Benign renal neoplasms are noncancerous growths that develop in the kidneys. Unlike cancerous tumors, these growths do not spread to other parts of the body and are typically not life-threatening, though they can sometimes cause health problems if they grow large or affect kidney function.
What Are Benign Renal Neoplasms?
A benign renal neoplasm is an abnormal growth of tissue in the kidney that does not behave like cancer. The word “benign” means that the tumor will not spread, or metastasize, to other organs or tissues in your body. These growths form when kidney cells begin to multiply in unusual ways, but unlike cancerous cells, they remain contained within the kidney area and grow much more slowly.[1]
Most benign kidney tumors are discovered by accident when doctors perform imaging tests like CT scans or ultrasounds for completely unrelated health concerns. This is because many of these tumors are small and do not cause any noticeable symptoms in their early stages. When symptoms do appear, they can include blood in the urine, pain in the lower back or between the ribs and hips, unexplained weight loss, fever, or a feeling of fullness in the abdomen.[6][7]
Even though benign tumors are not cancerous, they still require medical attention. They can grow large enough to press on surrounding tissues, cause bleeding, or interfere with normal kidney function. In some cases, doctors may recommend removing them through surgery, while in others, careful monitoring may be all that is needed.[1]
Types of Benign Kidney Tumors
There are several different types of noncancerous tumors that can develop in the kidneys. Each type has its own characteristics and behavior patterns. Understanding the differences helps doctors decide on the best approach to treatment.[1]
Papillary Renal Adenoma
Papillary renal adenoma is the most common type of benign kidney tumor. These growths are typically small and grow very slowly over time. Most people with papillary adenomas never experience any symptoms at all. Because they rarely cause problems, these tumors are usually found only when a person undergoes imaging tests for other medical reasons. Their small size and slow growth rate make them less concerning than other types of kidney masses.[1][6]
Oncocytoma
Oncocytoma is another type of benign kidney tumor that begins in the cells lining the collecting ducts of the kidney. These tumors can grow quite large compared to adenomas, and it is possible to have several oncocytomas in one or both kidneys at the same time. Sometimes, doctors discover an oncocytoma alongside a cancerous tumor, which makes careful evaluation important. Despite their size, oncocytomas typically grow slowly and do not spread to other parts of the body.[1][4]
Angiomyolipoma
Angiomyolipoma is a kidney tumor composed of three different types of tissue: fat cells, blood vessels, and smooth muscle. This combination gives the tumor its name. Angiomyolipomas often appear golden yellow in color because of their fat content, and they can range in size from about half an inch to around eight inches.[2]
These tumors frequently develop in people with a genetic condition called tuberous sclerosis, which causes noncancerous tumors to form in many organs throughout the body, including the eyes, skin, brain, lungs, heart, and kidneys. Even though angiomyolipomas are benign, they can cause serious complications. They have the ability to grow into and damage surrounding kidney tissue. When they grow larger than four centimeters, there is a risk of sudden and severe bleeding into the abdomen, which can be dangerous and require immediate medical attention.[1][2]
How Common Are Benign Kidney Tumors?
Benign kidney tumors are more common than many people realize. Studies show that most growths that develop in the kidneys are not cancerous. Among small kidney tumors, approximately 20 to 25 percent turn out to be benign when examined. This means that one in every four or five small kidney masses discovered is noncancerous.[6][10]
The frequency of kidney tumor discoveries has increased dramatically over the past several decades. This rise is not necessarily because more people are developing kidney tumors, but rather because medical imaging technology has become more advanced and more widely used. When people undergo CT scans or MRI tests for unrelated problems like back pain or abdominal discomfort, doctors often spot kidney masses that would have gone undetected in earlier times.[3][4]
Angiomyolipoma, one of the most common types of benign kidney tumors, affects fewer than one in every 100 people in the general population. However, it is much more common in people with certain genetic conditions. Women are more likely than men to develop angiomyolipomas, especially those between the ages of 40 and 60.[2]
Who Is at Risk for Benign Kidney Tumors?
Anyone can develop a benign kidney tumor, but certain factors can increase a person’s likelihood. Understanding these risk factors helps doctors identify who might benefit from closer monitoring or earlier screening.[6]
People with certain genetic conditions face a higher risk of developing benign kidney tumors. Tuberous sclerosis complex is strongly associated with angiomyolipomas, while other conditions like von Hippel-Lindau disease and neurofibromatosis type 1 also increase the risk. A condition called lymphangioleiomyomatosis, which primarily affects the lungs, is another risk factor for developing angiomyolipomas in the kidneys.[2]
Age plays a role in kidney tumor development. Simple kidney cysts, which are noncancerous fluid-filled sacs, become very common in people over the age of 50. The older a person gets, the more likely they are to develop these benign kidney changes.[1]
Gender also influences risk. Benign kidney tumors, particularly angiomyolipomas, occur more frequently in women than in men. The reason for this difference is not fully understood, but hormonal factors may play a role.[2]
Family history matters as well. People who have relatives with kidney tumors or kidney cancer may have a higher chance of developing kidney masses themselves. This connection suggests that genetic factors beyond specific named syndromes may contribute to tumor development.[7]
Signs and Symptoms of Benign Kidney Tumors
Many people with benign kidney tumors do not experience any symptoms at all. This is especially true when the tumors are small. Doctors often discover these growths only when performing imaging tests for other health concerns. However, as tumors grow larger, they may begin to cause noticeable problems.[6][7]
Blood in the urine, medically called hematuria, is one of the most important warning signs. Sometimes the blood is visible to the naked eye, making the urine appear pink, red, or brown. Other times, the blood can only be detected under a microscope during routine urine testing. Any appearance of blood in the urine should prompt immediate medical evaluation, as it can signal various kidney problems, not just tumors.[6][7]
Pain is another common symptom. People with kidney tumors may experience persistent discomfort in the lower back, not related to any injury. The pain can also occur between the ribs and hips in an area called the flank. This pain may be constant or come and go, and it might feel tender to the touch.[2][7]
Some people notice a lump or mass in the kidney area that they can feel through the skin. This typically happens only with larger tumors. Other symptoms can include unexplained weight loss, loss of appetite, feeling full quickly when eating, persistent fever that comes and goes, and general tiredness.[2][6]
Anemia, a condition where the body does not have enough healthy red blood cells, can develop in people with kidney tumors. Healthy kidneys produce a hormone called erythropoietin that helps the body make red blood cells. If a tumor affects this hormone production or causes blood loss through the urine, anemia may result. People with anemia often feel weak, tired, and short of breath.[6]
High blood pressure can also develop as a result of kidney tumors. The kidneys play an important role in regulating blood pressure, and when a tumor interferes with normal kidney function, blood pressure may rise.[2][7]
Growth Rate and Behavior of Benign Kidney Tumors
One of the key differences between benign and cancerous kidney tumors is how quickly they grow. Most benign kidney tumors grow very slowly, and some do not grow at all. This slow growth pattern makes them much less dangerous than cancerous tumors.[2]
Classic angiomyolipomas, for example, often remain stable in size for many years. When they do grow, the average growth rate is about one-sixteenth of an inch per year. This is so slow that it may take several years before any significant size change is noticed. However, there are exceptions to this pattern. Angiomyolipomas can grow more rapidly during pregnancy, likely due to hormonal changes. Women who have an angiomyolipoma and are considering pregnancy should discuss treatment options with their doctor before becoming pregnant.[2]
Papillary adenomas are typically discovered at small sizes and tend to stay small. Their slow growth is one reason doctors sometimes choose to monitor them rather than remove them immediately.[1]
The behavior of benign kidney tumors is generally predictable. They do not invade nearby organs in the aggressive way that cancerous tumors do. They do not travel through the bloodstream or lymphatic system to form new tumors in distant parts of the body. However, angiomyolipomas are an exception in one respect: even though they are benign, they can grow into and damage surrounding kidney tissue if they become large enough.[1]
Diagnosis of Benign Kidney Tumors
Diagnosing a benign kidney tumor involves several steps. Because most kidney tumors do not cause symptoms, especially when small, they are often discovered accidentally. When a doctor suspects a kidney tumor based on symptoms or incidental findings, several tests help determine the nature of the growth.[7]
Imaging Tests
Imaging tests are the primary tools for detecting and evaluating kidney tumors. A CT scan (computed tomography scan) is considered the gold standard for examining kidney masses. This test uses X-rays and computer technology to create detailed, three-dimensional images of the kidneys. A CT scan can show the size, shape, and exact location of a tumor, and it can help doctors determine whether the mass is solid or fluid-filled. Special contrast agents may be injected into a vein to make certain tissues more visible on the scan.[4][7]
MRI (magnetic resonance imaging) is another powerful imaging tool. It uses magnets and radio waves instead of radiation to create detailed pictures of soft tissues. MRI is particularly useful for people who cannot have CT scans, such as those with allergies to contrast agents or those with poor kidney function that might be harmed by the contrast material used in CT scans.[4][7]
Ultrasound uses sound waves to create images of the kidneys. It is especially good at determining whether a mass is solid or cystic (fluid-filled). Ultrasound is often the first imaging test performed because it is quick, inexpensive, and does not use radiation. However, it provides less detail than CT or MRI scans.[4][7]
For angiomyolipomas specifically, the presence of fat within the tumor on a CT scan is a key indicator. Because angiomyolipomas contain fatty tissue, they often appear different from other types of kidney masses on imaging tests, allowing doctors to diagnose them based on imaging alone in many cases.[4]
Biopsy
A biopsy involves removing a small sample of tissue from the tumor so it can be examined under a microscope. For kidney tumors, this is typically done using a thin needle that is inserted through the skin into the kidney under imaging guidance. The tissue sample can help doctors determine whether a tumor is benign or cancerous and what type of cells it contains.[4]
Biopsies are not performed in all cases of kidney tumors. Doctors may recommend a biopsy if the imaging results are unclear, if the person has cancer elsewhere in the body, if surgery would be particularly risky due to other health conditions, or if there is concern about a rare type of tumor. For many benign tumors, especially angiomyolipomas that show characteristic fat content on imaging, a biopsy may not be necessary.[4]
Blood and Urine Tests
Blood tests can provide important information about kidney function and overall health. They may reveal anemia or other abnormalities related to the tumor. Urine tests check for blood that may not be visible to the naked eye and can help evaluate kidney function. While these tests do not diagnose tumors directly, they provide valuable supporting information.[7]
Changes in Normal Body Function (Pathophysiology)
Understanding how benign kidney tumors affect normal body function helps explain why they can cause symptoms and why treatment is sometimes necessary, even though they are not cancerous.[1]
The kidneys perform several vital functions in the body. They filter waste products from the blood, regulate fluid balance, control blood pressure, and produce hormones that help make red blood cells and maintain bone health. When a tumor grows in the kidney, it can interfere with these normal functions in several ways.[6]
As a benign tumor grows, it takes up space within the kidney. This physical presence can compress normal kidney tissue, reducing the kidney’s ability to filter blood effectively. If the tumor grows large enough, it may damage or destroy functioning kidney tissue. However, because most benign tumors grow slowly, the remaining healthy kidney tissue often compensates for the loss, and overall kidney function remains adequate, especially since people have two kidneys.[1]
Angiomyolipomas, because they contain blood vessels, present a special risk. The blood vessels within these tumors are often abnormal and fragile. As the tumor grows larger than four centimeters, these blood vessels become more likely to rupture. When this happens, bleeding can occur suddenly into the kidney or surrounding areas. This bleeding, called hemorrhage, can be severe and may cause sharp, sudden pain, a drop in blood pressure, and require emergency treatment.[1][2]
The presence of a tumor can also trigger the kidney to produce hormones abnormally. For example, some tumors interfere with the production of erythropoietin, leading to anemia. The physical presence of a tumor might also affect the kidney’s role in blood pressure regulation, potentially causing high blood pressure to develop.[6]
When benign tumors cause blood to appear in the urine, this happens because the tumor disrupts the normal structure of the kidney’s filtering system or its blood vessels. Even though the bleeding may be microscopic, repeated or persistent bleeding can lead to anemia over time.[6]
Treatment Options for Benign Kidney Tumors
Not all benign kidney tumors require immediate treatment. The approach depends on the tumor type, size, location, growth rate, symptoms, and the patient’s overall health. Several treatment options are available, ranging from careful monitoring to surgical removal.[1]
Active Surveillance
Active surveillance means carefully monitoring the tumor over time without immediately treating it. This approach is often appropriate for small benign tumors that are not causing symptoms and are growing slowly or not at all. During active surveillance, patients undergo regular imaging tests, typically every three to six months initially, to check whether the tumor is growing or changing. If the tumor remains stable and causes no problems, no other intervention may be needed.[4][12]
This approach is particularly suitable for older patients, those with other serious health conditions that make surgery risky, or people with tumors smaller than three centimeters that are not causing any symptoms. Active surveillance allows patients to avoid the risks and recovery time associated with surgery while ensuring that any changes in the tumor are detected early.[4]
Surgery
Surgery is the main treatment for benign kidney tumors when intervention is needed. There are two primary surgical approaches. Partial nephrectomy involves removing only the tumor and a small margin of surrounding tissue while preserving the rest of the kidney. Nephrectomy means removing the entire kidney. Doctors prefer partial nephrectomy whenever possible because it preserves kidney function.[1]
Modern surgical techniques often use minimally invasive approaches, such as laparoscopic or robotic-assisted surgery. These methods involve smaller incisions, less pain, shorter hospital stays, and faster recovery times compared to traditional open surgery.[1]
Most people function well with just one kidney. If one kidney must be removed, the remaining kidney typically enlarges and increases its function to compensate. However, preserving kidney tissue is always preferable when possible, especially in people with existing kidney problems or other risk factors for kidney disease.[9]
Arterial Embolization
Arterial embolization is a treatment specifically used for angiomyolipomas. This procedure works by blocking the blood supply to the tumor, causing it to shrink. During the procedure, a doctor inserts a thin tube called a catheter into a blood vessel, usually in the groin, and guides it to the arteries feeding the tumor. Small particles or coils are then injected through the catheter to block these blood vessels.[1]
Embolization is less invasive than surgery and can be particularly useful for people who cannot undergo surgery due to other health problems. It is also used as an emergency treatment for bleeding angiomyolipomas.[1]
Other Treatments
For people who cannot have surgery, other treatments may be available. These can include procedures that destroy the tumor using extreme cold (cryotherapy) or heat (radiofrequency ablation). These techniques are typically performed through the skin using needles, making them less invasive than traditional surgery.[1]
Outlook and Prognosis
The outlook for people with benign kidney tumors is generally excellent. Because these tumors are not cancerous, they do not spread to other parts of the body, and they are typically not life-threatening. When treatment is needed, it is usually successful in resolving the problem.[1]
After surgical removal of a benign kidney tumor, the tumors do not usually come back. Most people recover well from surgery and return to their normal activities within a few weeks to months, depending on the type of procedure performed. Those who undergo partial nephrectomy typically maintain good kidney function with the preserved kidney tissue.[1]
For people under active surveillance, many benign tumors remain stable for years without requiring treatment. Regular follow-up imaging ensures that any changes are caught early, allowing for timely intervention if needed.[4]
Even people who need to have an entire kidney removed usually do well. The remaining kidney can handle the filtering needs of the body on its own. These individuals can lead normal, healthy lives, though they should take care to protect their remaining kidney by maintaining a healthy blood pressure, avoiding medications that can damage the kidneys, and staying well-hydrated.[9]
In rare cases, what appears to be a benign tumor may later be found to have cancerous characteristics. This is why careful evaluation through imaging or biopsy is important, and why follow-up care should continue even after treatment.[2]
