Why Does My Dog Have a Lump on His Testicle?

A lump on your dog’s testicle is most often a tumor. Testicular tumors account for roughly 90 percent of all tumors affecting male canine genitalia, making them one of the most common cancers in intact male dogs.¹ The reassuring part is that most of these tumors are benign and rarely spread. But not all lumps are tumors, and not all tumors behave the same way, so understanding what you’re dealing with matters for deciding how quickly to act.

The Three Main Tumor Types

Nearly all canine testicular tumors fall into three categories, and your vet will want to figure out which one your dog has because they behave quite differently.

Interstitial cell tumors (also called Leydig cell tumors) are the most common, making up about half of all cases in most large studies.² These grow from the cells that normally produce testosterone. They tend to be small, slow-growing, and almost always benign. Many are found incidentally during a routine exam or during neutering surgery, and the dog shows no symptoms at all. Malignant versions with spread to other organs exist but are genuinely rare.³

Seminomas are the second most common, accounting for roughly a quarter to a third of cases depending on the study population.¹ They arise from the germ cells that produce sperm. Most seminomas are also benign, though a small percentage can become malignant and metastasize. They tend to be larger than interstitial cell tumors and are more likely to cause a noticeable lump that you can feel or see.

Sertoli cell tumors make up roughly 17 to 26 percent of testicular tumors.⁴ These grow from the cells that normally support and nourish developing sperm. Sertoli cell tumors deserve the most attention because they are the most likely of the three to cause hormonal side effects and, in a minority of cases, to become malignant. They are also the ones most strongly associated with undescended testicles.

Some dogs develop mixed tumors, with more than one tumor type growing in the same testicle or in both testicles at once. About 7 percent of cases in one large study were classified as mixed.²

Why Undescended Testicles Raise the Risk Dramatically

If your dog has a retained testicle, one that never dropped into the scrotum and sits somewhere in the abdomen or inguinal canal, the risk of a testicular tumor jumps significantly. Research puts cryptorchid dogs at roughly 9 to 14 times the risk of developing a testicular tumor compared to dogs whose testicles descended normally.⁵ One case-control study estimated the relative risk at about 9 times higher.⁶

The connection is especially strong for Sertoli cell tumors and seminomas. In a Japanese retrospective study, over half of all testicular tumors developed in cryptorchid testes, and the link was statistically significant for Sertoli cell tumors, seminomas, and mixed germ cell tumors, though not for interstitial cell tumors.⁷ The retained testicle sits at a higher temperature than it would in the scrotum, and that abnormal environment is thought to promote the changes that lead to tumor formation. This is one of the strongest arguments for neutering dogs with undescended testicles even if breeding is not a concern.

One complication with retained testicles is that a tumor can grow silently inside the abdomen where you cannot feel it. By the time the dog shows signs, the tumor may be large or may already be producing enough hormones to cause systemic effects. If your dog was diagnosed with cryptorchidism as a puppy and was never neutered, any new abdominal symptoms or hormonal changes should raise a flag.

Signs That a Tumor Is Producing Hormones

Most testicular tumors are hormonally quiet and cause no symptoms beyond the lump itself. But Sertoli cell tumors, and occasionally the other types, can pump out excess estrogen. When that happens, your dog may develop a set of changes collectively known as feminization syndrome. About a quarter of dogs with Sertoli cell tumors develop this.⁸

The signs are hard to miss once you know what to look for:

Hair loss: Symmetrical thinning or complete loss of fur, often along the flanks, belly, and chest. The skin underneath may darken.
Mammary development: The nipples enlarge, and breast tissue may swell, giving the dog a distinctly feminine appearance.
Genital changes: The penis and the unaffected testicle may shrink. The prepuce can become pendulous and droopy.
Behavioral shifts: Some dogs become less interested in marking territory, may squat to urinate, or become attractive to other male dogs.

Feminization syndrome is driven by excess estrogen flooding the bloodstream.⁹ The hair loss alone often leads owners to consult a dermatologist before anyone thinks to examine the testicles. If your intact male dog is losing hair symmetrically and there is no obvious skin disease, a testicular tumor should be on the list of possibilities.

When Estrogen Becomes Dangerous

The feminization signs described above are cosmetically odd but not life-threatening on their own. The real danger from a hormonally active tumor is what excess estrogen does to bone marrow. Estrogen at high enough levels is toxic to bone marrow in dogs, and the result is a progressive failure to produce blood cells normally.¹⁰

This can show up as low platelets, leading to unusual bleeding or bruising. It can cause anemia, so your dog may seem weak, tired, or pale around the gums. And it can suppress white blood cell production, leaving the dog vulnerable to infections. In severe cases, the bone marrow essentially shuts down, which is a life-threatening emergency. This is one reason why a testicular lump in an intact dog is not something to casually monitor for months. If the tumor is producing estrogen, the longer it stays, the more damage it can do to the bone marrow.

The good news is that when the tumor is removed promptly, the bone marrow can recover. In one reported case of a golden retriever with a Sertoli cell tumor causing severe bone marrow toxicity, the marrow regenerated within four months of surgery.¹¹ But if the suppression has gone on too long before the tumor is caught, recovery becomes less certain.

Not Every Lump Is a Tumor

Although tumors are the most common cause, a lump or swelling on the testicle can also result from infection or inflammation. Epididymitis, an infection of the coiled tube that sits alongside each testicle, is underreported but clinically significant. It often shows up as a painful, swollen scrotum, and in acute cases dogs may have a fever, elevated white blood cell counts, and general signs of feeling unwell.¹² The swelling tends to come on more quickly than a tumor would and is usually painful to the touch, whereas most testicular tumors grow slowly and are not painful.

Bacterial infections from the urinary tract or prostate can travel to the testicle and epididymis. Brucella canis, a specific bacterium, is a well-known cause of reproductive tract infections in dogs and can produce testicular swelling. If your dog suddenly develops a hot, swollen, painful scrotum, infection is a strong possibility and antibiotics may be needed alongside or instead of surgery.

Orchitis, inflammation of the testicle itself rather than just the epididymis, can also create a lump-like feel. Trauma to the scrotal area, a bite wound, or even an autoimmune reaction can cause orchitis. The distinction between an infectious lump and a neoplastic one matters because treatment is different, but it often takes imaging or a needle sample to tell them apart.

Testicular Torsion

Torsion happens when the testicle rotates on its spermatic cord, cutting off blood flow. This causes rapid swelling, intense pain, and if not corrected quickly, tissue death. Clinical signs can include lethargy, hind-limb stiffness, vomiting, fever, and a visibly enlarged and painful testicle.¹³ In dogs, torsion is most commonly reported in undescended testicles, and some of those retained testicles already have tumors, which can make them heavier and more prone to twisting.¹⁴

Torsion is a veterinary emergency. Unlike a slowly growing tumor that gives you weeks to plan surgery, torsion requires rapid intervention to prevent irreversible damage to the testicle and to manage pain. If your dog is suddenly in acute distress with a swollen, hard testicle, do not wait for a scheduled appointment.

How Your Vet Figures Out What It Is

The diagnostic workup typically starts with a physical exam and ultrasound. Ultrasound can confirm the presence of a mass and give information about its size, location, and blood-flow pattern. However, standard grayscale ultrasound alone cannot reliably distinguish between the three main tumor types. More advanced techniques using Doppler to assess blood-flow patterns around and within the mass can help differentiate, since different tumor types tend to show different vascular signatures.¹⁵

For a definitive pre-surgical diagnosis, your vet may recommend fine-needle aspiration, essentially inserting a thin needle into the lump and pulling out a sample of cells for examination under a microscope. This technique is remarkably reliable for testicular tumors. One study found sensitivity of 95 percent for seminomas, 88 percent for Sertoli cell tumors, and 96 percent for Leydig cell tumors, with perfect specificity across all three types.¹⁶ That means a needle aspirate that says “seminoma” is almost certainly right. Final confirmation still comes from histopathology after the testicle is removed, but cytology gives your vet enough information to plan treatment and discuss the prognosis with you before surgery.

If there is any suspicion of a hormonally active tumor, blood work matters too. A complete blood count can reveal early signs of bone marrow suppression, and estrogen levels can be measured to assess whether a Sertoli cell tumor is flooding the system.

Treatment Is Usually Straightforward

Surgery to remove the affected testicle, called orchiectomy, is the standard treatment for canine testicular tumors and is considered curative for the vast majority of cases.¹⁷ Most vets will recommend bilateral castration (removing both testicles) because of the risk of a second tumor developing in the remaining one. However, unilateral orchiectomy, removing only the affected side, can be considered when the other testicle looks normal on ultrasound and the owner wants to preserve breeding ability or the dog’s working status.¹⁷

Because interstitial cell tumors and most seminomas are benign, surgery alone is typically all that is needed. Dogs with hormonally active tumors, especially those that have developed bone marrow suppression, need monitoring after surgery to make sure blood counts recover and estrogen levels normalize. Chemotherapy is rarely needed and is reserved for the uncommon cases where a tumor has metastasized. Radiation may play a role in select malignant cases but is not routine.

Prognosis after surgery is generally excellent. Most dogs recover quickly and live normal lives afterward. The dogs that face a harder road are those whose tumors were producing estrogen long enough to severely suppress the bone marrow before anyone caught it. For those dogs, recovery depends on how much damage the bone marrow has sustained and whether it retains the capacity to regenerate once the estrogen source is gone.

Age, Breed, and What Makes Some Dogs More Vulnerable

Testicular tumors are overwhelmingly a disease of middle-aged to older intact male dogs. They are rare in dogs under five and become increasingly common after that. If your young dog has a testicular lump, infection or a benign cyst is more likely than a tumor, though certainly not guaranteed.

Breed matters. Some breeds appear in testicular tumor studies at higher proportions than others, with boxers, German shepherds, and several terrier breeds showing up frequently. One population-based study found significant breed differences in both the overall proportion of testicular tumors and the distribution of specific tumor types.⁴ Breeds with higher rates of cryptorchidism naturally have higher rates of the tumor types linked to retained testicles.

The single biggest modifiable risk factor remains neutering. Early neutering eliminates testicular tumor risk entirely, since there are no testicles left for tumors to grow in. For intact dogs, regular physical exams that include checking the testicles are the best way to catch tumors early. Many owners do not routinely feel their dog’s testicles, and lumps can go unnoticed until they are quite large. Getting comfortable with a quick check during grooming or belly rubs can make a real difference in catching problems early when treatment is simplest.

Seminomas and the Connection to Human Cancer Research

An interesting wrinkle in this topic is that canine testicular tumors are studied not just for the sake of dogs but as a window into human reproductive cancers. Canine seminomas closely resemble a specific subtype of human testicular cancer called spermatocytic seminoma, based on shared patterns in protein expression and cell behavior.¹⁸ Researchers have also confirmed that two distinct subtypes of seminoma exist in dogs, paralleling a classification used in human medicine, and there is evidence that a precursor condition similar to a pre-cancerous state found in humans may also exist in canine testes.¹⁹

This cross-species research has practical implications. It means that advances in understanding how testicular tumors develop and behave in dogs can inform human oncology, and vice versa. It also means that the classification and treatment of these tumors in dogs is becoming more sophisticated over time, benefiting from the same molecular tools used in human cancer research. For dog owners, the practical takeaway is that your vet’s ability to diagnose and differentiate these tumors has improved significantly in recent years, even if the core treatment, surgical removal, has stayed the same.

When to Get to the Vet

Any new lump you feel on your intact dog’s testicle warrants a veterinary visit, but certain situations call for urgency. A sudden, painful swelling with fever suggests torsion or infection and should be seen the same day. A lump accompanied by hair loss, swollen nipples, or changes in your dog’s behavior or energy level points toward a hormonally active tumor that may already be affecting the bone marrow. And any rapidly growing mass, regardless of other symptoms, deserves prompt attention.

For a small, firm, painless lump that your dog does not seem bothered by, scheduling a visit within a week or two is reasonable. Your vet will likely recommend ultrasound and possibly a needle aspirate. Even if the lump turns out to be a benign interstitial cell tumor that poses no immediate threat, the information lets you and your vet make an informed decision about whether surgery is the right next step or whether monitoring is appropriate for your dog’s specific situation.

1
PubMed. A five-year cohort study on testicular tumors from a population-based canine cancer registry in central Italy (Umbria)
2
PubMed Central. Canine testicular tumors: An 11-year retrospective study of 358 cases in Moscow Region, Russia
3
PubMed Central. Malignant Leydig cell tumor in dogs: two cases and a review of the literature
4
PubMed. Breed differences in the proportional morbidity of testicular tumours and distribution of histopathologic types in a population-based canine cancer registry
5
Biology of Reproduction. Cryptorchidism and testicular cancer in the dog: unresolved questions and challenges in translating insights from human studies
6
PubMed. Canine cryptorchism and subsequent testicular neoplasia: case-control study with epidemiologic update
7
PubMed. A 12-year retrospective study of canine testicular tumors
8
Acta Scientiae Veterinariae. Feminization Syndrome in a Dog with Sertoli Cell Tumor
9
PubMed Central. Sertoli cell tumors associated with feminizing syndrome and spermatic cord torsion in two cryptorchid dogs
10
PubMed Central. Estrogen-induced myelotoxicity in dogs: A review
11
PubMed Central. Estrogen-induced myelotoxicity in a 4-year-old golden retriever dog due to a Sertoli cell tumor
12
PubMed Central. Ultrasonographic features of epididymitis in dogs: a case series
13
Clinical Theriogenology. A case of testicular torsion in an intact Cane Corso
14
PubMed. Testicular torsions in veterinary medicine
15
PubMed Central. Characterization of Testicular Tumor Lesions in Dogs by Different Ultrasound Techniques
16
PubMed. Cytologic features of testicular tumours in dog
17
PubMed Central. Clinical Management of Testicular Tumors in Dogs
18
PubMed. Testicular germ cell tumours in dogs are predominantly of spermatocytic seminoma type and are frequently associated with somatic cell tumours
19
PubMed Central. Canine testicular tumors: two types of seminomas can be differentiated by immunohistochemistry