Lumps on your cat: which ones are worrying and why you should not wait
In dogs, many skin lumps are harmless lipomas. In cats it is unwise to think that way: although many lumps are benign or simple abscesses, feline skin and mammary tumours are malignant more often, and treated early they have a much better outlook. A new lump on a cat should be checked, not watched for months.
Why the rule in cats is “check it early”
A cat with a lump rarely shows any other symptoms at first: it eats, plays and behaves normally. That makes it tempting to wait and see whether it grows. But in cats that wait has a price. Mammary tumours in female cats are malignant in the vast majority of cases, and skin tumours are malignant more often than in dogs. Many of these tumours can be cured with surgery if they are removed while small, and can no longer be removed with good margins once they grow. A fine needle sample in the consulting room, which barely bothers the cat, often reveals what it is. The prudent course is not to wait more than a few weeks with a new lump, and not at all with one that is growing.
The most common lump: an abscess
In cats with outdoor access, the most common lump is an abscess from another cat’s bite: it appears within a few days, is soft, warm and painful, often on the head, neck, legs or base of the tail, and usually comes with lethargy and fever. Sometimes it bursts and foul-smelling pus comes out. It is not a tumour, but it does need a vet to drain it, clean it and treat it with veterinary antibiotics and pain relief. Other common inflammatory lumps are granulomas caused by foreign bodies such as a grass awn, reactions to stings, skin cysts and bruises after a knock. Whether a lump is inflammatory will be shown by the examination and, if needed, cytology.
Lumps after a vaccine or injection
After a vaccine or an injection, it is normal for some cats to have a small lump at the site, which disappears within a few weeks. But in a very small percentage of cats, a sarcoma, a very invasive malignant tumour known as injection-site sarcoma, can develop over time at the site of an injection. To avoid missing it, the 3-2-1 rule is used: any lump at an injection site should be biopsied if it persists for more than three months, is larger than two centimetres or is still growing one month after the injection. For this reason, guidelines recommend vaccinating on the legs or tail, where surgery would be simpler, and not between the shoulder blades.
Lumps in the mammary glands
Any lump along a female cat’s mammary chain, however small, should be considered suspicious until proven otherwise. Mammary tumours are one of the most common cancers in female cats, especially those not spayed or spayed late, and those that have received hormonal treatments to suppress heat. They can also appear, more rarely, in males. The treatment of choice is wide surgery, often removing the entire mammary chain on the affected side or on both sides, and the prognosis is much better when the tumour is under two centimetres. Feeling your cat’s belly when you stroke her is a simple way to detect them early.
Other skin tumours and lumps
Several types of tumour appear on cats’ skin: basal cell tumours, often benign; mast cell tumours, which on a cat’s skin are usually low grade but can be multiple; squamous cell carcinoma, common on the ears, nose and eyelids of white cats that sunbathe; and fibrosarcoma. Lipomas do occur in cats, especially in overweight neutered cats, but they are less common than in dogs. Enlarged lymph nodes, which can be felt in the neck, armpits, groin or behind the knees, may indicate an infection or lymphoma. And in kittens, a lump at the navel is usually a hernia, which the vet will assess at a consultation.
What to note before you go
It will help both you and the vet to bring some details. Exactly where the lump is: take a photo with a coin or a ruler beside it for scale. How long you have noticed it, whether it has grown and how fast. Whether it is hard or soft, whether it moves under the skin or is fixed to what lies beneath, whether it is warm or painful, whether it bleeds or is ulcerated. Whether it coincides with a site where the cat was given a vaccine or injection, and when. Whether there are other lumps. And whether the cat has changed in any way: eating less, losing weight, more subdued. A lump map with photos and dates, especially in older cats, is a very useful tool.
What the vet will do
After the examination, the most common test is fine needle aspiration cytology: the lump is pricked with a very fine needle, cells are drawn out and looked at under the microscope. It is done in the consulting room, without anaesthetic in most cats, and in many cases shows whether it is an abscess, a cyst, a benign tumour or a malignant one. If the cytology is inconclusive or the lump looks suspicious, the next step is a biopsy, which gives a more precise diagnosis and the grade of the tumour. Before surgery for a malignant tumour, staging with chest X-rays, ultrasound or a CT scan may be needed, to find out whether it has spread and to plan the operation properly.
What it costs in Spain in 2026
As a guide: consultation, €40 to €60; fine needle cytology with laboratory work, €50 to €120; biopsy under anaesthetic or sedation with analysis, €150 to €400; chest X-rays, €60 to €150; ultrasound, €70 to €150; CT scan, €300 to €700. Removing a small skin lump with analysis, €200 to €500. A mastectomy of one mammary chain, €600 to €1,500, and of both chains, more. Surgery for an injection-site sarcoma requires very wide margins and often radiotherapy, and can cost several thousand euros at a referral centre. That is why, the smaller it is when found, the simpler and cheaper everything becomes.
What not to do
Do not wait for months to see if a lump goes away. Do not prick or burst it yourself, even if it looks like an abscess: you could make the infection worse or hurt yourself, and if it is a tumour it achieves nothing. Do not put creams or home remedies on it. Do not give it human painkillers or anti-inflammatories if it hurts: paracetamol is fatal to cats. And do not trust “it moves, so it is benign” or “it does not hurt, so it is nothing”: neither mobility nor pain distinguishes a benign lump from a malignant one. Only cytology or a biopsy can.
Prevention and detection
Spaying a female cat early, before or shortly after her first heat, greatly reduces the risk of mammary tumours. Avoiding hormonal treatments to suppress heat, for the same reason. Protecting white cats from the sun to prevent carcinoma of the ears and nose. Neutering males that go outdoors, to reduce fights and abscesses. Vaccinating according to current guidelines, at the recommended sites and with no more doses than necessary. And above all, knowing your cat’s body: run your hands over its whole body once a week while stroking it. What is noticed early is treated better.
A new lump on a cat should be checked early, because feline skin and mammary tumours are malignant more often than in dogs and are more curable when small. Fine needle cytology is quick and inexpensive. Remember the 3-2-1 rule for lumps after a vaccine.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.