Quite a few people are startled to hear, after an abdominal ultrasound or CT at a routine checkup, that there is a mass on their kidney. They had no symptoms and felt perfectly well, so the word cancer is hard to believe. Yet kidney cancer is, by its nature, a cancer with a fairly high rate of being found by accident. Understanding why can take some of the edge off the vague fear.
The kidneys sit deep toward the back, a pair tucked beneath the ribs. Because of that inward location, they cannot be felt by hand until a mass grows quite large, and nothing shows on the outside. On top of that, the kidneys have a built-in backup: if one side loses some function, the other takes over the work. So a small mass on one kidney often produces no change at all in the urine or in how a person feels.
The trio of signs commonly linked to kidney cancer, blood in the urine, flank pain, and a lump felt in the side, rarely appear together; and when they do, the disease is often already somewhat advanced. In other words, kidney cancer found after symptoms begin tends to be caught relatively late, while the cancer found incidentally on a screening, with no symptoms at all, is more often small and easier to treat.
This rise in incidental discovery owes much to how common imaging has become. A small mass that, in earlier times, would have gone unnoticed until symptoms appeared is now spotted in advance on an abdominal ultrasound or CT done for some other reason. In fact, a large share of newly diagnosed kidney cancers are found this way, without symptoms and at a comparatively small size.
When a mass is found, doctors usually examine its shape and blood flow in detail with a contrast-enhanced CT or MRI to tell a benign mass from cancer. Not every mass on the kidney is cancer; a simple fluid-filled cyst or a benign tumor is common. If the mass is very small and slow-growing, doctors may not operate right away but instead follow it with imaging over time.
Treatment is decided by weighing the mass's size and location together with kidney function. For a small cancer, the first preference is often to remove only the part containing the mass rather than the whole kidney, preserving as much kidney function as possible. Being found incidentally and small also means the range of options is wider.
The key point, in the end, is that there is no need to feel falsely reassured because there are no symptoms, nor to despair at the first mention of a mass. If you have been told a mass is visible on your kidney, the most practical path is to confirm exactly what it is with further testing and to discuss the next steps with your care team.
This article shares general medical information in plain language and does not replace individual diagnosis or treatment. Please discuss any specific decisions with your own care team.