Prevalence of Low-Attenuation Homogeneous Papillary Renal Cell Carcinoma Mimicking Renal Cysts on CT

Objective: The objective of our study was to determine the attenuation range of homogeneous papillary renal cell carcinomas (RCCs) on contrast-enhanced CT.

Materials and methods: This retrospective study was performed at two institutions from January 1, 2007, to January 1, 2017. Multiphasic CT studies with and without IV contrast material of 114 patients with pathologically proven papillary RCCs were independently reviewed by two sets of two abdominal radiologists. Seventy-two cases were excluded because of subjective lesion heterogeneity, leaving 42 homogeneous RCCs. Three ROIs were placed on all lesions for all CT phases, and the mean attenuations were calculated.

Results: Mean lesion size was 2.8 cm (range, 1.2-11.0 cm). The attenuation range for each CT phase was as follows: unenhanced, 14.7-50.7 HU; corticomedullary, 32.2-99.5 HU; portal venous, 40.8-95.1 HU; nephrographic, 17.9-90.8 HU; and excretory, 18.0-73.0 HU. Two of 114 (1.8%; 95% CI, 0.2-6.5%) RCCs were homogeneous and less than 30 HU on the portal venous or nephrographic phase. One of these RCCs was a solid hypoenhancing mass, and the other was a homogeneous cystic RCC. Of the cases with an unenhanced phase, three of 107 (2.8%; 95% CI, 0.6-8.8%) were both homogeneous and were less than 20 HU in attenuation.

Conclusion: Papillary RCCs are rarely both subjectively homogeneous and less than 20 HU at unenhanced CT and less than 30 HU at portal venous or nephrographic phase CT.