every week in practice, I see owners who discovered a lump on their pet weeks or months ago and finally decided to bring them in. The internal debate they describe is familiar: "I thought it was nothing. Then I thought it might be something. Then I got scared and waited longer." The anxiety around lumps is understandable — but waiting is rarely the right strategy.
This guide explains what veterinarians actually assess when we evaluate a skin growth, which features carry more concern than others, and what the diagnostic process looks like. The goal is to give you enough information to make good decisions without assuming any particular lump is fine.
Why "Just Watch It" Can Be a Problem
The intuition to wait and see before taking action is reasonable in many situations. With skin growths on pets, it's less useful than it might seem, for a few reasons.
First, benign and malignant growths can look nearly identical from the outside. A small, smooth, movable lump is more likely to be a lipoma — but it can also be a mast cell tumor, which has a wide range of behavior from indolent to aggressive. Second, many malignant tumors are more successfully treated when they're small and haven't yet involved deep tissue or regional lymph nodes. Third, the size of a lump alone isn't a reliable indicator of its significance — small mast cell tumors in certain locations cause more harm than large lipomas in others.
The practical guidance I give owners: evaluate new lumps, don't watch them.
How Veterinarians Assess Skin Growths
When I examine a new growth, I'm building a mental picture using several factors simultaneously.
Location
Certain anatomical sites carry higher statistical risk of malignancy. Growths on the digits (toes) are more likely to be subungual squamous cell carcinoma than benign — particularly in large, dark-coated breeds like Standard Poodles, Giant Schnauzers, Gordon Setters, Rottweilers, and black Labrador Retrievers, where it shows up disproportionately on the black-pigmented toes specifically. Growths on the spleen or liver detected incidentally have different significance than growths on the skin surface. Lumps in the area of previous injections (particularly in cats between the shoulder blades) carry concern for injection-site sarcoma.
In cats, any skin growth deserves more urgent evaluation than a comparable growth in a dog. Feline skin tumors are malignant more often than canine skin tumors — depending on the study, somewhere around half to two-thirds of feline skin tumors turn out malignant, compared to roughly a fifth to two-fifths in dogs. That gap is exactly why I get more twitchy about a new lump on a cat than the same-looking lump on a dog.
Size and growth rate
A lump that has been present and stable for over a year is statistically less likely to be aggressively malignant — though this is not a rule, and some tumors grow slowly before accelerating. A lump that has doubled in size over two to four weeks warrants prompt evaluation regardless of how benign it looks. Growth rate is one of the most informative features.
Surface and consistency
Smooth, soft, freely movable lumps — particularly in middle-aged to older dogs — are commonly lipomas (benign fat deposits). Firm, fixed, or ulcerated growths with irregular surfaces carry more concern. Growths that are ulcerated (have a raw or open surface), discolored, or that ooze fluid should be evaluated promptly.
Attachment to deep tissue
Gently move the growth under your fingers. Freely movable lumps that slide over the tissue beneath them are typically contained within the subcutaneous fat. Growths that appear fixed to the underlying muscle or fascia are more concerning because this suggests invasion beyond the subcutaneous layer.
Single vs. multiple
Multiple small lumps appearing simultaneously, particularly in a previously healthy animal, may suggest lymphoma, histiocytic disease, or systemic conditions rather than individual benign growths. A single new lump is more consistent with a focal process.
Common Benign Skin Growths
Lipoma
Lipomas are benign fatty tumors — by far the most common skin growth seen in dogs, particularly middle-aged and older individuals and certain breeds including Labrador Retrievers, Dobermans, and Miniature Schnauzers. They're soft, smooth, movable, and typically grow slowly over months to years.
Most lipomas don't require removal unless they're in a location causing mechanical problems (e.g., in an axilla disrupting movement, or growing very large). However, they cannot be reliably diagnosed as lipomas by feel alone — a fine needle aspirate confirming adipocytes (fat cells) is needed to rule out other possibilities.
Infiltrative lipomas are a subtype that invade into surrounding muscle rather than remaining encapsulated. These require wider surgical margins.
Sebaceous cysts and follicular cysts
Sebaceous cysts form when a hair follicle or sebaceous gland becomes blocked. They typically feel like small, round, fluctuant (fluid-filled) lumps just beneath the skin surface, sometimes with a central pore. Contents, if expressed, are white or grayish. Most are benign and don't require treatment unless they become inflamed, infected, or are in a problematic location.
Warts (papillomas)
Viral papillomas appear as small, irregular, cauliflower-like growths, most often in young dogs and in immune-compromised older dogs. They can appear on the skin or in and around the mouth. Most resolve spontaneously in young healthy dogs as immunity develops.
Histiocytoma
Histiocytomas are common, benign skin tumors in young dogs (typically under two years of age). They appear as raised, pink, button-like lesions — sometimes mistaken for more serious tumors. Most regress spontaneously within one to three months as the immune system mounts a response. Confirmation by fine needle aspirate is recommended to exclude look-alike lesions.
Skin tags (fibromas)
Soft, pedunculated (on a stalk) growths that hang from the skin surface. Extremely common in middle-aged and older dogs, particularly in skin fold areas. Almost always benign. Removal is straightforward if they're bothersome or in a location prone to trauma.
Common Malignant Skin Tumors
Mast cell tumor (MCT)
Mast cell tumors are the most common malignant skin tumor in dogs and are notorious for mimicking benign lesions. They can look like lipomas, skin tags, histiocytomas, or inflamed cysts. MCTs release histamine and other inflammatory mediators when manipulated, which can cause localized redness, swelling, and occasionally systemic effects.
Behavior ranges from slow-growing, indolent tumors that don't spread for years, to aggressively metastatic tumors that require combination therapy. Histological grade determines this, not appearance. A fine needle aspirate typically identifies mast cells (granulated cells with a distinctive appearance), which then triggers staging and treatment planning.
Location matters: MCTs on the muzzle, digit, and mucocutaneous junctions (lips, eyelids, and similar transition zones) tend to behave more aggressively and carry higher rates of regional lymph node involvement. I'll add a caveat here, because I had to update my own thinking on this one — the older teaching that preputial and scrotal MCTs automatically carry that same elevated risk has been pushed back on by more recent research, which hasn't found evidence those sites are inherently more aggressive once other prognostic factors are accounted for.
Squamous cell carcinoma (SCC)
SCC of the skin in dogs and cats appears as red, raised, ulcerated, and crusting lesions. In cats, SCC frequently affects sun-exposed areas: ear tips, eyelids, and the nose — particularly in cats with white or light-pigmented skin in these locations. Sun exposure is the major risk factor for cutaneous SCC in cats.
SCC on the digit in dogs (subungual SCC) often causes swelling and disruption of the toe nail and may initially be mistaken for an infection.
Injection-site sarcoma (feline)
Cats can develop fibrosarcomas or other sarcomas at the site of previous injections — vaccinations, microchips, or medication injections. These typically appear in the interscapular region (between the shoulder blades) as firm, rapidly growing masses. Any new growth at an injection site in a cat, particularly one that appears and grows within weeks of an injection, warrants immediate evaluation. This tumor type is locally invasive and tends to recur after incomplete removal.
Melanoma
Cutaneous melanoma in dogs most often occurs in areas of pigmented skin and varies widely in behavior — benign melanocytomas occur, but malignant melanoma is possible. Oral melanoma (discussed in our guide on growths that develop inside the mouth) is typically more aggressive than cutaneous melanoma.
The Diagnostic Process: What to Expect
Fine needle aspirate (FNA): The standard first step for most skin masses. A small needle is inserted into the mass, cells are collected on a slide, and the slide is examined under a microscope. FNA is quick, inexpensive, and provides useful information in many cases — lipomas, histiocytomas, and mast cell tumors are often diagnosable by FNA. However, some tumors don't exfoliate well, and FNA results of "no diagnostic cells" don't exclude malignancy.
Biopsy: Provides tissue for histopathology — the definitive diagnostic method. An incisional biopsy takes a small piece of the mass; an excisional biopsy removes the entire mass and examines it. For growths suspected of being malignant, your vet may recommend immediate excisional biopsy with wide margins rather than FNA first.
Staging: For confirmed malignancies, staging involves assessing whether the tumor has spread. This typically includes evaluation of regional lymph nodes (by FNA or biopsy), thoracic radiographs (chest X-rays to check for lung spread), and sometimes abdominal ultrasound or CT scanning depending on tumor type.
For comprehensive skin health monitoring as part of overall care, regular home skin checks and annual veterinary assessments are the best way to catch new growths early.
When to Act Immediately vs. Schedule Soon
Act immediately:
- Any lump in a cat (evaluate within one to two weeks)
- Rapidly growing lump — doubling in size over two to four weeks
- Ulcerated or bleeding growth
- Lump in a young dog (under two years, but histiocytoma usually resolves; still evaluate)
- Lump at a prior injection site in cats
- Lump on the toe of a dark-coated large breed dog
- Lump accompanied by systemic signs (loss of appetite, weight loss, lethargy)
Schedule within one to two weeks:
- Any new growth that's stable and has been present for under a month
- A growth that has changed in texture, surface, or color
- Multiple new growths appearing simultaneously
At next routine appointment:
- Stable lipomas that have been confirmed by FNA and aren't causing mechanical problems
- Known benign growths that are being monitored
Frequently Asked Questions
Can I tell if a lump is a lipoma by how it feels?
A soft, smooth, movable subcutaneous lump in a middle-aged or older dog has a reasonably high likelihood of being a lipoma — but not high enough to act on without verification. Mast cell tumors and other malignancies can have identical feel. A fine needle aspirate takes minutes and gives you an answer. I always recommend confirming rather than assuming.
My vet said to "just watch" a lump. Is that the right advice?
Sometimes watchful waiting is appropriate — for stable, previously confirmed benign growths, or for histiocytomas in young dogs that are expected to regress. What's important is that the lump has been evaluated, not just observed from a distance. If your vet recommends watching, ask what specific changes would prompt rechecking, and mark a calendar to return in four to six weeks regardless.
How much does it cost to have a lump tested?
Fine needle aspirate is generally inexpensive — a fraction of the cost of surgery. Biopsy with histopathology costs more but provides definitive information. In most cases, the information from a cytology or histopathology result guides decisions about whether treatment is needed at all, making it cost-effective even for growths that turn out to be benign.
My older dog is getting new lumps constantly. Do I need to test every one?
In older dogs — particularly prone breeds like Labrador Retrievers — new benign lipomas appearing regularly are common. After a pattern of confirmed lipomas has been established with testing, your vet may agree on a monitoring approach for small new lumps with clear lipoma characteristics, with FNA reserved for anything that doesn't fit the pattern. This is an individual decision made with your vet based on your dog's history.