Malignant mesothelioma with a pronounced myxoid stroma: a clinical and pathological evaluation of 19 cases.

Mesothelioma with a pronounced myxoid stroma has been identified as a morphological pattern that might portend a better prognosis. Reports on this type of cases are few. Its clinical and pathological features are not defined. In this study, we identified 19 such cases from a series of 234 pleural mesotheliomas and performed a comprehensive clinical and pathological analysis. The inclusion criteria were mesotheliomas with at least 50% of the tumor exhibiting a pronounced myxoid stroma that occupied more than 50% of the tumor volume (designated as myxoid mesothelioma). There were ten males and nine females, with a median age of 58 years. Forty seven percent of the patients had probable or definite asbestos exposure. Patients presented at various stages [International Mesothelioma Interest Group (IMIG) stage II, 4; stage III, 9; and stage IV, 6]. Most (16/19) patients were treated by extrapleural pneumonectomy with adjuvant radiation or chemotherapy or both. Overall, the median survival rate was 36 months (median follow-up time, 17 months), and the 2-year survival rate was 79%. Histologically, the tumor cell component was entirely epithelioid without significant cytologic atypia. The myxoid material stained positive for Alcian blue, and the staining diminished after treatment with hyaluronidase in 12 of 12 cases. The tumor cells showed a typical mesothelial immunophenotype. Ultrastructurally, all six tumors examined had typical mesothelial-type surface microvilli and a moderately electron-dense extracellular amorphous material that often formed a haze enmeshing the surface microvilli. Hyaluronic acid-type, fern-like crystals were noted in all cases. These findings show that myxoid mesotheliomas represent a group of epithelioid mesotheliomas that have retained the secretory activity of normal mesothelium. Patients may present at different stages, but survival appears to be superior to that of epithelioid mesotheliomas in general. Our study emphasizes the need for better attention to histologic subtypes, particularly in the context of prognostically or therapeutically oriented investigations of this lethal disease.