts to develop more effective diagnostic and therapeutic approaches, malignant pleural mesothelioma (MPM) prognosis remains poor. The assessment of tumor response to therapy can be improved by a deeper phenotypical classification of the tumor, with emphasis on its clinico-biological heterogeneity. The identification of molecular profiles is a powerful approach to better define MPM subclasses and targeted therapies.

EXPERIMENTAL DESIGN

Molecular subclasses were defined by transcriptomic microarray on 38 primary MPM cultures. A three-gene predictor, identified by quantitative reverse transcription PCR, was used to classify an independent series of 108 frozen tumor samples. Gene mutations were determined in BAP1, CDKN2A, CDKN2B, NF2, and TP53. Epithelial-to-mesenchymal transition (EMT) markers were studied at the mRNA and protein levels.

RESULTS

Unsupervised hierarchical clustering on transcriptomic data defined two robust MPM subgroups (C1 and C2), closely related to prognosis and partly to histologic subtypes. All sarcomatoid/desmoplastic MPM were included in the C2 subgroup. Epithelioid MPM were found in both subgroups, with a worse survival prognosis in the C2 subgroup. This classification and its association with histologic subtypes and survival were validated in our independent series using the three-gene predictor. Similar subgroups were found after classification of other MPM series from transcriptomic public datasets. C1 subgroup exhibited more frequent BAP1 alterations. Pathway analysis revealed that EMT was differentially regulated between MPM subgroups. C2 subgroup is characterized by a mesenchymal phenotype.

CONCLUSIONS

A robust classification of MPM that defines two subgroups of epithelioid MPM, characterized by different molecular profiles, gene alterations, and survival outcomes, was established.

Claudin-4 as a marker for distinguishing malignant mesothelioma from lung carcinoma and serous adenocarcinoma

We compared claudin-4 with Ber-EP4 and carcinoembryonic antigen as markers to distinguish mesothelioma from lung adenocarcinoma, poorly differentiated lung squamous cell carcinoma, and serous adenocarcinoma of the uterus or ovary. All mesothelioma specimens were negative for claudin-4, but 3 of 18 specimens were focally positive for Ber-EP4. In contrast, lung adenocarcinoma including poorly differentiated adenocarcinoma was highly positive for claudin-4, but expression of Ber-EP4 and carcinoembryonic antigen varied widely. Claudin-4 in poorly differentiated squamous cell carcinoma had a lower positive expression rate than in adenocarcinoma. Granular claudin-4 immunoreactivity was conspicuous in poorly differentiated squamous cell carcinoma; this immunoreactive pattern was also observed in mesothelioma. Claudin-4 was thus considered very useful marker for distinguishing mesothelioma and adenocarcinoma, even if histological specimens are small, as in biopsies that contain limited numbers of tumor cells. However, it should be mentioned that claudin-4 has a limit in discrimination between squamous cell carcinoma from mesothelioma.

Value of claudin-4 immunostaining in the diagnosis of mesothelioma

Claudin-4 (CL-4) is a tight junction-associated protein that is expressed in most epithelial cells but absent in mesothelial cells. The purpose of this study is to evaluate the utility of CL-4 immunostaining for assisting in the differential diagnosis of mesothelioma. Sixty mesotheliomas (40 epithelioid, 10 biphasic, and 10 sarcomatoid), 185 carcinomas of different origins that can potentially be confused with mesotheliomas, 37 soft-tissue sarcomas, and 5 melanomas were investigated for CL-4 expression. All 60 mesotheliomas were CL-4 negative. In contrast, 169 (91%) of 185 carcinomas expressed this marker. Five of 8 desmoplastic small round cell tumors and the epithelial component of all 5 biphasic synovial sarcomas were CL-4 positive, whereas none of the remaining soft-tissue sarcomas or melanomas expressed this marker. It is concluded that CL-4 is a highly specific and sensitive immunohistochemical marker for assisting in distinguishing epithelioid mesotheliomas from metastatic carcinomas to the serosal membranes.

N-cadherin expression in malignant germ cell tumours of the testis

BACKGROUND

Testicular germ cell tumours (TGCTs) are the most common malignancy in young men aged 18-35 years. They are clinically and histologically subdivided into seminomas and non-seminomas. Cadherins are calcium-dependent transmembrane proteins of the group of adhesion proteins. They play a role in the stabilization of cell-cell contacts, the embryonic morphogenesis, in the maintenance of cell polarity and signal transduction. N-cadherin (CDH2), the neuronal cadherin, stimulates cell-cell contacts during migration and invasion of cells and is able to suppress tumour cell growth.

METHODS

Tumour tissues were acquired from 113 male patients and investigated by immunohistochemistry, as were the three TGCT cell lines NCCIT, NTERA-2 and Tcam2. A monoclonal antibody against N-cadherin was used.

RESULTS

Tumour-free testis and intratubular germ cell neoplasias (unclassified) (IGCNU) strongly expressed N-cadherin within the cytoplasm. In all seminomas investigated, N-cadherin expression displayed a membrane-bound location. In addition, the teratomas and yolk sac tumours investigated also differentially expressed N-cadherin. In contrast, no N-cadherin could be detected in any of the embryonal carcinomas and chorionic carcinomas examined. This expression pattern was also seen in the investigated mixed tumours consisting of seminomas, teratomas, and embryonal carcinoma.

CONCLUSIONS

N-cadherin expression can be used to differentiate embryonal carcinomas and chorionic carcinomas from other histological subtypes of TGCT.

Application of immunohistochemistry in the diagnosis of epithelioid mesothelioma: a review and update

A large number of immunohistochemical markers that can assist in the differential diagnosis of epithelioid mesotheliomas are currently available. Because these markers are expressed differently in the various types of carcinomas that can metastasize to the serosal membranes and can potentially be confused with epithelioid mesothelioma, their selection for inclusion in a diagnostic panel largely depends on the differential diagnosis, as well as on which ones work the best in a given laboratory. Traditionally, the panels used in the differential diagnosis of epithelioid mesothelioma have consisted of a combination of positive mesothelioma markers and broad-spectrum carcinoma markers. At present, a wide variety of organ-associated carcinoma markers such as thyroid transcription factor-1 and napsin A for the lung, PAX 8 and PAX 2 for the kidney, and Müllerian-derived tumors; gross cystic disease fluid protein-15 and mammaglobin for the breast; and CDX2 for intestinal differentiation are available, which can assist in establishing the site of origin of an adenocarcinoma when included in a diagnostic panel. This article provides updated information on the composition of the panels of markers recommended in the various differential diagnoses.

Advances in malignant pleural mesothelioma therapy: targeting EphA2 a novel approach

Malignant pleural mesothelioma (MPM) is an aggressive neoplasm with a poor prognosis. MPM grows from the mesothelial cells lining the surface of the lung and chest wall called Pleura. Exposure to asbestos is mainly linked to the development of MPM. Approximately 80% of the tumors are pleural in origin, and up to 3000 people are diagnosed with MPM in the United States annually. The incidence of MPM is expected to rise in the coming decades particularly in the developing countries. Although there is an increase in the awareness of danger associated with the use of asbestos, its use is still prevalent in Australia and Asia because of its durability and low cost. This further warns and adds to the mortality and morbidity of patients with MPM globally. The traditional treatment strategies have shown only modest improvement towards the disease. MPM is difficult to treat because of the fact that the time between the exposure to asbestos and the appearance of symptoms is extremely delayed, and also due to tumor involvement with the pleural surface and the adjoining tissues such as the chest wall, pericardium and sub-diaphragmatic organs. Despite advances in the diagnostic and treatment approaches the median survival rate for MPM is between 9 to 17 months. The standard care with double agent has shown modest improvement however, multimodality approach using novel targets may have potential to achieve the improvement in the survival rate. In this review we give an update on the conventional treatment modalities and discuss about various molecular targets including receptor EphA2, a novel target gene which may be considered as a biomarker for the diagnosis and treatment of MPM.

Malignant biphasic peritoneal mesothelioma in a child: fine-needle aspiration cytology, histopathology, and immunohistochemical features along with review of literature

Peritoneal mesotheliomas in children are of rare occurrance. We herein report the clinical, radiological, and pathological findings of a rare case of malignant peritoneal mesothelioma occurring in nine-year-old female child. The child presented with abdominal distension and awareness of a painless mass in the abdomen which on radiology appeared as a large heterogeneous pelvic mass with peritoneal deposits at multiple sites. To the best of our knowledge, this is the first case of a peritoneal malignant mesothelioma on which fine needle aspiration (FNA) was performed as first line investigation of the primary tumor. The cytological features, major differential diagnoses, and the pitfalls therein are discussed. Histopathology revealed biphasic pattern of mesothelioma which is again a rare pattern. Immunochemistry was carried out on the cell block made from the FNA as well as the biopsy specimen essentially showed the same features. There was positivity for vimentin, EMA, and cytokeratin 5/6 while WT1, calretinin, and CEA were negative; however, D2-40 showed diffuse membranous positivity in the epithelial areas and cytoplasmic positivity in the spindle areas confirming a mesothelioma. We emphasize the use of immunochemistry on cell block material for a confident diagnosis of mesothelioma in such cases.

Peritoneal mesothelial hyperplasia associated with gynaecological disease: a potential diagnostic pitfall that is commonly associated with endometriosis

Aims

To describe the clinicopathological features of florid peritoneal mesothelial hyperplasia associated with gynaecological disease.

Methods

Review of a series of 44 cases where there was significant peritoneal mesothelial proliferation, usually on the surface of the ovary, which resulted in diagnostic problems, often including a consideration of malignancy.

Results

Florid mesothelial proliferation was associated with a variety of benign and malignant lesions, most commonly endometriosis. The most characteristic morphological appearance was that of small bland tubules and nests and cords of cells often embedded in fibrous tissue, sometimes with a linear arrangement. In a minority of cases, there were closely packed small glands and papillae resulting in mimicry of a serous proliferation; psammoma bodies were present in one such case. In several cases, the groups of mesothelial cells either exhibited true lymphovascular invasion or were surrounded by spaces closely simulating lymphovascular invasion. Although most cases were appreciated to represent a reactive mesothelial proliferation, several were referred with a presumed or possible diagnosis of mesothelioma, low-grade serous carcinoma, adenocarcinoma, adenomatoid tumour or Sertoli cell tumour. Positive staining with calretinin and negative staining with Ber-EP4 often helped to identify the cells as mesothelial rather than epithelial.

Conclusions

Florid peritoneal mesothelial hyperplasia can occur in association with a variety of gynaecological disorders. Recognition of this phenomenon and appreciation that it can produce worrisome histological patterns, particularly when associated with endometriosis, is key to reaching the correct diagnosis.

Expression of thyroid transcription factor-1 in colorectal carcinoma

Thyroid transcription factor-1 (TTF-1) is a member of the homeodomain transcription family expressed in epithelial cells of the thyroid and lung. Although nuclear TTF-1 is generally considered a specific marker for lung and thyroid neoplasms, it has been reported to be positive in other types of tumors including colorectal carcinoma (CRC). During metastatic adenocarcinoma workup for patients who had a history of CRC, we identified 4 positive TTF-1 cases using clone 8G7G3/1. Three of the 4 corresponding primary carcinomas were also positive for TTF-1. Therefore, we sought to retrospectively investigate the expression of TTF-1 in 100 CRC cases constructed in tissue microarray blocks and whole tissue sections of the 4 primary tumors corresponding to the 4 positive metastases. In tissue microarray cases, all cases had negative nuclear staining. Our results suggest that during immunohistochemical workup for adenocarcinoma, especially when the differential diagnosis includes the lung and CRC, TTF-1 results should be interpreted with caution as a small percentage of CRC expresses this marker. Positive nuclear TTF-1 in a metastatic carcinoma does not rule out CRC primary. Clinicopathologic correlation, tumor morphology, and a panel of immunohistochemical markers are essential to render the correct diagnosis.

The diagnostic utility of maspin in the distinction between malignant mesothelioma and pulmonary adenocarcinoma

Immunohistochemistry is frequently employed to differentiate between malignant mesothelioma (MM) and pulmonary adenocarcinoma (AC) infiltrating the pleura, but there is uncertainty as to which antibodies are most useful. The present study investigated the presence of the serine protease, maspin, in epithelioid MMs and evaluated the diagnostic utility of maspin for the differential diagnosis between epithelioid MM and pulmonary AC with pleural involvement. The results showed more frequent maspin immunostaining among AC cases compared with MM cases. Maspin positivity was significantly higher among AC cases with respect to both the extent and intensity of staining. A significant difference also existed between the two tumour types with respect to the overall maspin score. Despite these findings, the sensitivity and specificity of maspin positivity to detect AC were only 59% and 73%, respectively, indicating that detection of maspin is of no value for the differential diagnosis of AC and MM.

Duodenal metastasis of malignant pleural mesothelioma

Metastatic malignant mesothelioma of the pleura is uncommon at the time of initial diagnosis. The gastrointestinal lumen is rarely found at autopsy in patients with widespread disease. Here, we describe an extremely rare case of isolated duodenal metastasis of sarcomatoid mesothelioma of the pleura in a 73-year-old man, without memory of any direct exposure to asbestos. The possibility of gastrointestinal tract metastasis should be considered in the presence of anemia or positive occult blood test in patients with malignant pleural mesothelioma.

[Pseudo-mesotheliomatous adenocarcinoma of the lung]

Both pathologists and clinicians are challenged by the diagnosis of a particular variant of the peripheral adenocarcinoma with involvement of the pleura parietalis, the so-called pseudo-mesotheliomatous adenocarcinoma of the lung (PMAC), which is hard to differentiate from epithelioid mesothelioma on imaging and cytology, macroscopically as well as histologically. However, the exact diagnosis is not only crucial for the patient's therapy but also for insurance matters. Immunohistochemical evaluation represents a quick and a relatively cheap tool for which a few antibody panels have been proposed in recent years as being suitable to distinguish between these two entities. One of the positive markers for epithelioid mesothelioma most often suggested seems to be calretinin. We would like to report on a case of PMAC with the special feature of positive calretinin immunohistochemical staining. Using histochemistry and a few additional antibodies we were able to reliably characterize the tumor and provide the patient with appropriate therapy. This article gives a short overview of the possibilities available for distinguishing between these two entities in the context of a case report.

Transcription expression and clinical significance of TTF-1 mRNA in pleural effusion of patients with lung cancer

The aim of this study was to evaluate diagnostic utility of thyroid transcription factor 1 (TTF-1) mRNA in pleural effusions (PEs) of patients with lung cancer, especially primary pulmonary adenocarcinoma (PPA). Transcription levels of TTF-1 were detected by reverse transcription polymerase chain reaction (RT-PCR) in PEs of patients with lung cancer (56 cases) and with lung benign diseases (44 cases). The expression rate of TTF-1 mRNA was significantly higher in PEs of patients with lung cancer (73.2%) than with benign lung diseases (0%). In subclasses, the expression rate of TTF-1 mRNA was obviously higher in PEs of patients with PPA (93.0%) than with metastatic pulmonary adenocarcinoma (0%) and with primary pulmonary squamous cell carcinoma (12.5%). When compared with immunocytochemistry and cytology, TTF-1 mRNA gave the best diagnostic performance in diagnosing PPA: sensitivity of 93.0%, specificity of 100%, and accuracy of 96.6%, respectively. As a molecular marker of detecting pleural micrometastasis, TTF-1 mRNA may be helpful to diagnose the cancer cells in PEs of patients with lung cancer, especially with PPA.

Expression of epithelial-cell adhesion molecule (Ep-CAM) in small cell lung cancer as defined by monoclonal antibodies 17-1A and BerEP4

Small cell lung cancer (SCLC) comprises 15-20% of all lung cancer. Despite high initial response to chemotherapy chemoresistance at relapse leads to a less than 5% five-year survival rate. Adjuvant treatment with monoclonal antibodies (Mabs) against SCLC antigens may offer a therapeutic option. Mab 17-1A directed against the epithelial-cell adhesion molecule (Ep-CAM) has been extensively tested for therapy in patients with colorectal carcinoma. Expression of Ep-CAM in SCLC has been earlier described. However, reactivity of Mab 17-1A and another Ep-CAM-binding Mab BerEP4 has not yet been described for SCLC. Using immunohistology first with BerEP4 and confirmatory with 17-1A a homogenous and strong expression of Ep-CAM was seen in all SCLC specimens tested (n = 10). For comparison, heterogenous expression was seen in the non-SCLC tumors tested (n = 15). These findings strongly suggest that Mab 17-1A can be useful for adjuvant immunotherapy in patients with SCLC. For detection of Ep-CAM in tumor tissue in patients that might be considered for immunotherapy with Mab 17-1A, Mab BerEP4 appears to be preferred.

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.

Pathologic evaluation of unknown primary cancer

The pathologic approach to metastases of unknown primary cancer (UPC) is stepwise and uses the clinical context, morphology, and, where necessary, immunohistochemistry (IHC). This review covers the initial approach to a UPC biopsy; the diagnosis of malignancy and broad tumor typing into carcinoma, melanoma, lymphoma, or sarcoma; and further subtyping of carcinoma into germ cell (broadly included), squamous, neuroendocrine, and solid organ including liver and renal, and adenocarcinomas. Finally, for adenocarcinoma, the prediction of primary tumor site, including lung, pancreas, stomach, colon, ovary, prostate, and breast, is discussed. For each tumor type, the morphologic features are presented alongside established useful IHC markers, with a description of their staining patterns and common diagnostic dilemmas. Optimal tissue handling and IHC interpretation, quality assurance, and limitations also are discussed. The target readership is oncologists, but other clinicians and trainee pathologists also may find the content of use.

CXCR4+FOXP3+CD25+ Lymphocytes Accumulate in CXCL12-Expressing Malignant Pleural Mesothelioma

CXCL12 is a chemokine that binds to a G-protein-coupled receptor (CXCR4). CXCL12 is expressed in various tumors and is considered as playing an important role in tumor growth and invasion. The aim of this study is to investigate the expression of CXCL12 in human malignant mesothelioma (MM), the chemotactic effect of CXCL12 derived from MM, and the expression of CXCR4 in MM tissues in relation to regulatory T cells. CXCL12 expression was examined by immunostaining of tissue specimens from malignant pleural mesothelioma (MPM) and malignant peritoneal mesothelioma (MPEM). The MM group comprised 6 patients (4 men/2 women, MPM=4, MPEM=2, aged 56.0 ± 12.4 years) and the control (non-mesothelioma) group also had 6 patients (4 men/2 women aged 65.0 ± 6.7 years). CXCL12 mRNA expression was also examined by RT-PCR in MPM cell lines (H28, H2052, and H2058), while CXCR4 mRNA expression was examined by in situ hybridization in MPM tissue. CXCL12 was expressed in the cytoplasm of MM cells from all patients, but was not expressed in the control group. H2052 and H2058 cells expressed CXCL12 mRNA, but H28 cells did not. CXCL12 in MM tissue homogenate supernatant had a chemotactic effect on CXCR4-expressing THP-1 cells. CXCR4 mRNA was expressed by a part of LCA+CD3+ Foxp3+CD25+ T cells that were located adjacent to the border of CXCL12-expressing epithelioid MPM. These findings suggest that CXCL12 contributed to tumor-related inflammation by inducing the accumulation of CXCR4-expressing cells with regulatory T cell markers around MM.

Early photon tomography allows fluorescence detection of lung carcinomas and disease progression in mice in vivo

Imaging of targeted fluorescent probes offers significant advantages for investigating disease and tissue function in animal models in vivo. Conversely, macroscopic tomographic imaging is challenging because of the high scatter of light in biological tissue and the ill-posed nature of the reconstruction mathematics. In this work, we use the earliest-transmitted photons through Lewis Lung Carcinoma bearing mice, thereby dramatically reducing the effect of tissue scattering. By using a fluorescent probe sensitive to cysteine proteases, the method yielded outstanding imaging performance compared with conventional approaches. Accurate visualization of biochemical abnormalities was achieved, not only in the primary tumor, but also in the surrounding tissue related to cancer progression and inflammatory response at the organ level. These findings were confirmed histologically and with ex vivo fluorescence microscopy. The imaging fidelity demonstrated underscores a method that can use a wide range of fluorescent probes to accurately visualize cellular- and molecular-level events in whole animals in vivo.

Carcinoma of unknown primary origin

Carcinoma of unknown primary origin (CUP) is a heterogeneous group of cancers defined by the presence of metastatic disease with no identified primary tumor at presentation. Identifying patients with prognostically favorable disease is important, since they may derive substantial benefit, including prolonged survival, from directed treatment. In CUP cases, a focused search for the primary tumor is recommended. Whether CUP is a distinct molecular genotype-phenotype relative to metastases of known cancers is unknown. However, use of a robust immunohistochemical panel and emerging molecular data may permit development of a tailored treatment algorithm for CUP patients that will include appropriate use of targeted agents.

Claudin 4 identifies a wide spectrum of epithelial neoplasms and represents a very useful marker for carcinoma versus mesothelioma diagnosis in pleural and peritoneal biopsies and effusions

We evaluated the usefulness of the tight-junction associated protein Claudin 4 (CL-4) in the diagnosis of mesothelioma and mimickers, analyzing biopsies from 454 tumors, including 82 mesotheliomas, 336 carcinomas of different origin (278 primary, 58 metastatic to serosae), 36 nonepithelial spindle cell neoplasms, as well as 97 cytological samples from reactive effusions (12), mesothelioma (23) and metastatic carcinomas (62). CL-4 was consistently negative in normal and reactive mesothelium, as well as in all 82 mesotheliomas. In contrast, strong reactivity was found in 57/58 serosal metastasis, and in 245/278 primary carcinomas, with uppermost expression (150/153) in those most frequently involved in the differential with mesothelioma (lung, breast, gastrointestinal tract, pancreas, ovary, primary serous papillary carcinoma of peritoneum). On effusions, reactive and neoplastic mesothelial cells were regularly negative, while metastatic tumor cells stained positively in 60/62 (96.8%) cases. Among spindle cell neoplasms, only 2/9 biphasic synovial sarcomas and 4/4 follicular dendritic cell sarcomas stained positively. Results indicate that CL-4 reacts with the majority of epithelial neoplasms that often metastasize to serous membranes, representing a pancarcinoma marker with extremely high sensitivity and specificity. CL-4 may be considered a primary immunohistochemical reagent to rule out the diagnosis of mesothelioma.