The Bosniak classification groups cystic renal lesions into one of four categories based on CT/MR appearances. The differentiation between groups II and III is important as group II are typically ‘follow-up lesions’ and group III are ‘surgical lesions’. Features of a Bosniak III lesion include irregular thickened septa, measurable enhancement, coarse irregular calcification, multiloculation, nodularity, uniform wall thickening and margin irregularity. (Ref: Israel GM et al. Pitfalls in renal mass evaluation and how to avoid them. Radiographics 2008; 28: 1325–1338)