Abstract
A 72-year-old man with lung cancer underwent positron emission tomography CT (PET-CT) as a part of cancer staging. As an incidental finding, the PET-CT revealed a renal mass with metabolic and morphological characteristics of a malignant tumour. A diagnostic CT scan revealed a Bosniak III renal cyst, and malignancy could not be excluded. For correct Bosniak classification, a multiphasic contrast-enhanced CT was performed and the renal mass was finally diagnosed as a calyceal diverticulum. This case report summarises how calyceal diverticula may mimic serious pathology, leading to diagnostic difficulties.