It is not clear what causes some types of kidney masses as of July 2015, according to Mayo Clinic. Kidney cysts are fluid-filled masses that form on or in the kidneys. Simple kidney cysts rarely cause complications, but more complex cysts can impair kidney function.
Doctors are also unsure of the causes of kidney cancer, which tends to form solid masses or tumors, according to WebMD. Risk factors for kidney cancer include being male, smoking and being overweight. Genetic conditions and exposure to certain chemicals can also increase the risks of developing cancer.
Kidney cancer represents about 3 percent of the cancers in the United States, according to Cleveland Clinic. Over 28,000 residents of the United States are diagnosed with kidney cancer annually. In the early stages, many kidney cancers cause no symptoms. By the time symptoms appear, the cancer may have already spread.
Doctors often discover kidney masses when evaluating patients for other indications, according to the Washington University School of Medicine in St. Louis. For example, a doctor might order a computerized tomography, or CT, scan for a patient experiencing abdominal problems. Even though the CT scan may not reveal the cause of the pain, it might show a mass on the kidney. After the mass’ discovery, it often requires further testing to determine its type.