Two Cases of Papillary Renal Neoplasm with Reverse Polarity: Radiologic-Pathologic Correlation.

Papillary renal neoplasm with reverse polarity (PRNRP) is a distinct subtype of papillary renal tumors first described in 2019. Here, we present the imaging and histopathological findings of two cases of PRNRP. Case 1 involved a 71-year-old man with an incidental right renal mass. A 30 mm tumor in the right kidney exhibited iso- to slight hyperattenuation on non-contrast computed tomography (CT). Dynamic contrast-enhanced (DCE)-CT revealed a tumor with solid and cystic components. The solid portion showed mild enhancement, followed by gradual enhancement. Regarding magnetic resonance imaging (MRI), the cystic component revealed mild hyperintensity on T2-weighted imaging (WI) and hyperintensity on T1WI. The small, lobulated solid component showed hyperintensity on T2WI and no apparent hyperintensity on diffusion-weighted imaging, with high ADC values. Case 2 involved a 74-year-old woman with a gradually enlarging left renal mass. A 20 mm tumor in the left kidney showed heterogeneous mild hyperattenuation on non-contrast CT. On DCE-CT, a small solid component exhibited mild enhancement, followed by gradual enhancement. Retrospectively, the mass showed an 8 mm homogeneous nodule on CT images obtained 7 years earlier. Although the tumor appears heterogeneous on CT, a small PRNRP may exhibit homogeneous attenuation. Additionally, the solid portion can show T2 hyperintensity related to stromal edema. Recognition of these imaging variations is important for the correct diagnosis of PRNRP.