MRI features of the nested variant of urothelial carcinoma of the urinary bladder: report of four cases.

We report on four patients with the nested variant of urothelial carcinoma (NVUC) of the urinary bladder and focus on their magnetic resonance imaging (MRI) findings. MRI showed that all lesions had irregular wall thickening with little protrusion into the bladder lumen. All had extravesical invasion, and two had invaded other organs (uterus and seminal vesicle). On T2-weighted images, all tumors mainly showed relatively strong hypointensity similar to that of the muscularis propria, and in three cases there was also a thin hyperintense layer on the tumor surface, suggesting edematous mucosa. Diffusion-weighted images demonstrated different degrees of hyperintensity, which was faint in one case. Dynamic contrast-enhanced MRI was performed in two cases and both showed gradual contrast enhancement. It has been suggested that NVUC may produce unique MRI findings reflecting its pathological features. It would be useful for those who interpret bladder MRI to recognize this rare urothelial carcinoma variant.