Reproducibility of histological subtyping of malignant pleural mesothelioma

Malignant pleural mesothelioma (MPM) has a very poor prognosis. Although clinical stage is currently the only reliable prognostic factor, histologic subtyping reportedly also affects prognosis. Some studies propose reclassification of pleomorphic epithelioid as biphasic or sarcomatoid MPM. This study assessed prognostic significance and interobserver agreement in MPM subtyping of small biopsy specimens. We analyzed biopsy specimens, and clinical and survival data from records of 108 patients who were diagnosed between 2000 and 2010 at the Institute of Pathology University of Zagreb School of Medicine, of whom 98 had epithelioid MPM, six biphasic MPM, and four sarcomatoid MPM. Among epithelioid subtypes, 44 (44.9 %) were solid, 19 (19.4 %) tubulopapillary, 18 (18.4 %) acinar, six (6.1 %) adenomatoid, five (5.1 %) pleomorphic, four (4.1 %) trabecular, and two (2.0 %) micropapillary subtype. Interobserver reliability for histological diagnosis was found to be kappa = 0.72 (P < 0.001). Median overall survival for epithelioid MPM was 10.5 months with an interquartile range (IQR) of 5.8-28.0 months but significantly shorter for the pleomorphic subtype (3 [IQR 3.0-8.0] months; P = 0.034), but not significantly different from biphasic (6.5 [IQR 3.5-15.3] months) and sarcomatoid mesothelioma (4.0 [IQR 1.3-6.8] months; P = 0.270). We found strong reproducibility of MPM subtyping with good interobserver agreement. Furthermore, our results indicate that pleomorphic subtype to be a predictor of poor prognosis and support classifying it with sarcomatoid or biphasic MPM, as patients with the pleomorphic, biphasic, or sarcomatoid subtype show similarly poor overall survival.