A renal mass on the kidneys is a growth that arises from the kidney, according to the Division of Urology at Washington University School of Medicine. The most common renal mass is a cyst, which is a sac filled with fluid arising from the kidney. Cysts rarely cause problems.
Solid masses on the kidneys are suspicious for cancer, and radiologists can easily identify them. About 80 percent of the time solid masses are cancerous. One in five masses that show characteristics of cancer on a CT scan, MRI or ultrasound test will not be cancerous, but they are generally treated expecting cancer, according to the Division of Urology at Washington University School of Medicine.