Dog Skin Tumor Types — Benign vs. Malignant Checklist

Oncology/CancerQ&AMeongsiljang Veterinary Advisory Board

Dog skin tumors are classified as either benign or malignant, and you can often get a sense of their nature by observing their size, growth rate, and texture. Here’s a quick overview of the main types and the warning signs that indicate it’s time to visit the vet.

What Are Dog Skin Tumors? Key Factors Distinguishing Benign from Malignant

A veterinarian examining a small lump felt on the back of a Golden Retriever.

Dog skin tumors are abnormal masses of cells that grow on the skin’s surface or in the underlying tissues. They can arise from various cell types, including epithelial cells (such as those in the epidermis, hair follicles, and sebaceous glands), mesenchymal cells (such as fat and fibrous tissue), and pigment-producing cells. The most critical distinction is whether a tumor is benign or malignant. You should seek veterinary care if a mass grows rapidly, feels firmly attached to surrounding tissues, enlarges noticeably over a short period, or develops an irregular shape or surface. While a small, soft, and mobile lump is more likely to be benign, you should never rely on self-diagnosis for peace of mind. Appearance alone cannot definitively distinguish between benign and malignant tumors; accurate diagnosis requires fine-needle aspiration (FNA) or a tissue biopsy.

Benign vs. Malignant Tumors — Differences You Can Feel

Benign tumors grow slowly and are well-defined from surrounding tissues. They are typically round, soft, mobile to the touch, and usually painless. In contrast, malignant tumors tend to grow more rapidly and often invade firmly into adjacent tissues. They may also metastasize to other organs, such as lymph nodes, the lungs, or the liver. However, ulceration or bleeding can occur in both benign and malignant tumors, so these signs alone cannot determine whether a tumor is benign or malignant. Additionally, some tumors, like mast cell tumors, may appear benign on the surface but are actually malignant, making visual assessment unreliable. Therefore, it is safest to perform a fine-needle aspiration (FNA) or biopsy if the tumor is growing quickly, changing in appearance, or regardless of its size.

Comparison Table of Benign and Malignant Tumor Characteristics

Item	Benign Tumor	Malignant Tumor
Growth Rate	Slowly over months to years	Rapidly within weeks to months
Border	Distinct and well-separated	Blurred border with surroundings
Mobility	Moves easily when touched	Firmly fixed
Surface	Generally smooth and uniform	Bumpy or eroded
Pain	Mostly painless	May be painful when touched
Metastasis	None	Possible metastasis to lymph nodes/other organs

The features in the table are only general tendencies. Ulceration (erosion) can appear in both benign and malignant tumors, so it alone cannot serve as a criterion for distinguishing benign from malignant. In particular, mast cell tumors can be malignant even when they look benign, so regardless of appearance, they must be confirmed by fine-needle aspiration (FNA) or biopsy.

Common Benign Tumors — Lipomas, Histiocytomas, Sebaceous Adenomas

Among benign tumors, lipomas are quite common. They feel soft and round under the skin, frequently appear in senior dogs (typically 8 years or older), and tend to develop more often in spayed or neutered pets and those who are overweight. Histiocytomas are benign growths that commonly occur on the head and ears (including the ear flaps) of puppies under three years of age and usually resolve on their own. Sebaceous adenomas are small, wart-like bumps that develop in older dogs, particularly in Cocker Spaniels and Poodles. Pilomatricomas are also benign and feel firm to the touch in the hair follicle area. Even if a tumor is benign, follow-up examinations are necessary if its size changes rapidly.

Illustration of a benign lipoma forming a round mass beneath a dog’s skin

Seek Veterinary Care Immediately If You Notice These Signs

If any of the following signs are present, seek veterinary care within 24 hours. Early detection and surgical removal are the most critical factors in determining the prognosis for skin tumors. - Rapid growth: The mass becomes noticeably larger within 2 to 4 weeks. - Fixed attachment: The mass feels firmly anchored to surrounding tissues and does not move easily when touched. - Ulceration or bleeding: The surface is broken down, or there is discharge of blood or pus. - Color change: The mass turns dark red or purple. - Multiple lesions: Several similar lumps appear simultaneously. - Lymph node swelling: Lymph nodes near the mass are also enlarged.

Malignant Tumors to Watch For — Mast Cell Tumors, Squamous Cell Carcinoma, Melanoma

Mast cell tumors are relatively common malignant skin tumors in dogs. They can vary widely in appearance and may look benign, making it difficult to distinguish between benign and malignant based on appearance alone; therefore, cytological examination is essential. These tumors may swell or redden when touched and can cause systemic symptoms such as gastrointestinal ulcers and anemia. Squamous cell carcinoma often develops on lightly pigmented areas like the nasal planum, where the surface breaks down and forms ulcers. Malignant melanoma can appear as black masses in the mouth or around the nails and may metastasize to other areas, including the lymph nodes. Soft tissue sarcoma is a mesenchymal tumor that grows firmly embedded beneath the skin (subcutaneously).

A veterinarian explaining the results of a malignant skin tumor test to a dog owner.

Monthly At-Home Check — Step-by-Step Tumor Self-Examination

Once a month, gently run your hands over your dog’s entire body after a walk or during brushing. - Order: Head, neck, armpits, chest, abdomen, lower back, hips, legs, and tail. - What to check: Look for any new lumps, changes in the size of existing lumps, signs of pain when touched, or changes in color. - Record-keeping: If you find a lump, take a photo to document its size and location; this will be helpful for comparison during vet visits. Placing a coin or ruler next to the lump in the photo allows for accurate tracking of size changes. Checking on the same date each month helps ensure you don’t miss anything.

A pet owner gently feeling their dog’s body at home to perform a self-check for lumps or tumors.

Breed-Specific Risks — Breeds Predisposed to Tumors

Boxers, Labrador Retrievers, and Golden Retrievers are known to be predisposed to malignant skin tumors such as mast cell tumors. Elevated tumor risk has also been reported in certain breeds, including Bernese Mountain Dogs and Rottweilers. Cocker Spaniels and Poodles frequently develop sebaceous gland tumors (sebaceous adenomas). However, breed-specific predispositions can vary by region and population, so this information should be used for reference only. Senior dogs (typically aged 8 years and older) face an increased risk of skin tumors across all breeds. For predisposed breeds, it is advisable to schedule regular check-ups once or twice a year and develop the habit of monthly skin examinations. Factors such as spaying/neutering status and obesity may also influence the risk of certain tumors, such as lipomas.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Can a dog’s skin tumor go away on its own?

In young puppies, histiocytomas are typically benign tumors that often resolve on their own. However, other types of tumors do not disappear spontaneously. Waiting for a tumor to vanish on its own can be dangerous, as malignant tumors may progress and metastasize during that time. Any lump that changes size rapidly, persists for more than two weeks, or gradually grows should be evaluated with a cytology test, regardless of its shape or size.

Do lipomas always require surgery?

Lipomas are benign, so surgery isn't always necessary. However, we recommend removal if the tumor is large enough to interfere with movement, growing rapidly, or located in sensitive areas like joints or the armpit. Since it's difficult to distinguish lipomas from liposarcomas (malignant) based on appearance alone, it's safer to confirm the diagnosis through a cell test before deciding on treatment.

Does a fine-needle aspiration (FNA) cause pain?

A fine-needle aspiration is a diagnostic test in which a small sample of cells is drawn from the tumor using a thin needle. The discomfort is minimal, comparable to that of a routine vaccination. The procedure can be performed immediately in the clinic without anesthesia, and results are typically available within 1 to 3 days. Because it is less expensive than a biopsy, it is the most commonly used initial test for evaluating tumors.

Can tumors that have been surgically removed come back?

Highly invasive malignant tumors, such as mast cell tumors and soft tissue sarcomas, can recur even after surgical removal. It is essential to ensure wide surgical margins (adequate clearance of surrounding healthy tissue), confirm the tumor type and grade through histopathology, and determine whether additional treatments like radiation or chemotherapy are necessary. Regular follow-up examinations at six months and one year post-surgery are also crucial.

A new black spot has appeared on my pet. Is it just a harmless mole, or could it be melanoma?

It can be difficult to distinguish between simple pigmentation (spots) and malignant melanoma with the naked eye. If a spot grows rapidly, has an asymmetrical shape, or appears as a black mass around the nails or inside the mouth, it may be melanoma. Caution is also warranted if a black area becomes visible after a nail falls out. If you suspect melanoma, seek a cytology exam immediately.

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References

[1] Raskin RE, Meyer DJ. Small Animal Cytologic Diagnosis: Canine and Feline Disease, 2nd Edition. Saunders, 2016. Chapter: Cytology of Skin and Subcutaneous Tissue

[2] Jackson HA, Marsella R. BSAVA Manual of Canine and Feline Dermatology, 4th Edition. BSAVA, 2021. Chapter 34: Cutaneous Neoplasia

[3] Withrow SJ, Vail DM, Page RL. Small Animal Clinical Oncology, 6th Edition. Saunders, 2019