A lump on a cat is a cytology question before it is a surgery question. Fine needle aspiration (FNA) takes 60 seconds, uses a 22 to 25 gauge needle, and usually returns enough mast cells to make a decision; that slide decides whether the next step is a 2 cm excision, a staging ultrasound, or a 3 month recheck. Going straight to removal without the needle can mean paying for 2 surgeries instead of 1.
Three forms, three different workups
Cutaneous disease gives a firm, hairless nodule of 3 to 10 mm, most often on the head, neck, or flank, and some of them itch and ulcerate after 2 to 4 weeks of licking. The histiocytic form gives multiple 2 to 5 mm nodules in a cat under 2 years, spread over trunk and legs, and it looks like nothing until the slide says mast cell. Visceral and splenic disease gives no skin lesion at all: vomiting, weight loss, and a spleen that sits 2 to 4 cm past the rib cage. The form decides whether you start with a skin slide or an abdominal ultrasound.
The 60 second sample, done properly
Technique decides the yield more than the laboratory does. Skip suction: capillary pressure from 3 to 5 quick passes through the mass fills the needle hub, while suction ruptures cells and sprays granules across the slide. Air-dry 2 to 3 smears within 10 seconds, because mast cells degranulate on a wet slide and the granules then hide the nuclei. Ask for Giemsa or Diff-Quik, not a rapid stain read at 400x on the spot, since the metachromatic purple granules are the whole point. A mast cell measures 10 to 15 um with a round nucleus and 20 to 50 granules; eosinophils beside them are a normal escort, not a separate diagnosis.
Two limits matter before you trust a negative. A share of aspirates come back hemodilute or necrotic and get reported as non-diagnostic, which is a reason to repeat in 7 to 14 days, not to relax. And degranulation during sampling can make a field look like eosinophilic inflammation, so a report reading eosinophils only, from a 1 cm hairless nodule, deserves a 4 mm punch biopsy.
Reading the report: the fields that change the plan
- Cellularity: 3 to 5 well-spread fields at 400x is enough to call it; 1 field is not.
- Granulation: heavily granulated cells hide the nucleus; ask for a second slide at 1000x.
- Mitotic figures: counted per 10 high power fields, and the number belongs in the report.
- Nuclear size: a 2 to 3 fold spread in nuclear diameter between cells is worth a note.
- c-kit: an immunohistochemical stain, run on the excised block, not on the smear.
None of those 5 fields is a diagnosis on its own. Together they tell you whether a 6 mm nodule can be watched for 3 months or needs a 2 cm excision this month.
Staging: 6 fields, 1 visit
Staging answers 3 questions: how big, how many, and has it left the skin. Most of it fits into a single 90 minute appointment.
| Field | What it answers | Test and sample size | Number that matters |
|---|---|---|---|
| Primary lesion | How big is the index mass | Calipers, 3 dimensions in mm | Over 10 mm at first visit |
| Regional node | Has it reached the draining node | FNA, 22 gauge, 2 passes | Short axis over 5 mm |
| Abdomen | Liver or spleen involvement | Ultrasound at 5 to 7 MHz, 4 organs | Nodule over 5 mm |
| Chest | Lung spread before surgery | 2 view radiographs | Mass over 5 mm |
| Blood | Baseline and viral status | 1 mL in EDTA for count and chemistry | Eosinophils over 1.5 x 10^9/L |
| Buffy coat | Cells circulating in blood | 1 slide, 10 fields at 400x | 1 mast cell or more |
A normal sized node can still hold cells, so many clinicians aspirate the draining node anyway; a 4 mm node that reads positive changes the surgery from a wide excision to excision plus node removal. Splenic involvement is the field that most often surprises owners, because the skin lump was 5 mm and the cat had been vomiting for 3 weeks.
Margins: measure in mm, not in feelings
For a cutaneous mass the usual plan is a 2 cm lateral margin and 1 fascial plane deep, marked with a suture at 12 o clock before the first cut so the pathologist can orient the block. On a distal limb or an eyelid that margin may cost function, and then the sequence flips: 4 mm punch biopsy first, plan second. Photograph the site with a ruler taped to the skin, record the 3 dimensions in mm, and write the exact location in the chart, because a 6 month recheck is a comparison of numbers.
The recheck schedule that catches a regrowth
3 months for the first visit, then 6 and 12, then every 6 months. Each visit is weight, node measured in mm, the same photograph, and a question about vomiting or appetite. A nodule that grows 2 mm in 3 months is a different problem from one that has not changed in 12. A new lump elsewhere on the body gets its own FNA rather than being assumed to be the same disease.
Go now, not next month
A lump that doubles in 4 weeks, one that ulcerates and bleeds through a 5 mm defect, a mass on the eyelid or the nasal plane, vomiting with 5% weight loss, or a cat that stops grooming for 2 weeks all justify a visit inside 7 days. A 3 mm nodule that has not changed in 6 months can wait for a routine appointment. The distinction is growth rate and function, not size alone.
Not every lump is a mast cell tumor; a mammary mass in an intact queen follows a different workup, covered in mammary tumor signs, and the first visit looks identical from the outside. Surgical and staging costs vary by a factor of 3 between cities, so price the plan with cost estimates before you book.
Terms mentioned in this article
Frequently asked
Does a needle spread a mast cell tumor?
Needle tract seeding is reported but rare, and the trade is worth it: one 60 second aspirate can separate a mast cell tumor from a cyst, an abscess, or eosinophilic inflammation, which avoids an unnecessary 2 cm excision. Many surgeons still remove the needle tract with the wide margin when the mass turns out to be a mast cell tumor, so mention the aspiration site on the shaved skin before surgery.
What if the first aspirate comes back non-diagnostic?
Repeat in 7 to 14 days with a 22 gauge needle and 3 to 5 capillary passes, and ask for 2 to 3 air-dried smears rather than 1. If the second sample is still hemodilute or necrotic, move to a 4 mm punch biopsy, which gives a histology answer and a mitotic count that cytology cannot provide. A negative cytology never rules the disease out on its own.
How often should a removed lump be rechecked?
At 3 months, then 6 and 12, then every 6 months. Each visit records weight, the draining node in mm, and the same photograph with a ruler. A nodule that grows 2 mm in 3 months gets a fresh aspirate; one unchanged across 12 months stays on the routine schedule. New lumps elsewhere are sampled separately, because 2 mast cell tumors in one cat are 2 separate problems.