The Best Way to Diagnose Skin Tumors on a Dog

If you have a dog, the odds are that he will develop at least one skin mass (tumor) in his lifetime. How do you know if you should worry about it or not?

Most people will ask their vet to look at it, their vet will do so (they might feel it as well), and unless it has "aggressive" characteristics (warm to the touch, hard, fast growing, ulcerated or bleeding), they often just recommend monitoring.

Unfortunately, monitoring a mass when you don't have a diagnosis isn't good advice.

They're hoping that it's just a lipoma - a common benign tumor of fat that feels soft. The problem is that many cancerous tumors can feel EXACTLY like lipomas. If you elect to "monitor" a tumor that's cancer and return to have your vet look at it in a month, two things could have happened during that time.

(1) It might have grown so large that it is no longer amenable to surgery at all.

(2) It might have grown to the point that surgery will remove most (not all) of the cancer cells, and now radiation or electrochemotherapy are needed after surgery to kill the cells left behind in the skin after surgery (and prevent tumor recurrence).

(3) It might have spread to other organs during the month of monitoring, changing your dog's prognosis.

Unfortunately, I see this happen all the time. I don't quite understand the reluctance to sample skin tumors. It's usually a very easy process.

We typically measure the skin mass with a plastic caliper ($3 at Home Depot), describe the size, location and description of the mass in the medical record or on a body map (a drawing of a dog used to keep track of skin masses), and add the diagnosis when it's available.

Most skin tumors can be diagnosed with a simple needle aspirate. This involves just sticking a small needle into the mass a few times and extracting cells. One of our nurses scratches the patient while I'm taking the aspirate, so they don't even realize it's happening. The cells are smeared on a glass slide, 5-6 slides are made. I usually stain one slide and examine it under the microscope to make sure I have a good sample and to see if I can easily make the diagnosis. This whole process literally takes just 5 minutes (sometimes less).

If the mass is a lipoma, the fat looks like grease on a slide and doesn't even need to be stained. If the mass is anything but a mast cell tumor or lipoma, I usually send it to the pathology lab, for confirmation of a diagnosis.

If the mass is in a more challenging location or is painful (anal sac, bone), or if your dog has 20 masses that need to be evaluated, the process will take longer and involve sedation so your dog is more comfortable.

Final Recommendations:

If your dog has a lump on his skin, I would strongly advocate for an aspirate. Don't let anyone tell you "it's just a lipoma" when they haven't sampled it (our eyes are not microscopes). Some mast cell tumors look EXACTLY like lipomas, and these are cancerous tumors - the ONLY way to differentiate between the two is with an aspirate. This can be done VERY quickly and an aspirate is a very simple and non-invasive procedure.

I've seen too many clients come in and tell me that they were monitoring a mass for the last 6 months...and now it is too large for surgery, has spread to other organs, and there's not a whole lot I can offer their dog. Not to sound dramatic, but this can honestly make the difference between situations where a patient can be cured of a tumor vs a patient will die from a tumor. It can be very sad.

On the flip side, I've also had a client paying her regular vet for monthly monitoring of a skin mass for 8 months. She came to see me as a new consult for another issue, I suggested sampling the skin mass, which was just a benign cyst - I didn't even need to send it out. She had probably spent $600 monitoring the cyst over 8 months, which didn't even need to be monitored at all. If it was aspirated from the beginning (we charge $70 for this) her dog would have had 7 fewer vet visits and she would have saved over $500.

A few other things to keep in mind:

(1) If you are told a mass is a lipoma, but it is growing rapidly, have a second aspirate done. In some cases, a cancerous tumor can be surrounded by fat, resulting in a misdiagnosis.

(2) If the first aspirate is non-diagnostic, it's possible that either a different technique needs to be used for a successful aspirate (for example, sarcomas often require suction on a syringe during the aspirate) or a biopsy needs to be performed. A biopsy involves submitting a piece of the tissue to the lab, instead of just cells smeared on a slide. If your vet isn't comfortable with this, they might refer you to an oncologist.

If your general practitioner isn't comfortable taking aspirates on your dog's lumps and bumps (or insists that they are all lipomas without taking a sample), then just get a second opinion (go to another vet or an oncologist). Your dog's masses certainly might all be lipomas, but wouldn't you feel better if you knew for sure?

Dr. Lori Cesario

Board Certified Veterinary Oncologist

PS: I'm happy to now offer online oncology consultations. Learn more about how this service can help by visiting the Vet Cancer Consultants site.

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