Three numbers carry most of the injection-site question: 3 standard injection sites, a hand check every 3 months after any vaccine, and a 2 cm line that turns a lump from wait-and-see into a surgical referral. A small swelling in the first 7 days is ordinary. A firm lump still there at 90 days is not, and the difference is decided with a ruler and a calendar, not a guess.
What a sarcoma at the injection site is
Feline injection-site sarcoma (FISS) is a soft-tissue tumor that starts where a needle went in, most often 2-10 months after the injection and sometimes 3 years later. It is uncommon: clinic tracking programs launched in the mid-1990s put the risk at roughly 1 in 1,000 to 1 in 10,000 injections, and adjuvant (aluminum-containing) rabies and FeLV vaccines were the ones most often named. Vaccine brands and protocols have changed a lot since then, and most vets now see far fewer of these than the 1990s figures suggest.
It is locally aggressive, spreading along tissue planes, so a 2 cm shell of normal tissue has to come out with it, and the window for a good outcome is early: a lump removed at 2 cm with clean margins is a different problem from one removed at 5 cm after 8 months of watching. Only a biopsy diagnoses it.
The 3 injection sites, and why placement matters
Older practice put everything in the scruff, the worst spot for a lump: a mass there cannot come out with wide margins without a large area of skin and muscle. Current guidance spreads injections across 3 regions so a future lump can be traced to one product and removed with a limb.
The 3 sites in common use: the right hind limb, below the knee; the left hind limb, below the knee; and the right front limb, as far down as the elbow allows. Rabies goes right hind, FeLV goes left hind, and the 3-year core vaccines (FVRCP) go right front. Each product keeps its own site for life, so a lump on the right rear leg points at rabies and nothing else. If your clinic still injects in the scruff, ask them to record the exact spot on a body map: left or right, and how far below the spine.
Most vets use a 22-25 gauge needle, 1 inch long, and a 1 mL dose. Record product, lot number, date, site and gauge: that line is what makes a 6-month comparison possible.
The 2 cm rule, measured properly
The threshold that matters is 2 cm, and it is a diameter, not a feel. Measure the longest dimension and the one at 90 degrees to it, in centimeters, with a cheap plastic ruler or fabric tape, and write both numbers with the date. Do not measure by eye over fur; a lump that feels like a pea is often 1.5 cm once the fur is parted, and a 2 cm lump can hide inside a fluffy flank for months.
Four findings push a lump past the 2 cm rule into a surgical work-up, and any 1 of them is enough. It is 2 cm or larger at any point after the 3-month mark. It is still palpable 3 months after the injection. It is growing at any recheck after day 30. It is fixed to the muscle underneath, or the skin over it is ulcerated. In those cases the usual next step is a fine-needle aspirate plus imaging, then excision with 2-3 cm margins and deep fascia, planned by a surgeon who does this often. A needle biopsy alone can miss the diagnosis, because the center of these tumors is not uniform.
A hand check every 3 months
Start at 3 months after the last injection, then every 3 months for 12 months: 4 checks in the first year. Run fingertips in slow 1 cm steps over 5 zones — scruff, both shoulders, both flanks, down each hind leg to the knee. Press with 2 fingers, not 1, and compare left with right, the fastest filter you have. Ten seconds per zone; repetition matters more than pressure.
Log 3 numbers, not adjectives: size in cm, date, site. Photograph the lump beside a ruler every 30 days. Owners under-estimate growth over 3 months: 0.3 cm a month looks like nothing on the day and like 1 cm on the record.
| What you find | Size | Days after injection | Action |
|---|---|---|---|
| Soft, warm swelling | under 1 cm | 1-7 | note date and site, recheck at day 7 |
| Firm lump, unchanged | 0.5-1.9 cm | 30 | measure, photograph, recheck at 3 months |
| Firm lump still present | any size | 90 | vet visit within 2 weeks, aspirate |
| Lump 2 cm or larger | 2 cm and up | 90 or later | surgical referral within 14 days |
| Lump growing | any growth | 30 and up | book within 7 days |
| Ulcerated or draining | any size | 1 and up | same-week appointment |
When a lump needs a vet today
Same-day calls: a lump that appeared within 24 hours with facial swelling, vomiting or trouble breathing (that is a reaction, not a sarcoma; see the 24-hour reaction watch); a lump that is hot, painful and leaking fluid, which is more often an abscess; a fever above 39.5 C; or a cat that stops eating for 24 hours after a vaccine. Rapid growth of 1 cm in 2 weeks, or a lump that breaks open, needs an appointment in days, not months. If a previous lump was removed and the area swells again, say so at booking: recurrence at the same site changes the plan.
Cutting the number of injections instead
The other lever is needle count. Antibody titres can show core protection against panleukopenia and calicivirus is still high at 3 years, which lets some cats skip a booster; a vet-run titre check decides that, not a guess. A 3-year rabies product means 1 injection instead of 3. A cat that swelled after a previous dose gets a different product or site next time, and any swelling is recorded before the next appointment. If several products fall on one day, ask whether they can be split across 2 visits 3-4 weeks apart.
Five steps that fit on one card
- Record product, lot, date, site and gauge; keep the card with the certificate.
- Part the fur and measure with a ruler at 7 days, 30 days and 3 months; write size in cm and the date.
- Hand-check 5 zones every 3 months for 12 months, in 1 cm steps, left against right.
- Treat 2 cm, growth after day 30, or a lump at 3 months as a surgical question, not a watching brief.
- Ask for titre-based decisions and 3-year products: 0-1 injection in 12 months, not 2-3.
Two habits carry it: a ruler next to the vaccine card, and a date in the calendar 90 days out.
Terms mentioned in this article
Frequently asked
How big is too big for a lump after a vaccine?
The line is 2 cm across, measured with a ruler on the longest axis, and timing matters as much as size. A soft swelling under 1 cm in the first 7 days is ordinary. A firm lump that is 2 cm or larger at any point after the 3-month mark, or any lump still palpable 3 months after the injection, needs a vet visit and usually a fine-needle aspirate within 14 days rather than another 3 months of watching.
How often should I check the injection site at home?
Every 3 months for 12 months after the last injection, which is 4 checks in the first year, plus a look at day 7 and day 30. Sweep 5 zones with 2 fingers in 1 cm steps: scruff, both shoulders, both flanks, and down each hind leg to the knee. Write down size in cm with the date, and photograph the lump beside a ruler every 30 days so growth is a number instead of a memory.
Should I skip vaccines to avoid a sarcoma?
Do not skip on your own; change the plan instead. Ask about antibody titres for the core panleukopenia and calicivirus components, which can justify skipping a booster in some adult cats, and about 3-year rabies products that turn 3 injections into 1. Record every product, lot and site, split multiple products across 2 visits 3-4 weeks apart, and mention any previous swelling before the next appointment.