Differentiation of renal neoplasms from high-density cysts: use of attenuation changes between the corticomedullary and nephrographic phases of computed tomography.

Purpose: The current study evaluated attenuation changes for proven renal neoplasms between the corticomedullary and the nephrographic phases of a contrast-enhanced computed tomographic (CT) scan as a possible means for differentiating these tumors from the cysts of the kidney when high-density renal masses are detected on a CT scan that does not include a noncontrast baseline.

Methods: We retrospectively reviewed the CT scans performed on 20 patients with 21 biopsy-proven renal neoplasms, which had been done using standard contrast-enhanced computed tomography only. Attenuation values for the 21 renal neoplasms and for 23 simple cysts, used as an internal control, were measured and recorded from the 2 phases of enhancement. The difference in attenuation between the 2 phases of enhancement for each mass was calculated. Data for the neoplasms and cysts were compared with published data for enhancement changes for proven high-density renal cysts.

Results: The mean absolute value change in attenuation between the corticomedullary and the nephrographic phases for renal neoplasms was 22 Hounsfield units (HU) (range, 1.0-48 HU) and 1 HU (range, 0.1-3.3 HU) for simple cysts. Only one renal neoplasm (5%) changed to less than 10 HU.

Conclusions: Almost all renal neoplasms studied had an attenuation change of more than 10 HU, either increased or decreased, between 2 phases of a contrast-enhanced CT scan separated by 50 seconds. The results suggest that if the attenuation of a renal tumor changes by more than 10 HU between phases of a contrast-enhanced computed tomography, then the diagnosis of renal neoplasm is very likely. High-attenuation renal masses which change less than 10 HU between the corticomedullary and tubular phases are most likely high-density cysts, but neoplasm is possible.