Section 2 -

Peritoneal mesothelioma, foamy cell variant Anthony S.Y. Leong University of Newcastle Newcastle, Australia

Clinical History: A 69-year-old man first presented for general malaise and was found to have a large spleen. Peripheral blood counts were within normal limits and a 1265 g, 205 x 150 x 85mm spleen was removed. A 3mm thick layer of clearly demarcated tumor encased the spleen. The tumor was composed of sheets of large polygonal cells with abundant eosinophilic granular to clear cytoplasm. Nuclei were vesicular and contained prominent eosinophilic nucleoli. Mitotic activity was low (Figs 1, 2 and 3). Focal collections of pigment containing cells were present (Fig 4). Large foam cells were frequently present in the red pulp of the spleen (Fig 5). Immunostaining showed positivity for broad-spectrum cytokeratin with Cam 5.2 and the tumor was negative for S100. The diagnosis rendered was "metastatic carcinoma, clear cell type, a renal primary cannot be excluded". The patient represented in 44 months later with subacute intestinal obstruction and laparotomy revealed the greater omentum to be caked by a glistening opaque grey tumor with smaller deposits scattered in the peritoneum. An omental biopsy of 145 x 60 x 25 mm was obtained. Microscopic findings: The tumor cells were large and pleomorphic and showed abundant, foamy and eosinophilic granular cytoplasm with distinct cytoplasmic borders. They had vesicular nuclei with prominent eosinophilic nucleoli and occasional multinucleated cells were present. The tumor cells formed sheets, clusters and cords and focally micropapillary projections and gland-like structures were present. Immunostaining revealed positivity for calretinin, WT1, and CK5/6. EMA revealed distinct long microvilli on the surface of the tumor cells, often circumferential and aberrant in location, juxtaposed to collagen and stroma. Intra-cytoplasmic lumina were also displayed. D2-40 was positive, similarly highlighting surface microvilli. HBME1 stain was similarly positive. PAS/D, CEA, S100 and HMB45 were negative. The splenic foam cells were CD68 positive and did not bear any mesothelioma markers. Diagnosis: Peritoneal mesothelioma, foamy cell variant. Discussion: The diagnosis of mesothelioma, especially with unusual clinical presentation or histology, continues to be a challenge. Four main histologic subtypes of diffuse malignant mesothelioma are recognised. These include epithelial (including well-differentiated papillary), sarcomatous (including desmoplastic), biphasic and poorly differentiated, the majority (75%) of peritoneal mesotheliomas being of the epithelial subtype. In addition, unusual variants have been described and include lympho-histiocytoid, small cell, deciduoid, adenoid cystic, and a clear cell and signet ring (lipid-rich) pattern, with the localised mesothelioma and mesothelioma of tunica vaginalis being included as variant clinical subtypes. [1] Recently, a 56-year-old woman with no history of asbestos exposure was reported with a pleural mesothelioma of solid and tubulo-papillary pattern in which the tumor cells were large with vesicular nuclei and prominent nucleoli. [2] Cytoplasm was abundant and mostly eosinophilic but in about 40% of the neoplastic cells it was clear and foamy. Mitotic activity was low. The foamy nature of the tumor cells was shown to be due to collections of lipid droplets by electron microscopy. Foamy cell mesothelioma is very uncommon and should not be mistaken for its mimics. Foamy cell change has been recognised in carcinomas and foam cells may often be macrophages. Foamy macrophage-like neoplastic mesothelioma cells have been described in pleural effusions in three mesotheliomas in which subsequent histologic specimens revealed mesotheliomas of conventional histology with no foamy elements. [3] Rare foamy cells were described in a papillary mesothelioma of the tunica vaginalis [4] and clear cell change has been reported in mesotheliomas. [5, 6] Foamy macrophages have also been reported as a feature of a peritoneal mesothelioma which did not show foamy cell change. [7] More recently, an ultrastructural study of mesothelioma with clear cell features revealed that the most frequent cause of cytoplasmic clearing was the accumulation of large amounts of glycogen. Accumulation of cytoplasmic lipid alone or in conjunction with glycogen was a less common factor and other causes included marked mitochondrial swelling, the presence of intracytoplasmic vesicles and large numbers of intracytoplasmic lumens with microvilli. [8] The present case showed striking foamy and granular cells as a major component of the mesothelioma. This contributed to the failure to recognise the tumor initially and there were also striking foamy macrophages in the spleen. The differential diagnosis of malignant mesothelioma includes metastatic carcinoma and reactive mesothelium. Despite the increasing number of markers to separate these entities, some authors still feel that "immunohistochemical analysis still has a limited role in the diagnosis of malignant mesothelioma". [9] Calretinin, thrombomodulin, keratin 5/6, mesothelin and WT1 are more recent additions to the list of mesothelioma markers but each has varying sensitivity and specificity, none being 100% specific, with lowest sensitivity in sarcomatoid mesotheliomas. [10] As such, a panel of antibodies is essential for the diagnosis of mesothelioma (Table 1). Table 1: Antibody panel for mesothelioma, adenocarcinoma and reactive mesothelium vim CK5/6 Calret BerEP4 CEA Throm WT1 mesoth Mesothelioma + + + -/+ - + + + Adenocarcinoma -/+ +/- -/+ +/- -/+ +/- -/+ +/- Reactive mesoth +/- +/- -/+ - - +/- + +/- Calretinin is expressed in the majority of epithelial mesothelioma but is also common in small and large cell carcinomas of the lung. [11] Thrombomodulin and CK5/6 are also expressed in the great majority of keratinising and non-keratinising carcinomas of the lung. [9] TTF-1 is expressed exclusively by pulmonary carcinomas especially adenocarcinoma and thyroid tumors and should be included in the panel. [8]Importantly, calretinin and mesothelin may be positive in about 1/3 of thymic carcinomas [12] (and thymomas) which can also present in the pleural cavity. The diagnostic ultrastructural features that identify malignant mesothelioma from adenocarcinoma and reactive mesothelium include long microvilli that are devoid of core rootlets, aberrant long microvilli that are juxtaposed to stromal collagen, and microvilli coated with proteoglycan particles. Optimal staining for EMA can highlight such microvillous processes, [13] initially described as "thick membrane" staining. [14] Circumferentially disposed microvilli can also be demonstrated with EMA, correlating with the aberrant microvilli seen at ultrastructural level, a feature that is pathognomonic of malignant mesothelial cells. [15] Such long microvilli can also be demonstrated in intercellular and intracellular lumina with anti-EMA. In our hands, this marker is extremely useful for the identification of malignant mesothelioma, especially of the epithelial subtype. The more recent use of HMBE1 serves a similar purpose of highlighting the long microvillous processes. [16] D2-40 [17] and podoplanin [17] are two most recently developed markers for lymphatic endothelial cells that have been shown to mark mesothelial cells and mesotheliomas. The antigens are localised to the cell membrane and also produce a pattern of staining which highlights the long surface microvilli of mesotheliomas. References: Allen TC. Recognition of histopathologic patterns of diffuse malignant mesothelioma in differential diagnosis of pleural biopsies. Arch Pathol Lab Med 2005;129:1415-20. Cavazza A, Pasquinelli G, Agostini I, et al. Foamy cell mesothelioma. Histopathology 2002;41:369-71. Spriggs AL, Grunze H. An unusual cytologic presentation of mesothelioma in serous effusions. Acta Cytol 1983;27:288-92. Mikuz G, Hopfel-Kreiner I. Papillary mesothelioma of the tunica vaginalis propria testis. Case report and ultrastructural study. Virchows Arch A Pathol Anat Histol 1982;396:231-8. Ordonez NG, Mackay B. Glycogen-rich mesothelioma. Ultrastruct Pathol 1999;24:401-6. Dessy E, Falleni M, Braidotti P, et al. Unusual clear cell variant of epithelioid mesothelioma. Arch Pathol Lab Med 2001;125:1588-90. Kitazawa M, Kab]neko H, Toshima M, et al. Malignant peritoneal mesothelioma with massive foamy cells. Cod fish roe-like mesothelioma. Acta Pathol Jpn 1984;34:687-92. Ordonez NG. Mesothelioma with clear cell features: an ultrastructural and immunohistochemical study of 20 cases. Hum Pathol 2005;36:465-73. Roberts F, Harper CM, Downie I, Burnett RA. Immunohistochemical analysis still has a limited role in the diagnosis of malignant mesothelioma. A study of thirteen antibodies. Am J Clin Pathol 2001;116:253-62. Leong AS-Y, Leong FJW-M, Cooper K. Manual of Diagnostic Antibodies for Immunohistology. 2nd. London: Greenwich Medical Media, 2003. Miettinen M, Sarlomao-Rikala M. Expression of calretinin, thrombomodulin, keratin 5, and mesothelin in lung carcinomas of different types; an immunohistochemical analysis of 596 tumors in comparison with epithelioid mesotheliomas of the pleura. Am J Surg Pathol 2003;27:150-8. Pan CC, Chen PC, Chou TY, Chiang H. Expression of calretinin and other mesothelioma-related markers in thymic carcinoma and thymoma. Hum Pathol 2003;34:1155-62. Leong AS-Y, Vermin-Roberts E The immunohistochemistry of malignant mesothelioma. Pathology Annual 1994;29(II): 157-179. Leong AS-Y, Parkinson R, Milios J. "Thick" cell membranes revealed by Immunocytochemical staining: A clue to the diagnosis of malignant mesothelioma. Diagn Cytopath 1990;6:9-13. Leong AS-Y, Wick ME, Swanson P. Immunohistology and Electron Microscopy of Anaplastic and Pleomorphic Tumors. Cambridge: Cambridge University Press, 1997. Dahlstrom JE, Maxwell LE, Brodie N, et al. Distinctive microvillous brush border staining with HBME-1 distinguishes pleural mesotheliomas from pulmonary adenocarcinomas. Pathology 2001;33:287-91. Odonez NG. D2-40 and podoplanin are highly specific and sensitive immunohistochemical markers of epithelioid malignant mesothelioma. Hum Pathol 2005;36:372-80.