Value of PAX8, PAX2, napsin A, carbonic anhydrase IX, and claudin-4 immunostaining in distinguishing pleural epithelioid mesothelioma from metastatic renal cell carcinoma

An Overview of Advances in Rare Cancer Diagnosis and Treatment

Cancer stands as the leading global cause of mortality, with rare cancer comprising 230 distinct subtypes characterized by infrequent incidence. Despite the inherent challenges in addressing the diagnosis and treatment of rare cancers due to their low occurrence rates, several biomedical breakthroughs have led to significant advancement in both areas. This review provides a comprehensive overview of state-of-the-art diagnostic techniques that encompass new-generation sequencing and multi-omics, coupled with the integration of artificial intelligence and machine learning, that have revolutionized rare cancer diagnosis. In addition, this review highlights the latest innovations in rare cancer therapeutic options, comprising immunotherapy, targeted therapy, transplantation, and drug combination therapy, that have undergone clinical trials and significantly contribute to the tumor remission and overall survival of rare cancer patients. In this review, we summarize recent breakthroughs and insights in the understanding of rare cancer pathophysiology, diagnosis, and therapeutic modalities, as well as the challenges faced in the development of rare cancer diagnosis data interpretation and drug development.

Clear cell mesotheliomas with inactivating VHL mutations and near-haploid genomic features

Clear cell mesothelioma is uncommon and shows predominance of clear cells with resemblance to clear cell carcinomas. Clinicopathologic and molecular descriptions of clear cell mesothelioma remained limited. In this study, we identified an index patient with clear cell mesothelioma, confirmed by immunohistochemical and ultrastructural studies. Targeted next-generation sequencing revealed the presence of an inactivating VHL mutation. We then systematically searched for VHL-mutant mesotheliomas in a comprehensive genomic profiling database of 1532 mesotheliomas. Collectively, we identified a cohort of four VHL-mutant clear cell mesotheliomas, including three peritoneal and one pleural tumors from three females and one male, with age range of 47-68 (median 63) years. Histologically, each tumor showed a microcystic to tubulopapillary architecture with prominent clear cells. By next-generation DNA sequencing, each of the four clear cell mesotheliomas harbored inactivating VHL mutations, while lacking other alterations typical of mesotheliomas such as BAP1, NF2, SETD2, CDKN2A, CDKN2B, TP53, and PTEN. By using low-pass whole genome sequencing on the index case and targeted next-generation sequencing on the remaining three cases, we identified extensive loss of heterozygosity throughout the genome but consistently sparing chromosomes 5, 7, and 20, characteristic of genomic near-haploidization. In summary, clear cell mesotheliomas were characterized by inactivating VHL mutations and genomic near-haploidization and appeared to represent a distinct clinicopathologic and molecular category of mesotheliomas. Our findings implicate VHL in the pathogenesis of a subset of mesotheliomas, particularly those with clear cell morphology.

Napsin A is a highly sensitive marker for nephrogenic adenoma: an immunohistochemical study with a specificity test in genitourinary tumors

Nephrogenic adenomas are uncommon benign lesions that are typically cytologically bland, but degenerative and reactive changes may make it difficult to distinguish these lesions from malignant entities, such as urothelial carcinoma and prostatic adenocarcinoma. In this study, we explored whether napsin A, a sensitive marker for lung adenocarcinoma, may also have a role in distinguishing nephrogenic adenoma from other genitourinary lesions. Immunohistochemically, napsin A was expressed in all 43 nephrogenic adenomas (bladder: 38, prostatic urethra: 4, and ureter: 1; mean positive tumor cells: 72%, median: 80%, range: 15-100%) and showed regional variability in its expression pattern with a bias toward surface architectures (flat, papillary) compared with stromal architectures (tubular/glandular, microcystic). We also compared napsin A with other markers including PAX8, GATA3, p63, and 34BE12. Although napsin A matched PAX8 in terms of its sensitivity for nephrogenic adenoma (100%), napsin A stained a lower percentage of tumor cells than PAX8 (72% vs 99%, respectively, P = 1.0 × 10^-5). P63 was negative in all nephrogenic adenomas, whereas GATA3 showed variable staining in 25 cases (58%). All 43 nephrogenic adenomas showed variable 34BE12 staining. Finally, we profiled napsin A expression among 401 genitourinary tumors on tissue microarrays (n = 308) and full tissue blocks (N = 93) and observed napsin A positivity in 37 tumors (9%), which included urothelial carcinomas with the glandular/microcystic component differentiation (in the glandular/microcystic component in 4/6), bladder adenocarcinomas (primary: 4/4 and metastatic: 3/3), urinary tract clear-cell carcinomas (primary: 8/9, metastatic uterine primary: 1/1), and some renal tumors (17/174). All 81 pure urothelial carcinomas and 53 prostatic acinar adenocarcinomas were negative for napsin A. Our study indicates that napsin A is a highly sensitive marker for nephrogenic adenoma and can serve as a useful addition in immunohistochemical panels seeking to distinguish it from pure urothelial carcinoma and prostatic acinar adenocarcinoma but not clear-cell carcinoma or urothelial carcinoma with glandular differentiation.

Claudin-4 shows superior specificity for mesothelioma vs non-small-cell lung carcinoma compared with MOC-31 and Ber-EP4

The distinction of malignant mesothelioma from non-small-cell lung carcinoma (NSCLC) usually requires immunohistochemistry, but some broad-spectrum carcinoma markers stain mesotheliomas, and it remains unclear which broad-spectrum markers are most valuable for distinguishing these malignancies. Here, we directly compared the sensitivity and specificity of three broad-spectrum carcinoma markers, claudin-4, Ber-EP4, and MOC-31, for distinguishing NSCLC from mesothelioma. Immunohistochemistry was performed on tissue microarrays containing 68 epithelioid mesotheliomas, 31 sarcomatoid mesotheliomas, and 147 non-small-cell lung cancers (53 adenocarcinomas, 60 squamous cell carcinomas, 13 large-cell carcinomas, and 21 sarcomatoid carcinomas). For adenocarcinoma, squamous cell carcinoma, and large-cell carcinoma, claudin-4 staining was present in 103 of 126 cases (82%), MOC-31 staining was present in 112 of 126 cases (89%), and Ber-EP4 staining was present in 113 of 126 cases (90%); these values were not statistically different. Claudin-4 stained 0 of 68 (0%), MOC-31 stained 22 of 68 (32%), and Ber-EP4 stained 24 of 68 (35%) epithelioid mesotheliomas; thus, the specificities for NSCLC versus epithelioid mesothelioma were 100%, 68%, and 65%, respectively. Claudin-4 staining was present in 7 of 21 (33%), MOC-31 staining was present in 8 of 21 (38%), and Ber-EP4 staining was present in 5 of 21 (24%) sarcomatoid carcinomas. All three markers were negative in 12 of 21 (57%) sarcomatoid carcinomas. Sarcomatoid mesotheliomas were not stained with any of these markers. We conclude that claudin-4 has considerably greater specificity and comparable sensitivity to MOC-31 and Ber-EP4 for distinguishing NSCLC from epithelioid malignant mesothelioma. The use of all three markers may be necessary for sarcomatoid neoplasms, given their limited sensitivity.

The Diagnosis of Pleural Tumors Other Than Mesothelioma

CONTEXT

- Pleural pathology has been dominated by discussions relating to the diagnosis, prognosis, etiology, and management of malignant mesothelioma. However, there exists a diverse group of other neoplasms that involve the pleura; the most common by far is metastatic carcinoma, usually of pulmonary origin. Other metastatic tumors of varied histogenesis do occur but are less common. Primary pleural neoplasms other than diffuse malignant mesothelioma are either uncommon or rare and have received less attention.

OBJECTIVE

- To provide a review of those diverse tumors that can involve the pleura other than mesothelioma in order to facilitate their accurate diagnosis.

DATA SOURCES

- Review of relevant literature published via PubMed and other search engines.

CONCLUSIONS

- A wide variety of tumors can involve the pleura. In most cases, the approach of considering the morphologic features with appropriate immunohistochemistry, in the correct clinical context, allows for a confident diagnosis. For a number of those soft tissue tumors that are well recognized in the pleura, such as solitary fibrous tumor, desmoid-type fibromatosis, synovial sarcoma, and epithelioid hemangioendothelioma, novel markers now exist based on an understanding of the individual tumors' molecular characteristics. Primary pleural lymphomas are rare with poor prognosis. They represent localized specific diffuse large B-cell lymphomas, with either post-germinal center B-cell or plasma cell lineage, arising in the context of either immunodeficiency or immune sequestration and with viral infection.

CAIX and pax-8 Commonly Immunoreactive in Endolymphatic Sac Tumors: A Clinicopathologic Study of 26 Cases with Differential Considerations for Metastatic Renal Cell Carcinoma in von Hippel-Lindau Patients

Endolymphatic sac tumors (ELSTs) are rare, slowly growing temporal bone neoplasms which show a high association with von Hippel-Lindau (VHL) syndrome. The immunohistochemistry evaluation of these papillary-cystic neoplasms frequently raises the differential diagnosis with renal cell carcinoma, among other metastatic neoplasms, whether in VHL patients or not. A cohort of 26 patients with ELSTs were evaluated for histologic features, immunohistochemistry findings, and association with VHL. Standard immunohistochemistry evaluation was performed. Sixteen females and 10 males ranging in age from 10 to 69 years (mean 44; VHL mean: 32) at initial presentation, comprised the cohort of patients. Most (86%) experienced hearing changes or inner ear symptoms (vertigo, dizziness), with an average duration of symptoms for 39 months (range 2-240 months). The tumors were an average of 2.9 cm (range 0.4-8 cm), with 14 left, 11 right sided and one bilateral tumor. Nine patients had documented VHL, with 3 patients having a concurrent or subsequent clear cell renal cell carcinoma. Patients were followed an average of 6.2 years (available in 24 patients): 19 alive without disease, 7.5 years; 2 dead without disease, 1.2 years; and 3 alive with disease, 3.1 years. The neoplastic cells show the following immunohistochemistry findings: AE1/AE3, EMA, CK7, CAIX, GLUT1, VEGF: 100% of cases tested were positive; pax-8: 85% of cases positive; CD10 and RCC: 0% of cases reactive. Based on this cohort of 26 patients with ELST, 9 of whom had VHL, the strong pax-8 and CAIX should be used in conjunction with negative CD10 and RCC to help exclude a metastatic renal cell carcinoma. As CAIX is an enzyme overexpressed in hypoxia and hypoxia inducible factor is what VHL protein regulates, this is an expected, although previously unreported finding. Whether part of VHL or not, VHL mutations may be a somatic rather than germline finding in the tumors, a possible further explanation for the CAIX reaction.

Updates in Effusion Cytology

Effusion cytology plays multiple roles in the management of benign and malignant disease, from primary diagnosis to tissue allocation for ancillary diagnostic studies and biomarker testing of therapeutic targets. This article summarizes recent advances in pleural effusion cytology, with a focus on the practical application of immunohistochemical markers, cytogenetic techniques, flow cytometry, and molecular techniques for the diagnosis and management of primary and secondary neoplasms of the pleura.

The diagnostic role of BAP1 in serous effusions

The aim of this study was to analyze the diagnostic role of BAP1 in effusion cytology. Effusions (n = 258), consisting of 53 malignant mesotheliomas and 205 other cancers, the majority carcinomas (62 breast, 60 ovarian, 31 lung, 51 carcinomas of other origin, 1 melanoma), were analyzed for BAP1 expression using immunohistochemistry. BAP1 was lost in 46 (87%) mesotheliomas compared with 4 (2%) of 205 other cancers (P < .001), resulting in sensitivity and specificity of 87% and 98%, respectively. There was no significant difference between peritoneal (n = 14) and pleural (n = 39) mesotheliomas. The 4 carcinomas with loss of BAP1 included 1 ovarian, 1 breast, 1 uterine cervical, and 1 gastric carcinoma. The present study supports the role of BAP1 as a highly sensitive and specific marker for malignant mesothelioma in serous effusions and argues for inclusion of this test in all specimens in which this diagnosis is considered.

The pathological and molecular diagnosis of malignant pleural mesothelioma: a literature review

Malignant pleural mesothelioma (MPM), an asbestos-induced tumor, represents significant diagnostic challenges for pathologists. Its histological diagnosis is stepwise and should be based on morphological assessment, supported by clinical and radiological findings, and supplemented with immunohistochemistry (IHC) and, more recently, molecular tests. The main diagnostic dilemmas are the differential diagnoses with benign mesothelial proliferations and other pleural malignant tumors. The present review is an update regarding the morphological, immunohistochemical, and molecular features with respect to MPM diagnosis. Data sources include a survey of the biomedical literature from PubMed (http://www.ncbi.nlm.nih.gov/pubmed) and textbooks focusing on the pathological diagnosis of MPM and associated immunohistochemical and molecular markers. The histological findings of MPM could facilitate its diagnosis and provide important prognostic information. The immunohistochemical approach should rest on the application of a panel including positive (mesothelial-related) and negative markers with greater than 80% sensitivity and specificity, which need to be selected based on morphology and clinical information. Moreover, in challenging cases, fluorescent in situ hybridization (FISH) testing for the p16 deletion and IHC to evaluate the loss of BRCA1-associated protein 1 (BAP1) expression could be useful in distinguishing benign from malignant pleural proliferations.

Immunohistochemistry in Peritoneal Mesothelioma: A Single-Center Experience of 244 Cases

Context.—

Diagnosis of malignant mesothelioma is more common in the chest than it is in the abdomen. Most published immunohistochemistry data are more applicable to pleural than to peritoneal mesothelioma.

Objective.—

To clarify the practical utility of 17 immunohistochemistry markers in the differential diagnosis of peritoneal mesothelioma with an emphasis on stains for which there is either contradictory information or a paucity of literature.

Design.—

Consultation files of peritoneal mesothelioma diagnoses rendered from 1999 to 2014 were reviewed; 244 cases were identified. The results of immunohistochemistry markers performed were tabulated.

Results.—

Immunohistochemistry markers positive in peritoneal mesothelioma in order of sensitivity were calretinin (244 of 244; 100%), WT1 (205 of 218; 94%), CK5/6 (173 of 194; 89%), mesothelin (132 of 150; 88%), and D2-40 (78 of 97; 80%). Markers used to differentiate carcinoma from mesothelioma showed immunoreactivity in peritoneal mesothelioma: estrogen receptor (2 of 84; 2%), B72.3 (6 of 196; 3%), CK20 (5 of 116; 4%), CD15 (7 of 192; 4%), p63 (3 of 62; 5%), carcinoembryonic antigen (9 of 199; 5%), PAX8 (12 of 191; 6%), progesterone receptor (5 of 71; 7%), Ber-EP4 (17 of 209; 8%), and CD138 (9 of 91; 10%). BAP1 loss, increasingly used in the differential diagnosis of benign versus malignant mesothelial proliferation, occured in 55% (99 of 181) of peritoneal mesothelioma cases.

Conclusions.—

The results support the experience that there is no definitive marker to rule out malignant mesothelioma, including PAX8, estrogen receptor, progesterone receptor, and p63 immunoreactivity. The high rate of immunoreactivity for mesothelin may have a role as a predictive marker for immune targeting. BAP1 loss of 55% in this cohort of peritoneal mesothelioma confirms published observations, and BAP1 retention is seen in a significant proportion of neoplastic cases.

Pax8, Napsin A, and CD10 as Immunohistochemical Markers of Canine Renal Cell Carcinoma

Pax8, napsin A, and CD10 are useful immunohistochemical markers of human renal cell carcinoma (RCC); however, their diagnostic utility in canine RCC is unclear. Forty formalin-fixed paraffin-embedded renal cell carcinomas from dogs (15 papillary, 12 solid, and 13 tubular) and 10 metastases were evaluated for expression of Pax8, napsin A, and CD10. Thirty-nine (98%), 24 (60%), and 19 (50%) tumors expressed Pax8 (nuclear labeling), napsin A (cytoplasmic labeling), and CD10 (cytoplasmic and membranous labeling), respectively. Pax8 was expressed in 92% of solid, 100% of papillary, and 100% of tubular tumors. Napsin A was expressed in 58% of solid, 60% of papillary, and 62% of tubular RCC. CD10 was expressed in 33% of solid, 47% of papillary, and 62% of tubular RCC. Pax8 was expressed in 80% of the metastatic tumors, napsin A in 60%, and CD10 in 50%. Additionally, Pax8 immunoreactivity was stronger overall than that of napsin A or CD10. In summary, Pax8 is a more sensitive marker than napsin A or CD10 for primary and metastatic canine RCC; its nuclear and more intense reactivity also makes it easier to interpret. Tubular and papillary RCCs were more likely than solid RCC to express all 3 markers. These findings highlight the utility of Pax8 as an immunohistochemical marker in diagnosing all major subtypes of canine primary and metastatic renal cell carcinoma.

CD24 is highly useful in differentiating high-grade serous carcinoma from benign and malignant mesothelial cells

CD24 was previously shown to be overexpressed in high-grade serous carcinoma (HGSC) effusions compared to malignant mesothelioma (MM) in gene expression array analysis. The present study validated this observation in a large series consisting of both effusions and surgical specimens. Effusions (n = 206; 100 HGSC, 16 ovarian carcinomas of other histological types, 54 breast carcinomas, 36 MM) and surgical specimens (n = 182; 117 ovarian carcinomas, 65 MM) were analyzed for CD24 expression using immunohistochemistry. CD24 was expressed in 105/116 (91%) ovarian carcinoma and 16/54 (30%) breast carcinoma effusions, while it was uniformly absent in MM (0/36; 0%; P < .001). Reactive mesothelial cells were CD24-negative in all carcinoma specimens. Comparative analysis of 117 solid primary (n = 43) and metastatic (n = 74) ovarian carcinomas and 65 solid MM specimens showed CD24 expression in 46% (54/117) of the former compared to 3% (2/65) of the latter (P < .001). Comparative analysis of ovarian carcinomas at different anatomic sites showed significantly higher CD24 expression in effusions compared to solid ovarian and metastatic lesions (P < .001), with similar results when analysis was limited to HGSC (P < .001). In conclusion, CD24 is a highly sensitive and specific marker of ovarian carcinoma in the differential diagnosis from MM and reactive mesothelium in effusions. CD24 is similarly a specific marker in surgical specimens, though with lower sensitivity. The overexpression of CD24 in ovarian carcinoma effusions compared to solid lesions may be due to the acquisition of cancer stem cell characteristics.

Metastatic carcinoma of unknown primary: diagnostic approach using immunohistochemistry

Carcinoma of unknown primary origin (CUP) is one of the 10 most prevalent malignancies. CUP patients in whom a site of origin can be ascribed have better outcomes than those in which the primary tumor remains unidentified. Among the tools available to pathologists in approaching these lesions, immunohistochemistry is a reliable, inexpensive, and widely available resource. New markers continue to emerge, which, in combination with other historically useful antibodies, allow rapid and accurate identification of primary site in an increasing number of cases. This review discusses the approach to the diagnosis of CUP using immunohistochemistry and outlines some of the most useful markers with a particular focus on the utility of lineage-restricted transcription factors, including CDX2, NKX3-1, PAX8, SATB2, TTF-1, and SF1.

Immunohistochemistry in undifferentiated neoplasm/tumor of uncertain origin

CONTEXT

Immunohistochemistry has become an indispensable ancillary study in the identification and classification of undifferentiated neoplasms/tumors of uncertain origin. The diagnostic accuracy has significantly improved because of the continuous discoveries of tissue-specific biomarkers and the development of effective immunohistochemical panels.

OBJECTIVES

To identify and classify undifferentiated neoplasms/tumors of uncertain origin by immunohistochemistry.

DATA SOURCES

Literature review and authors' research data and personal practice experience were used.

CONCLUSIONS

To better guide therapeutic decisions and predict prognostic outcomes, it is crucial to differentiate the specific lineage of an undifferentiated neoplasm. Application of appropriate immunohistochemical panels enables the accurate classification of most undifferentiated neoplasms. Knowing the utilities and pitfalls of each tissue-specific biomarker is essential for avoiding potential diagnostic errors because an absolutely tissue-specific biomarker is exceptionally rare. We review frequently used tissue-specific biomarkers, provide effective panels, and recommend diagnostic algorithms as a standard approach to undifferentiated neoplasms.

Diagnostic utility of immunohistochemistry in distinguishing between epithelioid pleural mesotheliomas and breast carcinomas: a comparative study

Epithelioid mesotheliomas and breast carcinomas can present a variety of morphologic patterns. Because of this, breast carcinomas that metastasize to the pleura and lung may be confused with mesotheliomas. The aim of the present study is to compare the immunohistochemical markers currently available for the diagnosis of these 2 malignancies and to determine the best panel of markers that can be used to assist in discriminating between them. Sixty epithelioid mesotheliomas and 80 breast carcinomas (40 triple negative and 40 estrogen receptor positive) were investigated for expression of the positive mesothelioma markers calretinin, keratin 5/6, mesothelin, podoplanin, thrombomodulin, and WT1; the positive carcinoma marker claudin 4; and the breast-associated markers gross cystic disease fluid protein 15 (GCDFP-15), mammaglobin, and GATA3. All of the epithelioid mesotheliomas reacted for calretinin and keratin 5/6, 93% for WT1; 88% for podoplanin; 77% for thrombomodulin; 23% for GATA3; and 0% for claudin 4, GCDFP-15, and mammaglobin, respectively. Of the triple-negative breast carcinomas, 100% expressed claudin 4; 5%, keratin 5/6; 30%, GATA3; 18%, mammaglobin; 15%, GCDFP-15; 56%, mesothelin; 38%, calretinin; 18%, thrombomodulin; 5%, WT1; and 3%, podoplanin. Among the estrogen receptor-positive breast carcinomas, 100% were claudin 4 and GATA3 positive; 70% expressed GCDFP-15; 63%, mammaglobin; 13%, calretinin; 13%, thrombomodulin; 8%, WT1; 5%, keratin 5/6; 3%, mesothelin; and 0%, podoplanin. It is concluded that podoplanin and WT1 are the best positive mesothelioma markers for differentiating epithelioid mesotheliomas from breast carcinomas. An accurate differential diagnosis can be reached with the use of these two markers in combination with the breast-associated markers GCDFP-15, mammaglobin, and GATA3.

Napsin A expression in small cell carcinoma of the lung: a cytologic study with review of differentials

INTRODUCTION

Napsin A is a diagnostic marker for pulmonary adenocarcinoma and a useful alternative to thyroid transcription factor 1 (TTF-1). TTF-1 also stains pulmonary small cell carcinoma (SCCA). Napsin A expression in SCCAs is not as established as it is in non-SCCAs. We analyzed napsin A and TTF-1 expression in 36 previously confirmed cytologic cases of pulmonary SCCA. Ours is currently the largest cytologic series of such cases examined for napsin A expression.

MATERIALS AND METHODS

Thirty-six patients, (20 men, 16 women), age 43-87 years, mean 57 years, had primary or metastatic pulmonary SCCA diagnosed by fine-needle aspiration biopsies of mediastinum (n = 5); liver (n = 3); subcutaneous nodule (n = 1); lung (n = 6); and axillary, cervical, and mediastinal lymph nodes (n = 20), as well as a pleural effusion (n = 1). Napsin A and TTF-1 expression was tested. Also, previous expression (or lack thereof) with immunocytochemical stains pancytokeratin and neuroendocrine markers (synaptophysin, chromogranin, and cluster of differentiation marker CD56) were noted.

RESULTS

All cases of pulmonary SCCA were positive for pancytokeratin. TTF-1 was positive in 35 of 36 cases (97%), and napsin A was negative in all 36 cases (100%). All 36 cases expressed ≥ 1 neuroendocrine marker, including the TTF-1 negative case.

CONCLUSIONS

This study showed napsin A was negative in all pulmonary SCCAs. This stain may prove to be a useful exclusionary marker in distinguishing pulmonary SCCA from other poorly differentiated lung carcinomas with similar morphologic features, especially those with concomitant TTF-1 expression.

Claudin-4 immunohistochemistry is highly effective in distinguishing adenocarcinoma from malignant mesothelioma in effusion cytology

BACKGROUND

Adenocarcinoma can be challenging to distinguish from malignant mesothelioma in effusions, and this distinction often requires ancillary studies and clinical correlation. Immunohistochemistry for claudin-4, a tight-junction-associated protein, has recently been shown to distinguish adenocarcinoma from malignant mesothelioma, mostly in surgical specimens. Our aim was to validate and assess the immunoreactivity profile of claudin-4 in a large series of malignant effusions.

METHODS

We evaluated 159 malignant effusions (84 adenocarcinomas and 75 malignant mesotheliomas). Claudin-4 immunohistochemistry was performed on cell-block paraffin sections and scored for staining intensity, staining pattern (cytoplasmic versus membranous), and percentage of positive tumor cells. Appropriate positive and negative controls were used throughout.

RESULTS

All cases of mesothelioma were negative for claudin-4 (0 of 64). Eighty-three of 84 cases of adenocarcinoma were positive (99%); 1 case of serous carcinoma was negative. Most adenocarcinomas showed strong and diffuse membranous staining (71 of 84; 84%); 12 cases (14%) showed membranous staining of moderate intensity. The overall sensitivity for adenocarcinoma was 99% (83 of 84).

CONCLUSIONS

Claudin-4 immunohistochemistry effectively distinguishes adenocarcinoma from malignant mesothelioma with high sensitivity and specificity in the evaluation of malignant effusions.