Found a Lump? Why Your Vet Should Put a Needle in It
Most lumps on dogs are harmless, but nobody can tell which by touch. Why a needle sample beats waiting, what it can miss, and when to go back.
Written by thecanineledger
A veterinary cancer specialist wrote about missing a tumour in a dog she knew well. The dog had had more than ten lumps sampled over the years, all harmless fatty lumps. The next one wasn’t. Her conclusion: “Even an experienced cancer specialist like me cannot look at or feel a mass and know what it is. I am not a microscope” (Ettinger, dvm360, 2015).
While most lumps on dogs turn out to be harmless, the only way to know is to look at cells from the lump. This page explains why, how it’s done, and what it can and can’t tell you.
1. Most lumps are harmless. Some aren’t.
The most common lump in dogs is a lipoma, a soft lump of fat under the skin. In UK practice records, lipomas were diagnosed in nearly 2 in every 100 dogs in a single year, and in more than 5% of Weimaraners, Dobermanns, German Pointers, Springer Spaniels and Labradors (O’Neill et al., Canine Genet Epidemiol 2018).
But a lump that feels like a lipoma can be something else. Mast cell tumours, one of the most common skin cancers in dogs, can look and feel like almost anything. In English practice records they were diagnosed in 0.27% of dogs, with the highest rates in Boxers and Golden Retrievers (Shoop et al., Canine Genet Epidemiol 2015). → Mast Cell Tumours: What the Grade Means
Our note: That mast cell tumours “can look and feel like almost anything” is from general knowledge, not a source we quote.
2. “Keep an eye on it” isn’t a plan
Owners are often told to watch a lump. Ettinger points out the problem: “Keep an eye on it for how long? How much can it grow before we should do something?”
Her campaign, called “See Something, Do Something”, gives a simple rule:
“See Something-If a dog or cat has a mass that is the size of a pea (1 cm) and it has been there one month …”
“Do Something-Go to a veterinarian and get it aspirated or biopsied.”
The reason for acting early is surgery. When a lump is cancer, a small one is easier to remove completely. In Ettinger’s words, “what could have been removed with a simple surgery may now require a bigger surgery and sometimes more treatment such as radiation or chemotherapy afterward.”
What this rule is. It is expert advice from a board-certified veterinary oncologist, published in a veterinary trade magazine, not a trial. We didn’t find a study testing whether following it changes outcomes.
Our note: “Pea-sized and present for a month” is a common rule of thumb. Any new lump in an older dog, or any lump that changes quickly, is worth showing your vet sooner.
3. What a needle sample is
A fine-needle aspirate (FNA) is a quick test: the vet pushes a thin needle into the lump, collects a few cells, and spreads them on a slide to look at under a microscope. Most dogs don’t need sedation. Some results come the same day; others go to a laboratory.
How reliable is it? A study of 243 usable samples from lumps on dogs and cats compared needle results with the full tissue result. Overall, “the cytologic diagnosis was in agreement with the histopathologic diagnosis in 90.9% (221/243) of cases” (Ghisleni et al., Vet Clin Pathol 2006).
4. What a needle sample can miss
The same study shows the limits.
Some samples don’t work. Of 292 samples, 49 were excluded “due to poor cellularity of the cytologic specimen”: there weren’t enough cells to read.
A “not cancer” result is less certain than a “cancer” result. When the needle sample said cancer, it was almost always right: “a positive predictive value of 99.4%”. When it said not cancer, it was right less often: “a negative predictive value of 68.7%.” In that study, 21 of 67 lumps called non-cancerous by the needle turned out to be cancer.
So if a needle sample doesn’t give a clear answer, or the lump keeps growing, the next step is usually a biopsy: taking a piece of tissue for a full examination.
Our note: The 68.7% figure comes from one 2006 study. The abstract doesn’t say where its samples came from, so the figure may differ in general practice.
5. What to do
- Measure the lump and note the date. A photo next to a coin or ruler helps.
- Have it sampled if it is pea-sized or larger and has been there a month, or sooner if it is growing, changing, bleeding or bothering your dog.
- Ask what the result means, and what would make your vet want a biopsy.
- Keep a map of your dog’s lumps, so a new one stands out.
- Don’t assume a new lump is the same as the old ones.
Our note: The “what to do” list is ours. That most dogs don’t need sedation for a needle sample, and that some results come the same day, is from general knowledge.
6. What we could not verify
| Commonly said | What we found |
|---|---|
| “Soft, movable lumps are always fatty lumps” | Not safe to assume. An oncologist says no one can tell by feel. |
| “Sampling a lump makes cancer spread” | Not found in our sources. Ask your vet. |
| “Older dogs just get lumps” | True that lumps are common (lipomas in about 2% of UK dogs a year). Each new one still needs checking. |
| That early sampling improves survival | Not tested. The 1 cm, one-month rule is expert guidance, not trial evidence. |
Sources
- Ettinger S. See Something, Do Something: Why wait? Aspirate! (dvm360, 8 June 2015; trade magazine, expert opinion by a DACVIM (Oncology), not peer-reviewed; read in full)
- O’Neill DG, Corah CH, Church DB, Brodbelt DC, Rutherford L. Lipoma in dogs under primary veterinary care in the UK: prevalence and breed associations (Canine Genet Epidemiol 2018; abstract read)
- Shoop SJ, Marlow S, Church DB, et al. Prevalence and risk factors for mast cell tumours in dogs in England (Canine Genet Epidemiol 2015; abstract read)
- Ghisleni G, Roccabianca P, Ceruti R, et al. Correlation between fine-needle aspiration cytology and histopathology in the evaluation of cutaneous and subcutaneous masses from dogs and cats (Vet Clin Pathol 2006; abstract read)
Always confirm with your vet before acting on anything on this page.