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

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One hundred twenty-one cases of anaplastic carcinoma of the thyroid treated at M. D. Anderson Cancer Center, Houston, were reviewed. Anaplastic carcinoma is a rapidly growing neoplasm with a dismal prognosis. The mean survival of our patients was 7.2 areas of well-differentiated thyroid carcinoma elsewhere, supporting the hypothesis that anaplastic thyroid carcinoma arises from preexisting well-differentiated thyroid carcinoma. Twenty-four of 30 tumors analyzed (84%) stained for keratin, 28 (93.3%) stained for vimentin, and ten (33%) stained for epithelial membrane antigen. Younger patients lived longer than older patients, and patients whose disease was earlier-stage at presentation responded better than patients with metastases at presentation. Radical surgery alone did not significantly increase survival duration over less radical surgery. The role of multimodality therapy needs further evaluation. Cancer 66:321-330,1990. 10 months. A significant percentage of our patients (35%) had NAPLASTIC THYROID CARCINOMA accounts for A 5% to 14% of primary malignant thyroid neoplasm^.'-^ In contrast to papillary and follicular thyroid carcinoma, anaplastic carcinoma is one of the most aggressive neoplasms affecting humans. This is exemplified by a 5-year survival rate of 7.1% and a mean survival period of 6.2 months seen in a previous review from The

Mesothelin is a differentiation antigen that was first described as the antigenic target of the monoclonal antibody K1. Using this antibody, it was demonstrated that mesothelin is strongly expressed in normal mesothelial cells, mesotheliomas, nonmucinous ovarian carcinomas, and some other malignancies. Immunostaining with the K1 antibody was suggested to be useful in the diagnosis of mesothelioma in the early 1990s. This, however, could not be further explored until recently because of the lack of commercially available anti-mesothelin antibodies. In a recent investigation by this author, all epithelioid mesotheliomas and about 40% of the lung adenocarcinomas reacted with the 5B2 anti-mesothelin antibody, which has only recently become commercially available. It was concluded that immunostaining with this antibody has limited value in discriminating between these conditions. The aim of the current study was to further investigate the potential application of the 5B2 antibody in tumor diagnosis. Mesothelin expression was evaluated in formalin-fixed, paraffin-embedded samples of normal tissues and in 471 tumors of various origins. The carcinomas that most frequently exhibited strong mesothelin reactivity were nonmucinous carcinomas of the ovary (14 of 14 serous, 3 of 3 endometrioid, 6 of 8 clear cell, and 4 of 4 transitional cell carcinoma), and adenocarcinomas of the pancreas (12 of 14), the ampulla of Vater (3 of 3), endometrium (7 of 11), lung (14 of 34), and liver (7 of 19 cholangiocarcinomas). The carcinomas that did not express mesothelin included renal cell carcinomas, hepatomas, carcinomas of the thyroid, adrenal cortical carcinomas, prostatic adenocarcinomas, and carcinoid tumors. All germ cell tumors, with the exception of teratomas, were consistently negative for mesothelin. Because of the strong mesothelin expression in nonmucinous carcinomas of the ovary, but not in a variety of tumors with which these lesions may be confused (eg, clear cell carcinoma of the ovary versus endodermal sinus tumor or renal cell carcinoma, clear cell type; transitional cell carcinoma of the ovary versus TCC of the urinary tract), immunostaining for this marker could be useful in establishing the differential diagnosis. The strong mesothelin expression in the large majority of pancreatic ductal adenocarcinomas (12 of 14), but not in normal pancreas, confirms that this marker may have some diagnostic utility in discriminating between neoplastic and nonneoplastic pancreatic ductal epithelium. The mesothelin expression in about one-third of the cholangiocarcinomas, but not in hepatomas, suggests that this marker may have some utility in distinguishing between these two malignancies when they are poorly differentiated. In the group of small round blue cell tumors, only desmoplastic small round cell tumors exhibited mesothelin positivity (7 of 12). Of the soft tissue tumors, only the epithelial component of biphasic synovial sarcomas (9 of 9) expressed mesothelin. These findings indicate that, in some instances, mesothelin immuno...

Adenocarcinomas account for approximately 2% of primary epithelial malignancies of the urinary bladder. The clinicopathologic features of 72 cases treated at one institution are reported; 22 cases were evaluated immunohistochemically. Twenty-four tumors were urachal and 48 nonurachal. The cases were analyzed according to their stage at presentation, histologic type, and mucin staining; they were tested immunohistochemically to determine their reaction to carcinoembryonic antigen, Leu-M1, prostate-specific antigen, and prostatic acid phosphatase. Tumor stage was a highly significant predictor of outcome (P = 0.001). Nonurachal tumors tended to have a worse outcome than urachal, but the difference was not statistically significant (P = 0.07). Histologic type was not a significant predictor of outcome (P = 0.10). For adenocarcinoma of the urinary bladder, stage was the most significant predictive factor; separating urachal from nonurachal tumors was important, but mucin histochemistry and immunohistochemistry did not help in this distinction. On occasion, a few tumors may react with some polyclonal antibodies to prostate-specific antigen; thus these results must be interpreted with caution. In these instances, the possibility of using highly sensitive and specific monoclonal antibodies such as the one employed in this study should be considered.

These data suggest that the orthotopic injection of human prostate cancer cells into the nude mouse may provide a valuable model to study the biology and therapy of human prostate cancer.

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