Cat Lump Under the Skin: What It Could Be and When to Act

Running your hand along your cat's fur and feeling an unexpected lump can make your heart drop. The mind immediately jumps to the worst-case scenario — but the reality is that lumps under a cat's skin can range from completely harmless to something that needs prompt attention. Here's how to think about what you found.

First: Don't Panic, But Don't Ignore It

The most important thing to understand is this: you cannot tell by feel alone whether a lump is benign or malignant. Even experienced veterinarians can't determine the nature of a lump without laboratory tests. So while most lumps in cats turn out to be benign, every new lump deserves a veterinary evaluation.

Common Types of Lumps in Cats

Lipomas (Fatty Tumors)

Lipomas are benign, slow-growing masses of fat cells. They feel soft, moveable, and well-defined under the skin. They're more common in overweight cats and older cats. While they're almost always harmless, they can occasionally grow large enough to interfere with movement — and occasionally what looks like a lipoma turns out to be something else on biopsy.

Abscesses

An abscess is a pocket of pus under the skin, almost always caused by a bite wound — typically from a fight with another cat. Cats' mouths harbor bacteria that seed infections under the skin when they bite. Abscesses feel warm, painful to the touch, and may rupture and drain a foul-smelling fluid. They require veterinary treatment with antibiotics and sometimes drainage.

Cysts

Epidermoid cysts are small, encapsulated sacs filled with keratin (a protein). They feel like round, rubbery lumps and are generally harmless. They can occasionally become infected or rupture.

Vaccine-Associated Sarcoma (VAS)

This is a serious concern specific to cats. A rare but aggressive form of cancer — fibrosarcoma — can develop at vaccination sites, typically between the shoulder blades. Any lump that appears at a vaccination site and is:

• Larger than 2 cm
• Present for more than one month after vaccination
• Growing in size

...should be biopsied immediately. Vaccine-associated sarcomas are locally invasive and must be caught early for the best outcome.

Mast Cell Tumors

Mast cell tumors in cats most commonly appear on the head and neck. They can look like small, raised, hairless bumps. In cats, mast cell tumors are often benign (especially skin forms), but they can spread to internal organs. Veterinary evaluation and a biopsy are essential.

Basal Cell Tumors

These are the most common skin tumors in cats and are usually benign. They tend to appear on the head, neck, and shoulders, often as firm, elevated, hairless lumps in middle-aged to older cats.

Squamous Cell Carcinoma

This aggressive cancer most often affects areas with little pigmentation — the tips of the ears, nose, and eyelids — particularly in white or light-colored cats that spend time in the sun. It can appear as a crusty, non-healing sore or ulceration rather than a distinct lump.

Warning Signs: See a Vet Urgently If the Lump:

• Appeared suddenly or is growing quickly
• Is larger than 1–2 cm
• Has been present for more than four weeks
• Feels firm, fixed to underlying tissue, or irregular
• Is painful when touched
• Is located between the shoulder blades (potential VAS)
• Has ulcerated or is draining fluid
• Is accompanied by lethargy, weight loss, or loss of appetite

What To Do

1. Don't squeeze or attempt to drain it yourself — this can introduce infection or spread cancer cells.
2. Schedule a vet appointment. Even if it's been there for a while, lumps should be evaluated.
3. Your vet may recommend a fine needle aspirate (FNA) — a quick, minimally invasive test where a needle collects cells for examination. This is usually painless and very informative.
4. Some lumps require a biopsy — a small tissue sample sent to a pathologist for definitive diagnosis.

How Voyage Can Help

Not sure if your cat's lump warrants a same-day vet visit or can wait for a routine appointment? Voyage AI Vet can help you evaluate the situation and decide how urgently to act — available 24/7, starting at $4.99/month.

This article is for informational purposes only and is not a substitute for professional veterinary advice. Always have new lumps evaluated by a veterinarian — early diagnosis makes a significant difference in outcomes.