Most lumps found under cat skin are benign, and the ones that are not are curable when they are found at 1 to 2 cm rather than 5 cm. The decision is not made by how the lump feels on day 1, it is made by 2 measurements taken 14 days apart. A lipoma that grows 5 percent in a month is a lipoma; a firm mass that doubles in 4 weeks is a problem that needs a needle that week.
Three numbers drive every next step: longest diameter in millimeters, days since you first noticed it, and the change between 2 measurements. Photograph the lump with a coin beside it, because a phone photo without a scale is worthless in 6 weeks.
Measure 3 dimensions, not a feeling
Use a soft tape or calipers and record length by width by height in millimeters, not centimeters, because 3 mm of change disappears in a rounded centimeter. Measure in the same body position every time, mark the edges with a tiny clip of fur or a dot of eyeliner pencil, and repeat every 14 days. Volume is roughly length times width times height times 0.52, so a lump going from 10 by 10 by 8 mm to 13 by 13 by 10 mm has gone from 416 to 878 cubic mm, which is a 111 percent increase that a ruler in centimeters would never show.
5 lumps, told apart by feel and speed
| Type | Feel | Typical size (cm) | Growth in 4 weeks (percent) | Next step |
|---|---|---|---|---|
| Lipoma | Soft, movable under the skin | 1 to 5 | 0 to 10 | Measure and recheck at 14 days |
| Abscess | Warm, painful, fluid-filled | 1 to 4 | 50 to 200 | Drain and flush within 24 hours |
| Sebaceous cyst | Firm, round, stuck to skin | 0.5 to 2 | 0 to 15 | Fine needle aspirate |
| Mast cell tumor | Firm, raised, may ulcerate | 0.5 to 2 | 20 to 100 | Aspirate, then wide excision |
| Injection-site sarcoma | Firm, deep, fixed to tissue | 1 to 5 | 20 to 80 | Aspirate plus biopsy, then CT |
The 2 cm rule and the 4 week rule
Fine needle aspiration (FNA) is due when any 1 of 4 triggers appears: longest diameter over 2 cm at the first visit, volume doubling within 4 weeks, a lump still present 4 weeks after a vaccination, or any lump fixed to the tissue underneath. A 22 to 25 gauge needle, 3 to 5 passes, and a slide sent to the lab gives an answer in 24 to 48 hours for 60 to 200 dollars. About 20 to 30 percent of aspirates come back inconclusive, usually because the sample was blood or the cells were sparse, and those lumps move to biopsy or excision rather than another month of watching.
Vaccine-site lumps: 12 weeks of watching, then act
A small soft swelling in the first 24 to 72 hours after an injection is normal and resolves in 1 to 2 weeks. A lump that is still there at 4 weeks, or that is over 2 cm at any point, or that is still growing at 12 weeks, needs aspiration that week. Injection-site sarcoma is uncommon, quoted at roughly 1 in 1,000 to 1 in 10,000 injected cats, and it usually appears 1 to 12 months after the injection, most often between the shoulder blades, on the lateral chest wall, or on a hind limb. Surgery means 2 to 3 cm margins and 1 full fascial plane, and recurrence is still common even with clean margins, which is the argument for acting at 2 cm instead of 5 cm.
Abscess: 72 hours to swell, 7 to 10 days of antibiotics
A bite wound from a cat fight swells in 24 to 72 hours, reaches 1 to 4 cm, and comes with a fever of 38.5 to 40 C, pain on touch and often a bald patch where the cat licked. It needs lancing, flushing with 0.9 percent saline, and a drain for 2 to 5 days, plus amoxicillin-clavulanate at 12.5 to 25 mg per kg twice daily for 7 to 10 days. Ask for a feline leukemia and FIV test at the same visit, since a fight that caused the abscess is a real exposure. The full drainage and aftercare sequence is in the bite abscess guide.
Site matters: 5 places with different risks
Between the shoulder blades is the classic injection site and the hardest place to get 3 cm margins, which is why current guidance moves vaccines to the lateral chest wall, the lumbar area or a distal limb. The ear base carries a higher odds of squamous cell carcinoma in white cats. A nipple lump in a cat is serious, because roughly 85 to 90 percent of feline mammary tumors are malignant and the cat is often only 10 to 12 years old. The flank and the thigh collect lipomas and soft tissue sarcomas, and the eyelid collects small benign cysts that still need a look because of the 1 to 2 mm clearance to the eye. A cat that hides for 2 days after you find the lump is showing pain or nausea, not shyness, and the behavior notes in the hiding guide help separate the two.
The 5 signs that skip watch and wait
Do not watch these: a lump over 2 cm on day 1, growth over 20 percent in 14 days, ulceration or discharge, a lump that is fixed and cannot be moved over the muscle, or weight loss of 5 percent in 30 days with a new lump. Add a sixth for older cats: any new lump in a cat over 10 years old gets aspirated at the first visit, because the pretest odds change with age and the cost is only 60 to 200 dollars. Cats are also good at hiding these: a 3 cm mass on the flank can sit under fur for months in a longhaired cat, so run your hands over the whole body once a month, including the tail base and between the toes.
Do these 5 things in the next 14 days
- Measure the lump in millimeters in 3 dimensions and photograph it with a ruler.
- Write the first-seen date down; day 1 matters more than size.
- Repeat the measurement on day 14 and calculate the volume change.
- Book FNA this week if the lump is over 2 cm or doubled.
- Check the rest of the body, including the tail base and paw pads.
Frequently asked
How do I know if a lump on my cat is cancer?
You cannot tell by touch, and neither can a vet with certainty. What predicts it is behavior: a mass that doubles in volume within 4 weeks, is fixed to deeper tissue, is over 2 cm, or keeps growing 12 weeks after a vaccine needs a fine needle aspirate. That test costs 60 to 200 dollars and answers in 24 to 48 hours.
Are fatty lumps on cats common and harmless?
Lipomas are common in middle-aged and older cats and are usually harmless, growing 0 to 10 percent in a month and feeling soft and movable under the skin. The risk is assuming the next lump is another lipoma, so every new lump gets measured in 3 dimensions and rechecked at 14 days.
Should a lump at a vaccine site be removed right away?
Not on day 1. A soft swelling in the first 24 to 72 hours usually fades in 1 to 2 weeks. It needs aspiration if it is over 2 cm, still present at 4 weeks, or still growing at 12 weeks, and then surgery with 2 to 3 cm margins if the cytology is suspicious.