Prognostic significance of morphological growth patterns and mitotic index of epithelioid malignant peritoneal mesothelioma

Aims The prognostic significance of histological subtyping of epithelioid pleural mesotheliomas has been recently reported, but similar data are lacking for peritoneal mesotheliomas. The aim of this study was to investigate possible relationships between histological growth patterns of epithelioid peritoneal mesotheliomas, clinicopathological features, and patient outcome. Methods and results Eighty‐four cases of chemotherapy‐naive epithelioid peritoneal mesothelioma were classified into tubulopapillary, micropapillary, papillary, tubular, solid and trabecular growth patterns. Pathological features such as depth of invasion, lymphocytic host response, mitotic count, nuclear grade, lymphovascular invasion, lymph node metastasis and stromal desmoplasia were analysed. The most common histological patterns were solid (n = 37, 44%), tubulopapillary (n = 24, 29%), and micropapillary (n = 11, 13%). The overall median survival was 36 months. Patients with solid mesothelioma had shorter overall survival (median, 29 months) than patients with tubulopapillary and micropapillary growth patterns (median, 51 and 53 months, respectively; P = 0.053). A high mitotic index (>5 in 50 high‐power fields) was found to be associated with poor survival (P < 0.03). A moderate to severe lymphocytic host response was associated with longer median survival (P = 0.13). Conclusions Our study highlights the prognostic importance of the solid growth pattern among diffuse epithelioid peritoneal mesotheliomas, and reaffirms mitotic index as a predictor of overall survival.