A subset of renal cell neoplasms contains a mesenchymal stromal component, often resembling smooth muscle. To date, it remains debated whether these represent one or more distinct pathological entities. The foremost of these, renal cell carcinoma with angioleiomyoma-like stroma (also known as smooth muscle or leiomyomatous stroma), has some distinctive features, including immunohistochemical positivity for cytokeratin 7 and often a lack of genetic changes of clear cell renal cell carcinoma. It is debated whether this is related to the much more common clear cell papillary renal cell carcinoma, owing to the substantial similarity of their immunohistochemical phenotypes. The term renal angiomyoadenomatous tumour has been used by some authors in this context, but this is a source of controversy with some believing it is synonymous with clear cell papillary renal cell carcinoma. Smooth muscle-rich renal cancers appear to be enriched in tuberous sclerosis complex patients, but it is likely that these occur sporadically also. Recently, renal cell carcinomas with TCEB1 mutation and monosomy of chromosome 8 have also been reported. These have been noted to have fibromuscular stroma and cytokeratin 7 reactivity; however, absence of these genetic alterations in renal cell carcinomas with smooth muscle stroma suggests that this represents only one of likely several entities. Other uncommon patterns of renal neoplasms, such as clear cell renal cell carcinoma with degenerative fibrosis or haemangioma-like changes, mixed epithelial and stromal tumour with epithelial proliferation, angiomyolipoma with epithelial cysts, renal cell carcinoma with associated angiomyolipoma, and sarcomatoid renal cell carcinoma may cause diagnostic challenges for the pathologist. Although knowledge of renal cell carcinomas with a stromal component has dramatically increased recently, further study is necessary to understand the molecular pathology of these tumours and if they have implications for inherited tumour syndromes.

Pathology. 2019 Jun 29 [Epub ahead of print]

Sean R Williamson

Department of Pathology and Laboratory Medicine and Henry Ford Cancer Institute, Henry Ford Health System, Detroit, MI, USA; Department of Pathology, Wayne State University School of Medicine, Detroit, MI, USA. Electronic address: .