Lump on a Cat's Kidney or Liver: What It Means

Name: Lump on a Cat's Kidney or Liver: What It Means
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

A cat lump on the kidney or liver is an abnormal area of tissue inside an organ, and it is usually found by accident during an ultrasound or x-ray done for another reason. Some of these lumps are harmless fluid-filled cysts, while others are tumors that can be benign or malignant. The only way to know which one your cat has is through imaging, sampling, and sometimes surgery.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

A lump inside the kidney or liver cannot be seen or felt from the outside in most cases. It shows up on imaging, often when your vet is checking something else.
Benign tumors are not invasive and do not spread, while malignant tumors can invade nearby tissue and spread to distant organs [1].
A cyst is a fluid-filled pocket. A hepatobiliary cystadenoma is a benign biliary tumor that can look like a cyst on imaging and can grow very large [2].
The type of lump matters more than its size. A small carcinoma can behave worse than a large benign cyst.
Your main decisions are usually monitoring with repeat imaging, sampling the lump with a needle or biopsy, or surgery to remove it.

What a Lump on a Cat's Kidney or Liver Actually Is

When your veterinarian says there is a "mass" or "lump" in your cat's kidney or liver, they mean an area of tissue that looks different from the normal organ. It could be solid, partly solid, or filled with fluid.

A cyst is a fluid-filled sac. On ultrasound, a simple cyst looks like a dark, thin-walled pocket with no solid parts inside. A hepatobiliary cystadenoma is an uncommon benign biliary tumor in cats that can mimic a cyst on imaging, and it can grow to an enormous size. In one reported case, a cystic liver mass measured 163 by 148 by 136 mm [2]. That is larger than most cat livers, which shows how much room these benign lesions can take up before they cause obvious signs.

A tumor is a growth of cells. Tumors can be benign or malignant. Benign tumors are not invasive, do not spread to other areas of the body, and are easy to remove surgically. Malignant tumors can invade surrounding tissue and spread to distant organs [1]. Telling the two apart requires specialized knowledge and laboratory equipment, which is why a sample is often needed [1].

Kidney lumps in cats include renal cell carcinoma, a malignant tumor of the kidney. One report described an 8-year-old cat with a renal mass that turned out to be an ill-defined renal cell carcinoma with marked necrosis and suspected vascular embolization [3]. Liver lumps include cholangiocarcinoma, a malignant tumor of the bile ducts. One cat with cholangiocarcinoma first showed up with fluid around the lungs, and the cancer later involved the lungs, spleen, and intercostal muscles [4].

Signs Owners Notice With a Cat Tumor on the Abdomen

Many cats with an internal lump have no signs at all at first. The lump is found because your vet was looking for something else.

When signs do appear, they tend to be vague. You might see vomiting, poor appetite, weight loss, or a belly that looks bloated. In the cystadenoma case, the cat had several weeks of vomiting, inappetence, and abdominal distension [2]. Blood work showed a cholestatic pattern, meaning bile flow was blocked, with high alkaline phosphatase and gamma-glutamyl transferase activities [2].

Some signs point to a specific organ. A kidney tumor can cause blood in the urine, straining to urinate, or signs that look like kidney disease. If your cat is vomiting and you are not sure whether the kidneys are involved, see How to Tell If Your Cat's Vomiting Is a Sign of Kidney Disease. Cats with kidney problems may also hide pain well, and Cat Kidney Disease: How to Tell If Your Cat Is in Pain explains what to watch for.

A liver tumor can cause jaundice, which is yellow color in the gums, skin, or whites of the eyes. It can also cause fluid buildup in the abdomen. Sudden liver failure causes a sudden loss of liver function and is often associated with hepatic encephalopathy and clotting abnormalities [5]. Hepatic encephalopathy means the brain is affected by toxins the liver can no longer clear, and it can cause confusion, wobbling, or seizures.

Sometimes the first sign is not in the belly at all. In one cat with kidney cancer, swelling appeared around several toes on both front limbs and a small eyelid nodule showed up a few days later [3]. X-rays of the front limbs showed aggressive bone destruction in the toe bones [3]. This is unusual, but it shows that a kidney or liver lump can sometimes announce itself in a distant part of the body.

Causes and Risk Factors

The cause of most kidney and liver lumps in cats is not known. Cancer happens when cells grow out of control, and in most cases there is no single trigger you could have prevented.

Age matters. The cats in recent reports of these tumors were 7, 8, and 12 years old [3][4]. Middle-aged and older cats are more likely to develop organ masses, which is one reason routine senior blood work and imaging are useful.

Some lumps have a known relationship to other problems. One cat with transitional cell carcinoma of both ureters had a long history of urinary stones that had been managed medically for a year [6]. The stones and the cancer coexisted, which may have delayed the diagnosis [6]. This is a reminder that a cat with a history of urinary stones can still develop a tumor, and that stones do not always explain every sign.

Chronic inflammation may play a role in some cases. In the cat with kidney cancer, the fluid around the kidney showed chronic inflammatory changes without atypical cells [3]. Inflammation alone does not prove cancer, but it can be part of the picture.

Some masses are not cancer at all. A hepatobiliary cystadenoma is a benign tumor, and a simple cyst is not a tumor at all [2]. These are important because they can look alarming on imaging while behaving far better than cancer.

How a Cat Lump Near the Kidney or Liver Is Diagnosed

Diagnosis usually happens in steps, and each step answers a different question.

Imaging

Ultrasound is the most common first test. It shows whether the lump is solid, cystic, or mixed, and whether it involves one organ or several. X-rays can show a large mass or displacement of organs. In the cystadenoma case, abdominal radiographs revealed a cranial abdominal mass, and ultrasound showed a multiloculated cyst with thin septa and no mural nodules [2]. Contrast-enhanced computed tomography gave more detail and confirmed the size and cystic nature of the mass [2].

Sampling

Imaging cannot tell you the cell type. A veterinarian can perform a fine needle aspiration of cells or a biopsy, which removes a small amount of tissue from a tumor, for evaluation [1]. A fine needle aspirate is quick and often done with light sedation or none at all. A biopsy gives more tissue and is more likely to give a definitive answer.

Sometimes sampling does not give the answer on the first try. In the cat with kidney cancer, cytology of the fluid around the kidney showed inflammatory cells but no atypical cells, and the diagnosis only came after surgery and histopathology [3]. In the cat with cholangiocarcinoma, the initial fluid around the lungs did not show neoplastic cells, and the diagnosis came from an ultrasound-guided core liver biopsy [4].

Staging

Staging means checking whether the lump has spread. It usually includes blood work, chest x-rays, abdominal ultrasound, and sometimes advanced imaging. Staging matters because it changes what treatment makes sense. In the cat with cholangiocarcinoma, the cancer had spread to the spleen at diagnosis and later involved the lungs, spleen, and intercostal muscles [4]. Knowing that up front would change the conversation about surgery.

A Simple Comparison

Finding	What it usually means	Typical next step
Simple cyst, thin wall, no solid parts	Usually benign fluid pocket	Monitor with repeat ultrasound
Large multiloculated cyst, thin septa, no nodules	Can be a benign biliary tumor such as cystadenoma [2]	Consider surgery if it causes signs
Solid kidney mass with necrosis	Concerning for renal cell carcinoma [3]	Stage the chest and abdomen, consider surgery
Liver mass with spread to spleen or lungs	Concerning for cholangiocarcinoma [4]	Stage fully, discuss options with your vet

Treatment Options

Treatment depends on what the lump is, where it is, and whether it has spread.

Monitoring

If the lump looks like a simple cyst and your cat has no signs, your veterinarian may recommend repeat imaging in a few months. This is a reasonable choice for many benign-looking lesions. The risk is that a tumor could grow during the monitoring period, so keep the follow-up appointments.

Surgery

Surgery is the main treatment for a lump that can be removed. In the cystadenoma case, exploratory laparotomy allowed complete excision with preservation of hepatic tissue [2]. In the kidney cancer case, the owners chose laparoscopy, the cyst-like content was aspirated, the firm wall was fenestrated, and the ventral wall was resected as much as possible [3]. In the ureteral cancer case, the tumor was found during exploratory surgery, and the cat was euthanized because of irreversible kidney failure from the blockage [6].

After surgical removal, tumors should be evaluated by a laboratory to confirm the type of tumor and whether all of it was likely removed [1]. This is done by checking the margins. Dirty margins mean tumor cells reach the edge of the removed tissue, so tumor cells likely remain in the body. Narrow margins mean cells are close to the edge but not at it, so some cells could be left behind. Wide or clean margins mean cells are far from the edge, so it is unlikely that tumor cells remain [1].

Medical Support

For liver disease, supportive treatment aims to slow progression, minimize complications, and allow the liver time to regenerate and compensate. It may include fluids, medications to protect the liver, and dietary support [5]. If your cat has sudden liver failure, treatment may include intravenous fluids, vitamin supplementation, diet changes, antibiotics, and certain liver medications, and another goal is to prevent or treat hepatic encephalopathy [5]. Your veterinarian will calculate any medication dose from your cat's weight and health.

Chemotherapy and Palliative Care

Some cancers, especially lymphoma, respond to chemotherapy. Others do not. If the cancer has spread widely, palliative care focuses on comfort. In the cholangiocarcinoma case, the cat received palliative management, the disease progressed, and the cat was euthanized 98 days after the diagnosis was confirmed [4].

Recovery, Outlook, and Long-Term Management

The outlook depends almost entirely on the type of lump.

A benign cyst or a benign tumor that is fully removed has a good outlook in most cases. A hepatobiliary cystadenoma is benign, but it can be large and can require complex surgery, so recovery depends on how much liver tissue had to be removed and how well your cat handles anesthesia and surgery [2].

Malignant tumors are more serious. Renal cell carcinoma can spread, and the cat in one report had suspected vascular embolization, meaning tumor cells were likely entering blood vessels [3]. Cholangiocarcinoma can spread widely, and one cat had cancer in the lungs, spleen, and intercostal muscles at postmortem examination [4]. Transitional cell carcinoma of the ureters caused irreversible kidney failure from blockage in one cat [6].

After treatment, most cats need regular checkups with blood work and imaging. Your veterinarian will set the schedule based on the tumor type and how it behaved. If your cat has kidney disease along with a kidney lump, see Signs Of Kidney Disease In Old Cats for what to watch at home. If your cat has had urinary stones in the past, Kidney Stones in Cats: Diagnosing Nephroliths and Ureteroliths explains why stones and tumors can be confused.

Prevention

There is no proven way to prevent a kidney or liver lump in cats. Most of these growths have no known cause, and no diet or supplement has been shown to prevent them.

What you can do is catch problems earlier. Keep up with routine exams, especially as your cat gets older. Ask your veterinarian whether senior blood work and imaging make sense for your cat. Report any vomiting, appetite change, weight loss, or belly swelling promptly. If your cat has a history of urinary stones, keep follow-up appointments, because stones and tumors can occur together and one can hide the other [6].

Limitations and When to Contact a Veterinarian

This article cannot tell you what your cat's lump is. Only your veterinarian can do that, using your cat's exam, imaging, and lab results.

Contact an emergency clinic right away if your cat is struggling to breathe, has a swollen or painful belly that came on suddenly, is collapsing, has seizures or severe confusion, or is bleeding from the mouth or rectum. Sudden liver failure can cause hepatic encephalopathy and clotting abnormalities, and it needs immediate veterinary treatment [5].

Call your veterinarian the same day if your cat has yellow gums or eyes, repeated vomiting, stops eating, or you feel a new firm lump in the belly.

Schedule a routine visit if your cat is eating and acting normally but you noticed a change on a previous x-ray or ultrasound, or if your cat has a known mass that is being monitored.

Frequently Asked Questions

Does a lump on a cat's kidney always mean cancer?

No. Some kidney lumps are cysts or benign growths, and only a sample can tell the difference. Imaging gives clues, but a fine needle aspirate or biopsy is often needed to confirm the cell type [1].

Can a cat live a normal life with a liver mass?

It depends on the type of mass. A benign lesion that is removed or monitored can allow a normal life, while a malignant tumor that has spread carries a much shorter outlook. Your veterinarian can give you a realistic estimate after staging.

Is surgery always the best choice for an organ mass?

No. Surgery is a good option when the mass can be removed safely and has not spread. If the cancer has already spread widely, or if surgery would leave your cat with organ failure, palliative care may be kinder.

How often should a monitored lump be rechecked?

There is no single schedule that fits every cat. Your veterinarian will set the interval based on the size, appearance, and location of the lump, and they may shorten the interval if anything changes.

Can a benign lump turn into cancer later?

It can happen, but it is uncommon. One report described a benign bone growth in a cat that later transformed into a malignant tumor, which led the authors to suggest more vigilance and repeat staging after such a diagnosis [7]. Ask your veterinarian whether your cat's specific lump has any known risk of transformation.

References

Tumors of the Skin in Cats - Cat Owners. Merck Veterinary Manual.
Giant Hepatobiliary Cystadenoma Requiring Biliary Reconstruction in a Cat: A Case Report. Veterinary medicine and science, 2026.
Suspected unusual digital metastases of a primary renal cell carcinoma in a cat. The Canadian veterinary journal = La revue veterinaire canadienne, 2026.
Suspected Paramalignant Pleural Effusion as the Primary Presentation of Disseminated Cholangiocarcinoma in a Cat. Veterinary sciences, 2026.
Disorders of the Liver and Gallbladder in Cats - Cat Owners. Merck Veterinary Manual.
Approach to the diagnosis of bilateral ureteral transitional cell carcinoma in a cat. Brazilian journal of veterinary medicine, 2026.
Malignant transformation of a solitary osteochondroma to chondrosarcoma in a cat. Australian veterinary journal, 2026.