Kidney Tumors and Cysts in Dogs: Symptoms and Diagnosis

Name: Kidney Tumors and Cysts in Dogs: Symptoms and Diagnosis
Creator: Dr. Zubair Khalid

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

A dog tumor on kidney or a cyst on the kidney is an uncommon finding, but it does happen, and it usually turns up during an ultrasound or X-ray done for another reason. Some of these masses are benign fluid-filled cysts that can be monitored or drained, while others are malignant tumors that need surgery or other treatment. Knowing which one your dog has matters, because the outlook and the plan are very different.

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

Quick Answer

Kidney tumors in dogs are uncommon, and the most common primary malignant type is renal carcinoma, which starts in the kidney's tubular lining cells [1].
A simple renal cyst is a benign, fluid-filled pocket in the kidney. It can cause vomiting, increased thirst and urination, and abdominal pain, and it can be treated surgically with a good long-term outcome [2].
Many kidney masses cause no obvious signs at first. Blood in the urine, weight loss, lethargy, and straining to urinate are the most common clues [1].
Diagnosis usually combines blood work, urinalysis, ultrasound, and often CT. A sample of the mass (cytology or biopsy) confirms what type of tissue it is [3].
Treatment depends on the type and stage. Options include surgery to remove the kidney, chemotherapy, radiation, or a combination [1].

What Is a Kidney Mass in Dogs?

A kidney mass is any abnormal growth or pocket of tissue inside or on the kidney. Vets group these into two broad buckets: tumors (neoplasia) and cysts.

Tumors can be primary, meaning they started in the kidney itself, or metastatic, meaning cancer spread there from somewhere else in the body. The kidneys are actually a common landing spot for cancer that has spread, and lymphoma is the most common multicentric tumor that involves the kidneys [1]. Among primary kidney tumors in dogs, epithelial tumors are the most common type, while sarcomas and nephroblastomas are rarely reported [1].

The most common primary malignant kidney tumor is renal carcinoma. It usually affects only one kidney, sits at one pole of the kidney, and is well demarcated from the surrounding tissue. Its size can range from microscopic to several times the size of a normal kidney [1]. Renal carcinoma also spreads early, most often to the other kidney, the lungs, the liver, and the adrenal glands [1].

Cysts are different. A simple renal cyst is a well-defined, fluid-filled sac. In one reported case, a 10-month-old dog had a cyst that was presumptively congenital, meaning the dog was likely born with it [2]. Cysts are not cancer, but a large one can press on surrounding tissue and cause real symptoms.

Signs Owners Notice

Kidney masses are sneaky. Early on, many dogs show nothing at all, and the mass is found by accident on imaging done for an unrelated problem.

When signs do appear, they depend on which part of the urinary system is affected. Common clues include lethargy, blood in the urine, weight loss, and straining to urinate [1].

For cysts specifically, the signs can look like a urinary or stomach problem. In one case, a young dog had a four-week history of drinking and urinating more than normal, plus six months of vomiting, and the exam found abdominal pain and an enlarged right kidney [2].

A lump you can feel near the kidney

Owners sometimes describe a "dog lump near kidney" area, meaning a firm swelling in the front part of the belly. This is called nephromegaly, or an enlarged kidney, and it is a real exam finding. In the cyst case above, the vet felt exactly that on physical exam [2]. An enlarged kidney does not tell you whether the mass is a cyst or a tumor. Only imaging and sampling can do that.

Signs that point to spread

If a tumor has spread to the lungs, you might notice coughing, faster breathing, or tiring easily. If it has spread to the other kidney, kidney function can drop and you may see the classic signs of kidney disease, such as poor appetite, weight loss, and increased drinking. If your dog has these signs along with a known kidney mass, tell your vet right away.

Causes and Risk Factors

The honest answer is that we do not fully know why most kidney tumors form in dogs. What we do know is which types are more likely and how they behave.

Primary kidney cancer is uncommon in dogs overall [1]. Renal carcinoma is the most frequent primary malignant type, and it tends to spread early to the opposite kidney, lungs, liver, and adrenal glands [1]. Because of that pattern, staging the whole body matters as much as looking at the kidney itself.

Some masses are not primary kidney cancer at all. Lymphoma is the most common multicentric tumor that involves the kidneys, and metastatic lesions from cancer elsewhere can appear in one or both kidneys [1]. This is why a vet who finds a kidney mass will often look for a primary tumor somewhere else.

Cysts have a different story. Some are congenital, meaning the dog was born with them [2]. In cats, polycystic kidney disease is a hereditary disorder with progressive cyst formation that leads to declining kidney function, and cysts grow over time [4]. In dogs, the inherited forms are less well established in veterinary research, and your vet will judge each case on its own imaging and lab work.

Age matters somewhat. Renal carcinoma is typically a disease of middle-aged to older dogs, while congenital cysts can show up in dogs under a year old [2].

How Vets Diagnose a Kidney Mass

Diagnosis is a step-by-step process. No single test answers everything.

Physical exam and blood work

Your vet starts with a hands-on exam, feeling for an enlarged kidney or abdominal pain. Blood work and a urinalysis check kidney values, look for blood or protein in the urine, and screen for anemia or other problems. These tests tell you how well the kidneys are working, but they do not tell you what the mass is.

Imaging

Ultrasound is usually the first imaging step. It can show whether a mass is solid or fluid-filled, which helps separate a cyst from a tumor. In one case, ultrasound showed a well-defined, one-sided kidney mass containing anechoic (clear) fluid, which is consistent with a simple renal cyst [2].

CT gives much more detail. In a review of 32 dogs with confirmed kidney tumors, CT helped characterize lesion number, size, margins, enhancement pattern, mineralization, which side was affected, location within the kidney, bleeding, lymph node involvement, lung spread, and collateral vessels. Primary tumors were mainly one-sided masses, while multicentric lymphoma and metastatic lesions appeared as nodules in both kidneys. Collateral vessels were significantly more frequent in renal carcinoma than in multicentric lymphoma [3].

Sampling the mass

Imaging suggests, but tissue confirms. Vets can take a sample with a fine needle guided by ultrasound, or during surgery. In one case, a 12-year-old Border Collie had a highly vascular right kidney mass, and cytology of the mass showed mesenchymal cells most consistent with a soft tissue sarcoma, which was later confirmed on histology after the kidney was removed [5].

Sometimes the answer comes during surgery. In a 13-year-old Shih Tzu with a large, cavitated kidney mass that was suspected to be cancer, an intraoperative impression smear of the mass wall showed a benign renal cyst with no evidence of cancer or infection, and the final pathology agreed. That test guided the surgeon to a less aggressive procedure [6].

Staging

Because renal carcinoma spreads early, staging matters. Your vet will check the other kidney, the lungs, the liver, and the adrenal glands [1]. Chest imaging looks for lung spread. This staging decides whether surgery alone makes sense or whether other treatment should be added.

Finding	What it often means
One-sided solid mass with collateral vessels on CT	More likely renal carcinoma [3]
Nodules in both kidneys	More likely lymphoma or metastatic spread [3]
Clear fluid-filled, well-defined mass	Consistent with a simple renal cyst [2]
Mesenchymal cells on cytology	Suggests a soft tissue sarcoma type tumor [5]

Treatment Options

Treatment depends on the tumor type, the location, and whether it has spread. Options include chemotherapy, surgery, radiation, or a combination [1].

Surgery

Removing the affected kidney (ureteronephrectomy) is the main treatment for a tumor confined to one kidney. In the Border Collie case, the right kidney and ureter were removed, and histology confirmed the tumor type [5].

Surgery can also handle tumors that have grown into major blood vessels. In one case, a 14-year-old Papillon had a renal cell carcinoma with a tumor thrombus extending about 1.5 cm from the right kidney vein into the caudal vena cava. The dog had open surgery to remove the kidney and the clot, using two tourniquets, with a total occlusion time of 12 minutes and 13 seconds. There were no major complications, and the dog went home [7].

Cysts

Small, quiet cysts may only need monitoring. Larger cysts that cause symptoms can be drained, but drainage alone often does not last. In the young dog with a congenital cyst, the cyst was drained under ultrasound guidance but came back three months later along with the original signs. The vet then performed laparoscopic deroofing, fulguration, and omentalization, and the dog recovered well with normal kidney function two years later and no recurrence [2].

A similar approach worked in the Shih Tzu with a large benign cyst, where deroofing and omentalization were performed after cytology ruled out cancer [6].

Chemotherapy and other drugs

Chemotherapy is used for lymphoma and for some tumors that cannot be fully removed. In the Papillon with renal cell carcinoma and a blood vessel clot, the dog received toceranib after surgery to address possible microscopic leftover disease. Lung spread was found on day 553 after surgery, and the dog was still alive on day 722 [7]. Your vet calculates any drug dose from your dog's weight and health, so never use a dose from another pet.

Radiation is another option for some tumors, often combined with surgery or chemotherapy [1].

Recovery, Outlook, and Long-Term Management

The outlook varies widely, and that is the most honest thing to say. A benign cyst that is treated can have an excellent outcome, with normal kidney function two years after surgery in one reported case [2]. A malignant tumor that has already spread has a much more guarded outlook.

After surgery, your dog will need monitoring of the remaining kidney. Kidney function can decline over time. In the Shih Tzu treated for a benign cyst, chronic kidney disease was diagnosed four months after surgery, though the dog was asymptomatic and the cyst did not come back [6].

Long-term care usually means regular blood work, urinalysis, and imaging. If your dog's kidney values trend upward, your vet may manage it like chronic kidney disease, which you can read about in Canine Chronic Kidney Disease: Staging, Diagnosis, and Management. If your dog was born with cysts, the inherited form is covered in Canine Polycystic Kidney Disease: Diagnosis and Management. And if kidney development itself is the concern, see Canine Renal Dysplasia: Diagnosis and Management.

Watch for the return of any original signs, such as vomiting, increased drinking, or blood in the urine. These can signal a cyst coming back or a tumor progressing.

Prevention

There is no proven way to prevent kidney tumors in dogs. The causes are not well established in veterinary research, so no diet, supplement, or lifestyle change has been shown to stop them from forming.

For cysts, some are congenital, meaning a dog is born with them [2]. You cannot prevent something a puppy is born with, but you can catch it early. Routine wellness visits with blood work and a urinalysis help your vet spot changes before your dog feels sick.

If your dog has a known kidney cyst, keep every recheck appointment. Cysts can grow and cause symptoms again, and monitoring is how you stay ahead of that.

Limitations and When to Contact a Veterinarian

This article cannot tell you what your dog's specific mass is. Only your veterinarian can do that, using your dog's exam, lab work, and imaging.

Contact your vet the same day if you notice blood in the urine, vomiting that will not stop, your dog drinking and urinating far more than usual, or a new firm swelling in the belly.

Go to an emergency clinic now if your dog is straining to urinate and cannot pass urine, is breathing fast or hard at rest, collapses, or has a swollen, painful belly that came on suddenly.

Schedule a routine visit if your dog has a known kidney mass and you notice weight loss, poor appetite, or lower energy than usual. Bring a note of when each sign started, because timing helps your vet decide what to test next.

Frequently Asked Questions

Is a kidney mass in dogs always cancer?

No. Some kidney masses are benign cysts filled with fluid, and one reported case was confirmed as a benign renal cyst with no evidence of cancer or infection [6]. Imaging and a tissue sample are the only way to know which type your dog has.

Can a dog live a normal life with one kidney?

Many dogs do well after one kidney is removed, as long as the remaining kidney works well. Your vet will check kidney values regularly after surgery, because function can change over months.

Does a kidney cyst need to be removed?

Not always. Small cysts that cause no signs may only need monitoring. Cysts that cause vomiting, pain, or increased drinking often need drainage or surgery, and drainage alone can fail, since one cyst returned three months after it was drained [2].

How fast do kidney tumors spread in dogs?

Renal carcinoma can spread early, most often to the other kidney, lungs, liver, and adrenal glands [1]. That is why staging tests happen before treatment is planned, even when your dog seems comfortable.

Can a kidney tumor be treated without surgery?

Sometimes. Chemotherapy and radiation are options, and treatment can combine surgery, chemotherapy, and radiation depending on the tumor type and location [1]. Your vet will recommend a plan based on the confirmed tumor type and how far it has spread.

References

Neoplasia of the Urinary System in Dogs and Cats - Urinary System. Merck Veterinary Manual.
Long-term follow-up of a juvenile simple renal cyst in a dog treated by laparoscopic deroofing, fulguration, and omentalization. Journal of the American Veterinary Medical Association, 2022.
Computed tomographic features of renal neoplasms in dogs: introduction of the collateral vessel sign in veterinary medicine. American journal of veterinary research, 2026.
Contribution of polycystic kidney disease to the development of chronic renal dysfunction in cats. Veterinary research forum : an international quarterly journal, 2026.
Clinical, Cytologic, Histopathologic, and Diagnostic Imaging of a Malignant Peripheral Nerve Sheath Tumor in the Renal Pelvis of a Border Collie Dog. Veterinary clinical pathology, 2025.
Use of intraoperative impression smear cytology to guide successful treatment of a large renal cyst in a dog: a case report. Journal of veterinary science, 2022.
Surgery and toceranib for renal cell carcinoma with caval thrombus in a dog. The Canadian veterinary journal = La revue veterinaire canadienne, 2026.