When to see a vet urgently
A lump should be examined without delay, and the following signs need urgent help:
- Heavy or continuous bleeding from the tumour area.
- Severe pain, swelling or deteriorating general health associated with an ulcerated area.
- Difficulty breathing, collapse or inability to stand.
Do not wait for monitoring criteria to be met in this situation. You can find a vet near you.
Overview
Fibrosarcoma is a firm tumour most often found on the back, limbs or between the shoulder blades. It grows into and destroys surrounding tissue. Metastases are fairly uncommon, but the tumour often regrows even after surgery.
A particular form is post-vaccination fibrosarcoma, which develops at the site of a vaccine or medicine injection in a small proportion of cats.
This is called feline injection-site sarcoma (FISS), because it has also been found after injections other than vaccines. It may behave more aggressively than soft tissue sarcomas arising elsewhere. However, a lump appearing after vaccination is not automatically cancer: appearance or touch alone cannot confirm the diagnosis.
Local growth and metastatic spread are different problems. A tumour can be difficult to remove even without detected metastases. Conversely, metastatic risk is not the same for every fibrosarcoma, so uncommon should not be interpreted as meaning that staging is unnecessary.
Causes
The exact cause is not always established, but the main proposed factors include:
- Hereditary predisposition.
- Chronic inflammation of skin and subcutaneous tissue.
- Carcinogenic factors, including poisons and radiation.
- Reactions after injections, in the post-vaccination form.
- Viral infections, with FeLV and FIV as predisposing factors.
The list is not a proven set of causes for every fibrosarcoma. Injection-site sarcoma development is incompletely understood, and chronic local inflammation is considered a possible trigger. FeLV and FIV have not been shown to explain ordinary injection-site sarcoma.
A separate, often multifocal form of feline fibrosarcoma occurring in young animals is associated with feline sarcoma virus. This does not mean that every fibrosarcoma is viral. Tumour type and background are assessed separately.
Risk factors
- Middle-aged and older cats, most often over 7 years old.
- Cats with chronic skin inflammation.
- Repeated injections and vaccinations at the same site.
- Hereditary predisposition in some breeds.
This list cannot provide a simple breed-based risk classification. Possible hereditary susceptibility has been studied, but an individual cat's disease cannot be predicted from breed. Nor does every long-standing skin inflammation lead to a tumour. A lump is examined even in a young cat or one without a known injection history.
Typical symptoms
Fibrosarcoma develops gradually and may appear as:
- A firm, immobile lump beneath the skin.
- Gradual enlargement of the lump.
- Absence of pain at first.
- Ulceration of overlying skin as the tumour grows.
- Restricted movement if the tumour is near a joint.
- General wasting and weakness at a late stage.
Lack of pain or a normal appetite does not rule out a tumour. A small subcutaneous change may be found incidentally during stroking, and extend deeper than the surface suggests. Record the discovery date, location, observed growth and previous injections there. There is no need to squeeze or massage a lump to assess it.
Its everyday effects may include reduced movement, avoiding jumps or changes in grooming. Cats hide pain, so also observe behaviour with familiar people and choice of resting places. Report changes even if your cat does not vocalise in pain.
Complications and consequences
- Recurrence after surgery.
- Growth into deep tissues, including muscles and bones.
- Ulcers and secondary infections.
- Metastases to internal organs, less common but possible.
Local recurrence does not always mean that a visible lump was left behind at the original operation. Microscopic tumour cells can extend into apparently healthy surroundings. A small superficial removal and a wide, preplanned operation are therefore different procedures. Treating a recurrent tumour can be harder than treating the first one.
Diagnosis at the vet
To confirm the diagnosis, investigations include:
- Palpation and clinical examination.
- Cell or tissue sampling, the diagnostic gold standard.
- X-rays and ultrasound to assess spread.
- CT or MRI for surgical planning.
- Blood tests to assess general health.
A cell sample can provide useful information, but does not always contain enough cells to confirm a sarcoma. Diagnosis usually relies on histological tissue examination, in which a pathologist assesses tissue structure. Sampling method and site are planned to fit possible later surgery.
The tumour should therefore not be removed as a trial through the smallest possible incision before planning. CT or MRI can reveal extension into muscles and other tissues and help define the area for removal. The visible lump's edges may not be the tumour's true boundaries.
Lung imaging and other investigations where necessary look for metastases. Nearby lymph nodes are assessed and may be sampled. Blood tests help evaluate general health and treatment suitability; normal blood results alone do not rule out a tumour. The pathology report from any previous removal and details of the surgical area are needed for further planning.
Treatment
Surgery is central when the tumour can be removed. The aim is sufficiently wide removal both around and beneath it, rather than simply shelling out the lump that can be felt. The first carefully planned procedure often offers the best chance of local control. Referral to a team experienced in cancer surgery may be appropriate before removal.
Location can make complete removal difficult. A limb tumour may require consideration of amputation, while wide excision is assessed at other sites. Planning also includes wound closure, recovery and expected function after surgery.
Radiotherapy may be used before or after surgery to improve local control. Chemotherapy is considered individually, for example for aggressive disease. The goals and expected benefits are discussed separately: not every tumour can be cured, and no single method guarantees freedom from recurrence.
If removal is not possible or intensive treatment does not suit your cat, symptom-relieving care remains possible. This focuses on pain control, ulcer care, eating and maintaining ordinary activities. Benefit is assessed through your cat's wellbeing, rather than tumour size alone.
Home care and monitoring
After surgery, prevent wound licking and follow movement and medicine instructions. Keep the wound dry and report increasing swelling, discharge or opening. Do not add your own creams or cleaning agents. Ask the practice for help if protection or medicine administration is difficult.
Continue checking the surgical area after healing, because recurrence may appear as new thickening or a lump. Agree a follow-up plan during treatment. Record appetite, movement, grooming and interest in familiar activities so pain and treatment effects are not judged solely at appointments.
Prognosis
Prognosis depends on tumour type, location, size, tissue findings and completeness of removal. Injection-site sarcoma and other fibrosarcomas should not automatically be given the same outlook. Metastases, previous recurrences and overall health also affect options. Research provides guidance but does not guarantee a particular survival time for an individual cat.
Prevention
- Avoid vaccinations and injections in the neck: WSAVA and ABCD guidelines recommend lower limb or tail sites for feline vaccines so a possible tumour can be completely removed.
- The 3-2-1 rule: a post-injection lump should be biopsied if it persists for over 3 months, exceeds 2 cm or is still growing after a month.
- Avoid unnecessary injections.
- Attend regular veterinary examinations and monitor all body lumps.
- Support overall health and immunity.
The tail is not as established a recommended injection site as the lower limb; more research is needed. Your vet chooses the site according to the product's approved route and possibilities for later treatment. Site selection may make surgery easier, but does not guarantee complete tumour removal.
Any one 3-2-1 criterion is enough to justify further investigation. The rule does not mean every lump should be monitored at home for months. A growing, painful or ulcerated lesion is examined earlier. Vaccination and injection sites are recorded so changes can be compared with earlier information.
Do not omit necessary vaccines through fear of cancer. Vaccine benefits and infection risk are assessed individually, while unnecessary injections are avoided. General health care does not replace injection-site selection or investigation of a new lump.
Important for cat owners
Fibrosarcoma is a malignant tumour requiring careful attention. Remember that every lump that grows or persists beyond 3–4 weeks should be shown to a vet. The earlier diagnosis and treatment, such as surgery or radiotherapy, begin, the better the chance of slowing growth and extending your cat's life.
This article provides general information and does not replace veterinary care. Contact a vet if you notice signs of illness in your cat.
Frequently asked questions
Is every post-vaccination lump cancer?
No. However, palpation cannot reliably distinguish an inflammatory reaction from a tumour. Persistence, size and growth guide sampling, and a symptomatic lesion is examined promptly.
Is a fine-needle sample enough for diagnosis?
Not always. Sarcomas may shed few cells, so a tissue sample may be needed. Sampling is planned with possible tumour removal in mind.
Why is healthy tissue removed around the tumour too?
Tumour cells can extend beyond the visible lump. Sufficiently wide removal aims to include these areas and reduce local recurrence risk.
Does normal health mean a lump does not need examination?
No. A local tumour may initially be painless while your cat otherwise seems bright. A new or growing subcutaneous lump is assessed without waiting for general symptoms.
This article is for general information only and is not a guide to treatment. If you notice the first signs of illness, contact your vet.
Paw4care editorial team
Sources
- Feline injection-site sarcoma, ABCD, accessed 2026-11-03.
- Medical Oncology: Feline Injection Site Sarcoma, North Carolina State University, accessed 2026-11-03.
- 2024 guidelines for the vaccination of dogs and cats, World Small Animal Veterinary Association, accessed 2026-11-03.
- Connective Tissue Tumours in Animals, Merck Veterinary Manual, accessed 2026-11-03.
- 2026 AAHA Oncology Guidelines: Section 1, Overview of Common Cancers, AAHA, accessed 2026-11-03.
- Care of Surgical Incisions in Cats, VCA Animal Hospitals, accessed 2026-11-03.
- 2023 AAFP/IAAHPC Feline Hospice and Palliative Care Guidelines, American Association of Feline Practitioners and IAAHPC, accessed 2026-11-03.
- Chronic Pain Assessment in Cats, American Animal Hospital Association, accessed 2026-11-03.
- Initial Triage and Resuscitation of Small Animal Emergency Patients, Merck Veterinary Manual, accessed 2026-11-03.