Non-contrast imaging characteristics of papillary renal cell carcinoma: implications for diagnosis and subtyping

INTRODUCTION: Current radiographic guidelines suggest unenhanced renal lesions < 20 Hounsfield Units (HU) are overwhelmingly benign, requiring no further evaluation. We evaluate our experience with papillary renal cell carcinoma (pRCC) presenting with low pre-contrast attenuation and the relationship of attenuation with histologic pRCC subtype. MATERIALS AND METHODS: We reviewed our institutional kidney cancer database for patients with pT1 or pT2 pRCC between 2003-2017. Tumors were categorized by papillary subtype by expert uropathologists. Preoperative CT images were analyzed at six regional tumor locations. Low, presumably benign, unenhanced median attenuation was defined as ≤ 20 HU. We calculated the frequency of pRCC with low attenuation and assessed the relationship between attenuation and pRCC subtype using logistic regression. RESULTS: Sixty-one patients with evaluable imaging were included. Median tumor size was 6 cm (1.7 cm-15.3 cm) with 39% (n = 24) type-1 and 61% (n = 37) type-2. Half of all pRCC tumors (n = 30) exhibited very low pre-contrast attenuation (< 20 HU), risking misdiagnosis as benign using current guidelines. Of these, 80% (n = 24) were type-2 with significant biological potential. Overall, type-2 tumors demonstrated a lower pre-contrast attenuation than type-1 (median HU: 19.8 (1.5-42.3) versus 29.6 HU (10-45.8), p < 0.01; max HU: 25.3 versus 36.5 HU, p < 0.01). After adjustment, lower pre-contrast HU was an independent predictor of pRCC subtype associated with a 5.5-fold increase of being type-2 (OR = 5.47, p < 0.01). CONCLUSION: pRCCs may exhibit very low attenuation on pre-contrast CT. This appears more common among the more aggressive type-2 subtype. These data suggest that low attenuation (< 20 HU) alone on non-contrast CT imaging is insufficient as a single parameter to rule out malignancy.