I believe the answer to your query is in the second portion of your question. Renal cell carcinoma (RCC) is diagnosed in more than 30,000 North Americans each year, leading up to 12,000 deaths. The general population risk factors include a history of smoking, obesity and certain occupational hazards. Renal tumours, however, account for only 2-3% of all newly diagnosed malignancies, so a general screening program wouldn't be efficacious. US is a safe -- no ionizing radiation is used -- inexpensive imaging modality for assessing the abdominal viscera. Visualization of the kidneys, however, isn't always optimal due to adjacent gas-containing bowel loops, and the retroperitoneal structures can be missed entirely. Incidental renal lesions are often discovered during US exams performed for other reasons. The vast majority of such findings are benign, representing cortical cysts, parapelvic cysts or solid angiomyolipomas. Approximately 30% of renal tumours are discovered incidentally, in patients without symptoms. Once indeterminate lesions are seen, or a renal tumour is suspected, a computed tomography (CT) scan is the best imaging modality to further investigate. The scan should be performed both without contrast (looking for intralesional fat -- such as in a benign angiomyolipoma, calcification and stones) and with dynamic contrast enhancement. It will characterize the renal lesion and provide excellent visualization of the retroperitoneal anatomy, allowing local staging of RCCs, as well as looking for metastatic disease. Renal magnetic resonance imaging (MRI) is now also a viable alternative or complimentary modality to CT, performed without and with contrast, providing multiplanar views of the kidneys and surrounding structures. Although still experimental, the use of US contrast agents (such as intravenously administered microbubbles) may also prove useful in the evaluation of indeterminate small renal masses seen on CT and MRI. The one area where renal screening is indicated is in select patient populations at much greater risk for RCC. Von Hippel-Lindau disease -- central nervous system (CNS) hemangioblastomas, retinal angiomas, pancreatic neuroendocrine tumours -- carries a 28-45% risk of RCC. Tuberous sclerosis -- CNS tubers, dermal angiofibromas, cardiac rhabdomyomas -- has a 2-5% risk. Hereditary leiomyoma and RCC syndrome -- cutaneous and uterine leiomyomas -- presents with a 15-30% RCC risk and the likelihood of RCC in Birt-Hogg-Dube syndrome -- lung cysts, pneumothoraxes -- is 8-15%. There are also patients with hereditary papillary RCC, familial renal oncocytoma and medullary carcinoma of the kidney (associated with the sickle cell trait). The preferred modality for screening the kidneys in these individuals is CT. US may be used in young people under 18, but it's not adequate for screening adults, since many lesions are too small to be detected at an early stage by this method. If you have patients with these syndromes and diseases, you should discuss the case with your local urologist to ascertain the best screening approach and protocol. MM [References: Lockhart ME, Smith JK. Radiol Clin North Am 2003;41:863-75. Robbin ML et al. Radiol Clin North Am 2003;41:963-78; Choyke PL. Radiol Clin North Am 2003;41:1037-51; Radiology: Imaging Diagnosis Intervention. Vol. 4, Ch. 115, p. 1-10, 2003; Radiology: Imaging Diagnosis Intervention. Vol. 4, Ch. 121, p. 8-8, 1999.]