Most lumps on a cat over 11 are harmless, and the ones that are not are usually curable when they are 1 cm and not when they are 3 cm. The routine that works is boring: hands on the cat once a week, a caliper and a notebook, a photo with a ruler in frame, and a rule that any lump which grows 20 percent in 2 weeks gets a needle in it. Three types cover most of what you will find, and only a sample separates them.
The 3 types you will actually find
A lipoma is soft, moves under the skin, and feels like a grape under a blanket; it is benign fat and it grows by 1 to 2 mm a month at most. A mast cell tumor in a cat is usually a small firm pink nodule, sometimes with a hairless top, and in cats it can behave far better than the same tumor does in a dog, but the only way to know is a sample. A cyst is fluid-filled, often bluish, and can rupture and leave a scab. Two others deserve a mention because they change the timeline: an injection-site sarcoma is firm, fixed to deeper tissue, and appears 1 to 6 months after a vaccine at the shoulder, hip, or flank; a squamous cell carcinoma is an ulcerated crusty sore on a white ear tip or nose, and 10 minutes of summer sun a day for 8 years is enough to cause it.
Measure in millimetres, not in words
Small is not a description; 8 mm by 6 mm by 4 mm is. Use a cheap plastic caliper, measure 3 dimensions, and write the date. Map each lump on a body diagram: left flank, 4 cm behind the last rib, 2 cm below the spine. Take a photo from 30 cm with a ruler lying beside the lump, in the same light, so the frame gives scale. The notebook matters more than the caliper, because a lump that has been 10 mm for 18 months is a different animal from one that went from 6 to 12 mm in 3 weeks.
The 1 cm and 20 percent rules
Record every lump of 1 cm or more, and every lump under 1 cm that is new. Then apply 1 growth rule: 20 percent increase in any dimension over 2 weeks earns a fine needle aspirate. That is not a biopsy: a 25 gauge needle, 3 or 4 passes, and 5 minutes in the consult room. Results come back in 1 to 3 days at most labs. A lump that doubles in 4 weeks, or one that goes from soft and movable to firm and fixed, skips the wait and goes straight to a sample.
| Lump type | Typical feel | Usual growth | Size that triggers a test (cm) | Act within (days) |
|---|---|---|---|---|
| Lipoma | soft, movable, under the skin | 1 to 2 mm a month | 2.0 | 30 |
| Mast cell tumor | firm, raised, pink, hairless top | 2 to 5 mm a month | 1.0 | 7 |
| Cyst | fluid-filled, bluish, may leak | 3 to 8 mm a month, then stable | 1.5 | 14 |
| Injection-site sarcoma | firm, fixed, deep | 5 to 10 mm a month | 1.0 | 3 |
| Squamous cell carcinoma | crusted, ulcerated, on ear or nose | slow, then bleeds | 0.5 | 7 |
How often to check, by lump age
- Weekly: a 3 minute hands-on pass over ears, jaw, neck, shoulders, flanks, belly, legs, tail base. Same order every time.
- Every 14 days: measure any lump on the record with the caliper, and photograph it.
- Every 30 days: full photo set with a ruler, and compare against last month side by side.
- Every 6 months: the vet does the same pass at the wellness visit, with the notebook open.
- Any time: a lump that was not there 2 weeks ago gets measured the same day, not next month.
Red lines that mean this week
Ulceration, bleeding, or a scab that will not heal; a lump fixed to the muscle underneath so the skin will not slide over it; any lump of 3 cm or more; 3 or more new lumps within 4 weeks; a lump at a vaccine site that is still there at 3 months; weight loss of 5 percent in 6 months with a new lump; drooling or trouble swallowing with a lump under the jaw. Any 1 of those is a visit within 7 days, and 2 of them is a visit within 48 hours. A cat that hides, stops grooming, or stops jumping onto the bed has a pain signal on top of the lump, and that also counts.
What the vet will do, and what it costs
First a fine needle aspirate of the lump and of the nearest lymph node; second, if the aspirate is not conclusive, a biopsy under local or general anesthesia; third, if the answer is malignant, staging with chest x-rays and an abdominal ultrasound. Ask for the cytology report by email, and ask 3 questions: is it benign, does it need to come out, and what is the margin needed if it does. Wide margins mean 2 to 3 cm of normal tissue around the lump, which is why a 1 cm lump on a leg is a smaller operation than the same lump at 3 cm. An aspirate, a biopsy, and staging sit in different bands on the cost tables, and the aspirate is almost always the cheapest step in the whole path.
What not to do while you watch
Do not squeeze, lance, or apply hot compresses to a lump: a mast cell tumor can degranulate and swell, and a cyst that is squeezed can abscess. Do not start a supplement or a homeopathic drop and count it as treatment, because the 2 weeks it buys are the 2 weeks the growth rule needed. Do not skip the measurement because the lump feels the same; hands are bad at 3 mm. Do not assume a lump is a lipoma because the last one was. And do not stop the weekly pass once a lump is on the record, because a second lump is common and appears 2 to 8 months later.
A 4 week start
- Day 1: 3 minute pass, measure everything you find, draw the body map, take photos with a ruler.
- Day 7: repeat the pass. Anything new goes on the record with a date.
- Day 14: measure again. A 20 percent change in any dimension books an aspirate.
- Day 21: weigh the cat to 10 g and add the number to the same page.
- Day 28: compare the photo set. A stable 8 mm lump stays on 30 day checks; a 12 mm one does not.
Weight belongs on the same page as the lumps, because a lump plus 5 percent weight loss in 6 months changes the urgency. The weekly routine is set out in weight monitoring for senior cats, and if the cat is drinking and urinating more alongside a new lump, the intake notes in water fountains for senior cats give you the numbers to bring.
Terms mentioned in this article
Frequently asked
How do I know if a lump on my cat is a lipoma?
You cannot know by feel alone. A lipoma is soft, movable, and grows 1 to 2 mm a month, which is a good sign but not proof, since some mast cell tumors also feel soft. A fine needle aspirate takes 5 minutes in the consult room and answers it in 1 to 3 days for the price of the cheapest test on the path.
How fast do cat tumors grow?
Benign fat grows 1 to 2 mm a month, cysts can add 3 to 8 mm a month and then stop, and a mast cell tumor often adds 2 to 5 mm a month. An injection-site sarcoma is faster at 5 to 10 mm a month and feels fixed. The 20 percent in 2 weeks rule exists because normal lumps do not do that.
Is it safe to wait 2 weeks before a vet visit?
For a soft, movable lump under 1 cm with no ulceration, 14 days of measurement is reasonable. It is not reasonable for a fixed lump, any lump of 3 cm, a bleeding or ulcerated lump, a lump at a vaccine site still present at 3 months, or a new lump with 5 percent weight loss in 6 months.