A lump on your dog’s testicle is most commonly a tumor. Testicular tumors are among the most frequent cancers in intact (unneutered) male dogs, typically showing up around age 10. The reassuring news is that only about 15% of these tumors spread to other parts of the body, and surgical removal is usually curative. That said, not every lump is cancer. Infections and other inflammatory conditions can also cause swelling that looks or feels like a mass.
The Three Most Common Testicular Tumors
Dogs develop three main types of testicular cancer, and a single dog can have more than one type at the same time. In one large veterinary study, 38 dogs had multiple tumor types present simultaneously.
Interstitial Cell Tumors
These are the most frequently diagnosed testicular tumors in dogs. They arise from the hormone-producing cells between the sperm-making tubes and tend to appear as soft, yellowish nodules that sometimes contain areas of bleeding. Despite being the most common type, they rarely cause hormonal side effects or spread beyond the testicle. Many are found incidentally during a routine exam because the dog shows no symptoms at all.
Seminomas
Seminomas grow from the cells that produce sperm. They appear as pale, homogenous masses that range from soft to moderately firm and can grow large enough to visibly distort the shape of the testicle. They don’t typically produce hormones, so your dog’s behavior and appearance usually stay normal aside from the lump itself.
Sertoli Cell Tumors
These tumors develop from the supporting cells inside the testicle and tend to feel noticeably firm compared to the other types. That firmness comes from dense fibrous tissue within the mass. Sertoli cell tumors get the most attention because they’re the most likely to produce estrogen, which can cause a distinctive set of changes in your dog: enlarged nipples, symmetrical hair loss on both sides of the body, a drooping penile sheath, attraction from other male dogs, and shrinkage of the opposite testicle. If you’re noticing a lump plus any of these signs, a Sertoli cell tumor is a strong possibility.
When It’s Not Cancer
Orchitis (inflammation of the testicle) and epididymitis (inflammation of the tube that stores sperm) can both produce swelling that mimics a tumor. These conditions are usually caused by a bacterial, fungal, or viral infection, though immune-related inflammation without infection also occurs. The key difference from a tumor is that orchitis typically causes obvious pain. Your dog may walk stiffly, resist being touched near the groin, or lick the area excessively. You might also notice reddened or irritated scrotal skin and swelling that feels warm.
Sometimes the epididymis (a small, coiled structure sitting on top of each testicle) becomes prominent enough to feel like a distinct lump, especially if the testicle itself has shrunk from prior damage or hormonal changes. This can be alarming but isn’t necessarily a new growth.
Other possibilities your vet will consider include testicular torsion (a painful twisting of the testicle), a hernia pushing into the scrotal area, or a blood-filled swelling from trauma.
Undescended Testicles Raise the Risk
If your dog appeared to have only one testicle as a puppy, the other one likely never descended from the abdomen. This condition, called cryptorchidism, significantly increases the risk of cancer developing in that retained testicle. Because the undescended testicle sits inside the body where it’s warmer than the scrotum, it’s more prone to abnormal cell growth. These hidden tumors can be harder to detect since you can’t feel them from the outside, and they may only become apparent when hormonal symptoms like hair loss or nipple enlargement develop.
What Happens at the Vet
Your vet will start by carefully feeling both testicles and the surrounding structures to identify exactly where the lump is and whether it’s firm, soft, or painful. Ultrasound is the primary imaging tool for evaluating testicular masses. It can reveal the size, shape, and internal characteristics of the lump, check blood flow patterns, and help rule out torsion or hernias. However, ultrasound alone cannot reliably distinguish between tumor types.
Fine-needle aspiration, where a thin needle is inserted into the mass to collect cells for examination under a microscope, is sometimes performed and has high diagnostic accuracy for testicular tumors. It’s not done as routinely in veterinary medicine as in human medicine, but it’s a low-risk option when the diagnosis is uncertain. If orchitis is suspected, your vet will likely test for Brucella canis, a bacterial infection that can spread to humans, and may culture a semen sample.
A definitive diagnosis for most testicular lumps comes after surgical removal, when the tissue is sent to a pathologist.
Treatment and Recovery
For testicular tumors, the standard treatment is surgical removal of the affected testicle. Because the metastasis rate is only around 15%, surgery alone is curative for the vast majority of dogs. Most vets recommend neutering (removing both testicles) at the same time, since dogs that develop one testicular tumor are at higher risk of developing another in the remaining testicle.
Recovery from the surgery typically takes about two weeks. During the first day or two, your dog will likely be groggy from anesthesia. Pain and anti-inflammatory medications are prescribed to keep him comfortable, and antibiotics may be added if there’s any concern about infection. Your dog will need to wear a recovery cone the entire time to prevent licking or chewing at the incision. Activity should be restricted to short, leashed walks for bathroom breaks only, with no running, jumping, or rough play. Check the incision daily for redness, heat, unusual smell, discharge, or swelling. A recheck appointment at two weeks is standard for suture removal and to confirm healing.
For orchitis caused by infection, treatment depends on the underlying cause. Bacterial infections are treated with targeted antibiotics, though if the damage to the testicle is severe or the infection doesn’t resolve, surgical removal may still be recommended.
What You Can Watch For at Home
While you wait for your vet appointment, take note of a few things that will help your vet narrow down the diagnosis. Feel the lump gently: is it hard and firm, or soft and squishy? Does your dog flinch or pull away when you touch it? Compare the two testicles for size differences. Look at the rest of your dog’s body for symmetrical hair loss, enlarged nipples, or any skin changes. Note whether the lump appeared suddenly (more suggestive of infection or injury) or has been gradually growing (more typical of a tumor). These details, combined with your dog’s age and neuter status, give your vet a strong starting point before any tests are run.