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Mast Cell tumor?

Species: Dog
Breed: Lab Mix
Age: 2-5 years
This past Saturday afternoon I was petting my dog and I noticed a small bump under her left front leg ("armpit" area). I looked at it and it was somewhat red - it looked like a bug bite that had been scratched until it bled a little bit. I put Zymox topical cream (without hydrocortisone) on it, and when I checked it again in the evening it had calmed down considerably.

I felt around for the bump on Sunday, but didn't find it, and didn't spend a lot of time investigating it (I didn't think much of it). Then, yesterday morning, I felt it and it was the size of a small marble. It was quite red - now it looked like an angry, big bug bite.

I know it is not an actual bug bite because it's January and we live in Massachusetts. I panicked and we went to the vet and had it aspirated. The vet we saw was not my normal vet because she wasn't in yesterday.

We are now waiting for the cytology results and I'm really scared. The vet is sure it's a mast cell tumor. It bled a lot when he aspirated it and was very swollen last night after the aspiration, but this morning it had calmed down considerably. It is not really red anymore (light pink), and is scabbing over

It is a little less than 1 cm in diameter and does not feel like it is attached to anything beneath the skin. It moves with the skin. I have emailed two pictures, but it's not easy to get a good shot of it because it's in such an awkward place.

Is this all consistent with mast cell tumor? Could it be anything else? The vet we saw said he couldn't think of any other possibilities. I know we will just have to wait for the cytology results to come back and go from there, but I feel like it is impossible for me to sit around doing nothing. She is still her playful, adorable self. She still has her appetite, and it just breaks my heart to think that she will have to go through something so awful.

Also, if it is a mast cell tumor, can you give me an idea of what will happen? What is the likelihood that it has already spread? How likely is it to be very malignant? (I read online that tumors in the axillary region tend to be more aggressive than others - is this true?).

Thank you for your time. I"m really scared :(


Online vet, Dr. Marie

Dr. Marie replied:

Thanks for sending me photos of this lump. I've included one here:

mast cell tumor

There definitely is a chance that this is a mast cell tumor. I have seen a number of labs of this age have mast cell tumors.

There are other possibilities. It could be an inflamed cyst or small wound of some sort. It could even be a type of lump called a histiocytoma. A histiocytoma is a lump that is benign and just goes away on its own.

If this is a mast cell tumor then there are different grades. The vast majority of mast cell tumors that I see are grade 1. What this means is that once surgery is done to remove the tumor then that's it...it should not come back or spread or cause any other problems. Surgery is usually not a huge deal although it will require a general anesthetic.

There are some mast cell tumors that can be more serious though - either grade 2 or 3. The fine needle biopsy that has been done will tell us if this is a mast cell tumor but it likely won't tell us if it is grade 1, 2 or 3. So, when the tumor is removed the vet will send it away and the pathologist will be able to grade it.

I have had some cases where a mast cell tumor was a grade 2 or 3 and I have had some cases where there has been spread to other organs. But the vast majority of mast cell tumors that I have seen have been grade 1 and have been cured by surgery.

I hope this answers your questions but please let me know if have more questions.

Dr. Marie.


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Disclaimer: Although Dr. Marie is a qualified veterinarian, the information found on this site is not meant to replace the advice of your own veterinarian. AskAVetQuestion.com and Dr. Marie do not accept any responsibility for any loss, damage, injury, death, or disease which may arise from reliance on information contained on this site. Do not use information found on this site for diagnosing or treating your pet. Anything you read here is for information only.

Customer reply:

Thank you for this information. The vet just called me and said that the cytology did not show any mast cells, but there was a lot of inflammation.

He said the report does not rule out the possibility of a neoplasia, but he recommends a week of antibiotics to see if it responds. If it doesn't, they want to remove it and send it for histopathology.


Does this seem appropriate to you? I will be able to pick up a copy of the pathology report this afternoon when I go to get the antibiotics, but I wanted to make sure this is not too conservative of an approach. I absolutely do not want to anesthetize my dog unnecessarily, but I also don't want to "wait and see" on a potentially aggressive mass. I would really appreciate your input.

Is it possible that the mast cells would not have shown up on the slides, or is that less likely?

Also, I have been cleaning it and putting neosporin on it and it has been getting smaller (slowly). This morning a little bit of pus came out of it. I don't know if this indicates anything.


Online vet, Dr. Marie

Dr. Marie replied:

Well this is great news that no mast cells were seen on the cytology! A mast cell tumor is very easy to diagnose with cytology. If the pathologist doesn't feel that it is a mast cell tumor then it is extremely unlikely to be one.

If you're seeing some pus then this also confirms that antibiotics could be helpful.

It sounds like your vets are doing a great job!

Dr. Marie.



Customer reply:

Thank you for your reply. I am very glad to know that mast cell tumors are easy to diagnose with cytology, because I always worry there could be some sort of mistake.

I'm picking up the antibiotics after work, so hopefully it's just an infection and will resolve quickly.

Thanks again!!


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Dr. MarieDr. Marie is a veterinarian who practices in a busy animal hospital in Ottawa, Ontario. She created Ask A Vet Question as a resource for good, accurate veterinary advice online. Dr. Marie treats dogs, cats, hamsters, guinea pigs, and rats. She has been a vet since 1999.

Is an online vet visit just as good as a trip to your veterinarian? No! But, many times, asking an online veterinarian a question can help save you money. While Dr. Marie can't officially diagnose your pet or prescribe medications, she can often advise you on whether a vet visit is necessary. You can also ask Dr. Marie for a second opinion on your pet's condition.