OBJECTIVE. Small renal masses (< 4 cm) can be difficult to accurately classify as benign or malignant, particularly when they appear T1 hyperintense on MRI. This intrinsic signal, potentially related to intralesional hemorrhage, may limit evaluation of signal intensity on DWI. The purpose of this study was to test whether apparent diffusion coefficient (ADC) measurements may distinguish malignancy. MATERIALS AND METHODS. This single-center retrospective study identified patients with a T1-hyperintense renal mass less than 4 cm on MRI. Malignant lesions were pathologically proven; a benign mass was established by a predefined hierarchy of pathologic proof, follow-up ultrasound, or follow-up imaging showing more than 5 years of stability. T1 hyperintensity, defined as a signal intensity equivalent to or greater than the adjacent renal cortex, was confirmed by a senior abdominal radiologist. Two additional abdominal radiologists independently measured ADC of the lesion, which was normalized to the ADC of the background ipsilateral kidney and represented as ADCratio. RESULTS. The final cohort included 58 benign and 37 malignant renal lesions in 95 patients. Interrater agreement for ADC measurements was almost perfect (κ = 0.836-0.934). ADCratio was significantly lower in malignant compared with benign lesions (0.65 ± 0.29 vs 1.03 ± 0.32; p < 0.001). Malignant lesions were significantly larger than benign lesions (2.66 ± 0.86 cm vs 1.50 ± 0.65 cm; p < 0.001); however, after controlling for lesion size, ADCratio remained a significant predictor of malignancy (p < 0.001). CONCLUSION. ADCratio was highly reproducible for T1-hyperintense small renal masses and was significantly lower in malignant compared with benign renal masses.
Keywords: DWI; MRI; hemorrhagic cyst; proteinaceous cyst; renal cell carcinoma.