The most common sign of testicular cancer in dogs is a lump or noticeable size change in one or both testicles. You may feel a firm mass during a belly rub, notice one testicle looks larger than the other, or see swelling in the scrotal area. Some dogs show no obvious testicular changes at all, especially if the tumor is in an undescended testicle hidden inside the abdomen. Knowing what to look and feel for can help you catch the problem early, when treatment is most straightforward.
What You Might Feel or See
The earliest and most reliable sign is a physical change in the testicle itself. When you gently feel your dog’s scrotum, a healthy testicle is smooth, firm but slightly yielding, and roughly symmetrical with the other side. A tumor often shows up as a hard lump or mass within the testicle, or as one testicle that has grown noticeably larger. Some tumors stay small (under a centimeter), while others can grow to 3 or 4 centimeters and distort the entire shape of the testicle.
Not all testicular tumors feel the same. Some are solid and firm, while others feel softer or even fluid-filled. You might also notice that one testicle seems heavier, or that your dog reacts when you touch the area. Scrotal skin changes like darkening or thickening can sometimes accompany the growth. The key thing to watch for is any asymmetry or new lump that wasn’t there before.
Hormonal Signs You Wouldn’t Expect
Some testicular tumors produce estrogen, which causes a surprising set of changes known as feminization syndrome. This happens most often with one particular tumor type (Sertoli cell tumors), especially when the affected testicle never descended into the scrotum. About 70% of these tumors in abdominal testicles are hormonally active.
The signs of feminization are distinctive and sometimes the only visible clue that something is wrong:
- Symmetrical hair loss that typically starts around the genitals and spreads to the belly, flanks, and chest. The hair falls out evenly on both sides and isn’t itchy.
- Darkened skin in the areas where hair has thinned.
- Enlarged nipples or mammary tissue, which looks unusual on a male dog.
- A swollen, droopy prepuce (the sheath covering the penis).
- Behavioral changes like squatting to urinate in a female posture or attracting attention from other male dogs.
These hormonal signs can develop gradually over months. In documented cases, dogs presented with progressive weight loss, widespread hair loss, and changes in urination posture before anyone suspected a tumor. If your intact male dog is losing hair symmetrically without itching, that combination alone warrants a vet visit.
Dogs at Higher Risk
The single biggest risk factor is cryptorchidism, the condition where one or both testicles fail to drop into the scrotum. Cryptorchid dogs are 9 to almost 14 times more likely to develop testicular cancer than dogs with normally descended testicles. For certain tumor types, the risk is even more dramatic: retained testicles carry a 23-times greater risk for Sertoli cell tumors and a 16-times greater risk for seminomas compared to scrotal testicles.
This is especially important because tumors in undescended testicles are hidden inside the abdomen or inguinal canal, where you can’t feel them during a routine check. The only signs may be those hormonal changes described above, or vague symptoms like lethargy and weight loss.
Age plays a significant role too. The median age for cancer diagnosis in dogs overall is about 8.8 years, and testicular tumors follow a similar pattern, appearing most often in middle-aged to older intact males. Certain breeds tend to develop cancers at younger ages. Boxers, Bulldogs, French Bulldogs, Boston Terriers, and Vizslas have median cancer diagnosis ages that are two or more years younger than expected for their size. If your dog is one of these breeds, paying attention earlier is worthwhile.
The Three Tumor Types
Testicular cancer in dogs comes in three main forms, and they behave quite differently from each other.
Seminomas grow from the sperm-producing cells. They tend to appear as solid, whitish or pale yellow masses. They can occur as single or multiple lumps within the testicle and don’t always have clear borders, which can make them harder to distinguish from normal tissue by touch alone.
Sertoli cell tumors develop from the cells that support sperm development. These are the ones most likely to produce estrogen and cause feminization. They tend to form firm, well-defined nodules that can occupy more than half the testicle. Both Sertoli cell tumors and seminomas have a higher potential for spreading to other parts of the body.
Interstitial cell tumors (also called Leydig cell tumors) arise from testosterone-producing cells. These are the most common type and also the least aggressive. They’re typically soft, yellowish-brown, and well-defined. They sometimes contain fluid-filled or hemorrhagic areas. While metastasis is rare with these tumors, it has been documented.
It’s also possible for a dog to have more than one type of tumor in the same testicle, which researchers have documented in multiple studies.
How Vets Confirm the Diagnosis
If you bring your dog in with a suspicious lump or hormonal changes, your vet will start with a physical exam, carefully feeling both testicles for masses, size differences, or texture changes. For cryptorchid dogs, they’ll check the inguinal area and abdomen.
Ultrasound is the primary imaging tool for evaluating testicular masses. Standard ultrasound can detect most tumors and show their size, location, and internal structure. More advanced techniques using contrast-enhanced ultrasound can identify tumors that standard imaging might miss, particularly small ones or those with poor blood flow. Your vet may also recommend blood work to check hormone levels, especially if feminization signs are present.
The definitive diagnosis comes after surgical removal, when a pathologist examines the tissue under a microscope. This determines the exact tumor type and whether it has features suggesting it could spread.
Treatment and Outlook
Surgical removal of the affected testicle (or both testicles) is the standard treatment and is often curative. For most testicular tumors, especially interstitial cell tumors and early-stage seminomas, surgery alone resolves the problem. The feminization signs caused by hormone-producing tumors typically reverse after the tumor is removed, though hair regrowth can take several weeks to months.
The prognosis for dogs with testicular cancer is generally favorable when the tumor is caught before it spreads. Sertoli cell tumors and seminomas carry a higher risk of metastasis than interstitial cell tumors, which is why early detection matters. If a tumor has spread to lymph nodes or other organs, additional treatment may be needed and the outlook becomes less predictable.
For dogs with cryptorchidism, veterinary guidelines recommend early neutering to prevent testicular tumors from developing in the retained testicle. This is particularly relevant because those hidden tumors can grow undetected for a long time.
What to Check at Home
If your dog is an intact male, get in the habit of gently feeling both testicles every few weeks. You’re looking for anything new: a hard spot, a size difference between sides, a change in shape, or any reaction from your dog suggesting pain. Both testicles should feel roughly the same size and have a smooth, consistent texture.
Pay attention to your dog’s body more broadly. Symmetrical hair loss without itching, enlarged nipples, changes in how your dog urinates, or unexplained weight loss are all worth noting. These signs develop slowly, so comparing recent photos of your dog to older ones can sometimes make subtle changes more obvious. Starting regular checks around age 7 is a reasonable baseline, or as early as age 4 for higher-risk breeds.