Rare histological subtypes of renal cell carcinoma in everyday diagnostic practice

Introduction: The fourth edition of the WHO Classification of Tumours of the Urinary System and Male Genital Organs (2016) contains new renal tumour entities. These new subtypes of renal cell carcinoma (RCC) were introduced based on morphological criteria, some genetic features, and clinical characteristics with prognostic implications. We present three patients with rare renal tumours belonging to newly recognized or still emerging categories of RCC. All cases were diagnosed based on careful morphological examination with immunophenotyping, and patho- clinical correlation. The first case is an example of acquired cystic disease - associated renal cell carcinoma with heterogeneous architecture as well as specific intra- and intercytoplasmic microlumens. The second tumour - a tubulocystic renal cell carcinoma - was composed of multiple, various-sized cysts divided by fibrovascular septa and tubules lined focally with hobnail cells. The third case presents very rare sporadic eosinophilic, solid, and cystic RCC. This tumour contained macro et microcystic, areas intermixed with solid fields composed of large, in part multinucleated eosinophilic cells. Inflammatory infiltrations accompanied the neoplastic stroma. New subtypes of RCC, although rare, can be encountered in everyday practice. It is important to perform careful differential diagnosis and classify such tumours according to the recent guidelines.