Histopathological features of epithelioid malignant pleural mesotheliomas in patients with extended survival

Diffuse malignant mesothelioma (DMM) of the pleura is a rare and aggressive disease, wherein the long-term survival (LTS) rate is low. The epithelioid subtype is the most prevalent form of DMM with the best prognosis. To study prognostic histopathologic factors associated with extended survival in epithelioid DMM, we examined 43 tumors from patients with survival more than five years (LTSs) and compared the findings with 84 tumors from a reference group (RG) with average survival. We analyzed the tumors considering previously published histopathological prognostic features and attempted to identify additional morphological features predictive of extended survival. Most of the LTS tumors presented with nuclear grade I (n = 34, 90%) and a tubulopapillary growth pattern (n = 30, 70%). One LTS tumor had necrosis. In contrast, nuclear grade II (n = 49, 61%) and solid growth pattern (n = 59, 70%) were more frequent in the RG, and necrosis was present in 16 (19%) tumors. We also evaluated the association of asbestos lung tissue fiber burden quantified from autopsy samples with histopathological features and found that elevated asbestos fiber was associated with higher nuclear grade (P < 0.001) and the presence of necrosis (P = 0.021). In univariate survival analysis, we identified the following three novel morphological features associated with survival: exophytic polypoid growth pattern, tumor density, and single mesothelium layered tubular structures. After adjustments, low nuclear grade (P < 0.001) and presence of exophytic polypoid growth (P = 0.024) were associated with prolonged survival. These results may aid in estimating DMM prognosis.