Testicular cancer in dogs is a tumor in one or both testicles, and it is one of the most common tumors in older male dogs that haven't been neutered. The reassuring part: most of these tumors are found by touch, few of them spread, and for most dogs neutering surgery is the cure.
Two things change that picture. A testicle that never came down into the scrotum is far more likely to grow a tumor, and some tumors make estrogen, which in a minority of dogs can damage the bone marrow. If one of your dog's testicles looks or feels different, book a vet visit.
How Common Is Testicle Cancer in Dogs?
Testicular tumors are the most common tumors of the male dog's reproductive tract, and they are mostly a disease of older intact (not neutered) males. The average age at diagnosis is about 10 years, according to Veterinary Partner (VIN), and the Veterinary Society of Surgical Oncology notes they are uncommon in dogs younger than 6.
You don't hear about them often because most pet dogs are neutered young. Any intact male can get one, and VCA lists Boxers, German Shepherds, Afghan Hounds, Weimaraners, Shetland Sheepdogs, Collies and Maltese among the breeds that may be predisposed. There is no single known cause; age, genetics and environment all seem to play a part.
Retained Testicles Raise the Risk
Normally both testicles drop into the scrotum when a puppy is young. When one or both stay inside the belly or get stuck in the groin, vets call it cryptorchidism; the Merck Veterinary Manual describes it as the most common disorder of sexual development in dogs. More in our post on canine cryptorchidism.
A retained testicle is a much more likely home for a tumor. The Veterinary Society of Surgical Oncology (VSSO) puts the risk at about 9 times higher for a Sertoli cell tumor and about 16 times higher for a seminoma in the retained testicle compared with the descended one.
These tumors can also start young: a 2026 review in Animals notes cases in cryptorchid dogs as young as 10 months. Because you can't see or feel a testicle inside the belly, the first sign may be a belly mass or swelling. They also tend to be found later, and they more often make hormones. Because the condition is inherited, the Merck Veterinary Manual advises that cryptorchid dogs be neutered (both testicles removed) and not used for breeding.
Thought your dog was neutered? If you adopted an adult male and only ever saw one testicle, or none, ask your vet to confirm he was fully neutered. A testicle hidden inside can go unnoticed for years.
The Three Types of Testicular Tumors
Dogs get three main types, named after the cell they grow from. One dog can have more than one type, even in the same testicle, and other kinds exist but are rare. Most don't spread: VCA puts the chance of spread at less than 15% for Sertoli cell tumors and seminomas, and interstitial cell tumors rarely spread.
Sertoli cell tumors
Sertoli cells nurse developing sperm, and they can produce estrogen, the main female hormone. A Sertoli cell tumor can make enough to make a male dog look and act more like a female. Sertoli cell tumors and seminomas are the two types most often found in retained testicles.
Seminomas
Seminomas start in the germ cells that make sperm. Most cause no illness and turn up when a vet feels an enlarged testicle. Feminization is rare.
Interstitial (Leydig) cell tumors
Leydig cells make testosterone. Tumors that grow from them are usually small, benign and found by chance, and they show up almost only in testicles that have descended normally. Some make extra testosterone, which can lead to swelling near the anus from an enlarged perianal gland or a hernia, according to Veterinary Partner.
Signs of Testicular Cancer in Dogs
Many dogs show no obvious signs, which is why so many of these tumors are found on a routine exam. When signs appear, they fall into three groups.
Changes you can see or feel:
- One testicle bigger than the other
- A firm lump, or several, in a testicle
- A testicle that feels irregular or bumpy
- General swelling of the scrotum
- A swollen belly or belly mass in a dog with a retained testicle
These tumors are often painless, and long coats can hide subtle changes.
Feminization (estrogen) signs, mostly from Sertoli cell tumors:
- Enlarged nipples and mammary glands
- A sagging prepuce (the sheath around the penis)
- Hair loss on both sides of the body, often the belly, inner thighs and chest
- Darkened, thin skin and hair that is slow to grow back after clipping
- A red stripe of inflammation along the midline of the prepuce
- The other testicle shrinking
- Squatting to urinate instead of lifting a leg
- Less interest in mating
- Other male dogs showing sexual interest in him
Where the tumor sits matters. VSSO reports feminization in roughly 1 in 6 dogs when the Sertoli cell tumor is in a scrotal testicle, about half when the testicle is stuck in the groin, and about 70% when it is inside the belly.
Signs the cancer may have spread (uncommon): weight loss, poor appetite, lethargy, vomiting, coughing or trouble breathing, and straining to pee or poop if lymph nodes near the prostate are enlarged.
None of these signs is specific to cancer; hair loss alone has many causes.
Bone Marrow Damage: The Serious Complication
When a tumor makes too much estrogen, the hormone can suppress the bone marrow, where red blood cells, white blood cells and platelets are made. The 2026 Animals review estimates that this happens in roughly 10-20% of dogs with feminizing Sertoli cell tumors.
Watch for:
- Pale gums
- Tiredness and low energy
- Bruising, nosebleeds or bleeding that is hard to explain
- Fever or repeated infections
This is what changes the outlook. Dogs with severe bone marrow damage can be very sick, and recovery after surgery can be slow, partial or, in some dogs, not happen at all. They need hospital care, which can include blood transfusions. Catching a feminizing tumor before it reaches this stage is the strongest reason not to wait on a changed testicle.
When to Call Your Vet
Call now:
- A testicle that suddenly becomes swollen and painful, or sudden belly pain in a dog with a retained testicle. A tumor can make a testicle twist, which is an emergency. See our guide to testicular torsion in dogs.
- Pale gums, or bruising or bleeding you can't explain.
See your vet soon:
- One testicle bigger, firmer or lumpier than the other
- A swollen scrotum
- Feminization signs such as enlarging nipples or symmetrical hair loss
- A swelling in the belly or groin of a dog with a retained testicle
Mention at your next visit:
- You're not sure he was fully neutered, or you've only ever seen one testicle
- He is an older intact male and hasn't had his testicles checked in a while
Know what's normal for your dog and tell your vet about any change in size or firmness.
How Vets Diagnose Testicular Tumors
Most testicular tumors are found when the vet feels the testicles during a routine physical exam. From there, your vet may suggest:
- A rectal exam to check the prostate and nearby lymph nodes
- Ultrasound of the testicles and belly, to find a retained testicle or look for spread
- X-rays of the chest and belly to look for spread
- Bloodwork and a urinalysis, including blood cell counts
- Hormone tests or a sample from the prepuce if feminization is suspected
- A needle sample of the testicle, mainly for breeding dogs
The tumor type is usually confirmed after surgery, when a veterinary pathologist examines the removed testicle under the microscope (histopathology). Your vet may also stage the disease, a search for spread, with imaging such as a CT scan, partly because cancers elsewhere in the urinary or reproductive tract can spread to a testicle.
Treatment: Neutering Surgery
Castration, the same surgery as a routine neuter, is the treatment of choice. Vets usually remove both testicles, because a tumor may be present in both even if only one looks abnormal; published reports put tumors in both testicles at about 6-15% of affected dogs. If a testicle is retained in the belly, removing it needs abdominal surgery.
For some breeding or working dogs, a vet may discuss removing only the affected testicle when the other looks normal on exam and ultrasound, with closer follow-up afterward.
If the cancer has spread, chemotherapy or radiation therapy may be options, and your vet may refer you to a veterinary oncologist. Recovery is much like any neuter. Your vet will send you home with aftercare instructions, usually including a cone to stop licking.
Outlook for Dogs With Testicular Cancer
For most dogs, the outlook is very good. When there is no spread and no major hormone effects, surgery is often the only treatment needed; Veterinary Partner describes the cure rate as extremely high. Interstitial cell tumors, Sertoli cell tumors that haven't spread or affected the bone marrow, and seminomas without estrogen effects all carry an excellent prognosis.
Feminization signs usually fade over the following weeks to a few months once the estrogen source is gone.
The outlook is guarded for dogs whose cancer has spread or whose bone marrow is badly damaged, and it depends on the tumor type and where it has spread. For a broader look at how vets talk about tumors, see our pet owner guide to neoplasia in dogs.
Can You Prevent Testicular Cancer in Dogs?
Neutering is the most effective prevention. A dog without testicles can't develop a testicular tumor, with very rare exceptions reported years after castration. The best age to neuter depends on your dog's size, breed and health, so it is a conversation to have with your vet. Our post on why to spay or neuter your pet covers the wider pros and cons.
If you are keeping your dog intact:
- Make sure a retained testicle is removed, and don't breed a cryptorchid dog.
- Keep up with regular checkups as he gets older, so your vet can feel his testicles.
Frequently Asked Questions
Is testicular cancer painful for dogs? Often not. A growing tumor can cause discomfort from swelling, and sudden pain may mean the testicle has twisted.
Can a neutered dog get testicular cancer? The risk is close to zero once both testicles are gone. A retained testicle that was never found and removed can still develop a tumor.
My dog has one testicle bigger than the other. Is it cancer? Not necessarily. Infection, inflammation and torsion can also enlarge a testicle, but a tumor is a common cause in older intact dogs, so any change is worth a vet visit.
How long can a dog live with testicular cancer? Most dogs treated with surgery before the cancer spreads are cured. With spread or bone marrow damage, survival depends on the details, so ask your vet or a veterinary oncologist.
Originally written by Akosua Kumi Nyarko. Updated by the I Love Veterinary editorial team.
Sources
- Veterinary Partner (VIN): Testicular Cancer in Dogs, Jacqueline Brister DVM
- VCA Animal Hospitals: Testicular Tumors
- Pereira M et al., Clinical Management of Testicular Tumors in Dogs, Animals 2026
- BluePearl Pet Hospital: Testicular Tumors in Dogs
- Veterinary Society of Surgical Oncology: Testicular Sertoli Cell Tumor
- Veterinary Society of Surgical Oncology: Testicular Seminoma
- Merck Veterinary Manual: Reproductive Disorders of Male Dogs
- Merck Veterinary Manual: Male Genital Abnormalities of Animals
- petMD: Testicular Tumors in Dogs
- PetPlace: Testicular Tumors in Dogs
- Quartuccio M et al., Sertoli cell tumors associated with feminizing syndrome and spermatic cord torsion in two cryptorchid dogs, J Vet Sci 2012
This article is for general information and is not a substitute for advice from your veterinarian.
