The CT findings in 32 patients with pathologically proven metastases to the kidney were compared to findings in 74 patients with renal cell carcinoma. Fourteen CT criteria were chosen to describe and characterize the lesions and 2 radiologists evaluated the CT images retrospectively according to these criteria. Renal metastases were characterized as small, multiple, bilateral, wedge-shaped, less exophytic, and located within the renal capsule. Renal cell carcinomas were single, unilateral, nonwedge-shaped, and exophytic, and easily transgressed the renal capsule. The sensitivity of CT to discriminate renal cell carcinoma from renal metastasis was 93.2% for renal cell carcinoma, and to discriminate renal metastasis from renal cell carcinoma was 75.0% for renal metastases by computer posterior probabilities. This study indicates that CT is useful for distinguishing these clinically important tumors. By using posterior probability, some unnecessary biopsies may be avoided.
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