Mixed epithelial and stromal tumour (MEST) of the kidney: report of 14 cases with male and PEComatous variants and proposed histopathogenesis

SummaryAimsThis article adds new cases and variants of MEST with discussion of the histopathogenesisMethods and resultsFourteen MEST were originally diagnosed as cystic nephroma which represents an incidence of 1.6% of renal neoplasms in adults. In females, the stromal component showed areas of mullerian differentiation with positive immunoreactivity for oestrogen (ER) and progesterone receptors (PR) and CD10. Immunoreactivity for HMB45 was identified in a single case having a leiomyomatous appearance. The epithelial component displayed features of mullerian epithelium and reactive renal tubular cells. In two male cases, MEST consisted of fibrous and smooth muscle stroma and cysts lined only by reactive renal tubular cells. Immunoreactivity for ER and PR was focal and weak.ConclusionsMEST represents a tumour developing from mullerian-like stromal cells in the kidney. The neoplastic stroma encroaches on the renal tubules and has the potential to stimulate the growth of the renal tubules by contact, with development into cysts. Furthermore, the mullerian stroma likely induces the renal tubules to differentiate into mullerian- like epithelium. Melanocytic differentiation of the stroma may occur which represents the PEComatous variant. MESTs in males were histopathologically slightly different from those in females due to the different hormonal milieu.