This page was reviewed under our medical and editorial policy by
Maurie Markman, MD, President, Medicine & Science at CTCA.
This page was updated on May 20, 2022.
The kidneys are two bean-shaped organs attached to the back wall of the abdomen and tucked underneath the ribs. Kidney cancer, also called renal cancer, develops when cells there begin to grow in an uncontrolled way.
Kidney cancer is often difficult to detect in its early stages. In fact, before a tumor has grown or the disease has spread, kidney cancer is most commonly discovered by an X-ray or ultrasound performed for another reason.
Many factors contribute to this delayed diagnosis, including the location of the kidneys—deep inside the body, where small tumors are less likely to be seen or felt during a physical exam. Once the disease has grown or advanced beyond the kidneys—generally to nearby lymph nodes or to the lungs, bones or liver—signs are more likely to develop, but they’re often mistaken for less serious conditions.
The most common sign of kidney cancer is blood in the urine (hematuria), which may appear rusty or dark red. Other symptoms of kidney cancer may include:
Kidney cancer symptoms for females are the same as kidney cancer symptoms for males. Although these symptoms may indicate a kidney tumor, they also may be caused by other, less serious health issues. Some kidney cancer patients experience none of these signs, and others experience different symptoms entirely.
Before symptoms of kidney cancer are noticeable, a laboratory test or an imaging study may reveal a possible diagnosis.
For example, a routine urine test may find traces of blood that the naked eye can’t see. Signs of kidney cancer also may be detected during a computed tomography (CT) scan, which consists of X-rays taken at different angles and processed by a computer into 3D images. This imaging test may show a growth in the kidney—and may even be able to tell a noncancerous cyst from a solid cancer tumor if dye is injected into a vein beforehand.
Magnetic resonance imaging (MRI) produces detailed images of the kidneys using a computer and magnetic fields in place of X-rays. During an MRI, a dye called gadolinium may be used to increase tumor visibility.
Finally, an ultrasound uses sound waves to look for signs of a kidney tumor, and determine whether a mass in the kidney is a fluid-filled cyst or a solid tumor.
The possible significance of light/dark spots in a scan of the kidneys varies widely depending on the imaging technique. When a dye is used during a scan, such as a computed tomography (CT) scan or magnetic resonance imaging (MRI), both dark and light spots on the kidneys could potentially indicate cancer. In these scans, the dye accumulates and shows up as a bright spot in places that may contain cancer. If the dye is blocked from reaching certain parts of the kidney, those areas may appear as dark spots in the images.
This technique helps differentiate between tumors and cysts. The dye doesn’t collect in a cyst and only builds up in a tumor.
The meaning of black vs. white spots on an ultrasound is different because the technique uses sound waves to reveal the density of any structures inside. Usually, cancerous tumors in the kidneys are solid, which means that cancer will appear as a lighter spot on an ultrasound image because the sound waves bounce off more solid structures. Benign cysts in the kidneys tend to appear as black spots on an ultrasound because the waves pass through them and don’t bounce back.
Your doctor can explain the potential implications of any light or dark spots revealed by an imaging scan. The findings of an imaging test alone, particularly an ultrasound, may not be enough to make a definitive diagnosis.