Swollen? Mass? Growth? in mammary region of Rottie

[LMR83]

First, I just want to say that I am extremely careful at checking over my dogs on a regular basis… so careful that my vet jokes when I first arrive at the office as to whether a lump/bump is a legitimate lump or bump, or something that only I would find.

Second, today is Sunday, and the item of concern was noticed last night. I have an appointment with one of her vets on Tuesday (for a post-surgery check-up), though I will be calling tomorrow morning to see if I can get in earlier.

My girl, Missy, is approximately 9 years old. She is a rescued Rottie, she was spayed at the age of 3 or 4 when she was rescued. She was exposed to some nasty chemicals (as she was a Hurricane Katrina survivor). Since then she has had a spleenectomy, a couple of minor spots removed (benign), one small melanoma removed from her back thigh (clear margins, indicated as little concern), some pinching of her spinal cord in her neck (that has gone away), and recently an ACL surgery (not TPLO). She has always been in extremely good shape (almost thin for a typical Rottie, but appropriate for her size... she has gained a little weight since ACL surgery, but is only a bit heavy, not overweight).

On Saturday night I noticed that she has a strange spot on her belly. It is on the opposite side from her ACL surgery (which was 3 months ago). The strange spot is along her left side, very close to nipple 4 & 5. It almost feels like she has fluid starting an inch or so behind nipple 5, a bit "thicker" around nipple 5, and then thinning again to a more fluid like consistency to nipple 4. There are no marks on her skin. The "fluid" beneath her skin is not really as thin as fluid, but not as thick or strong as muscle, or a typical mass. The most dense portion (around nipple 5) is about half an 1-2 cm thick, though not at all defined. Given her age and breed my first concern was a mammary tumor. Most of what I have seen about mammary tumors indicates that they are relatively formed and even the soft tumors are more clearly formed and identifiable on palpation.

I realize I am not a vet, and I am not asking for a diagnosis. Has anyone had experience with a mammary tumor that presented like this (very soft, not liquid, but very soft… and very undefined margins), or something related to the lymph system? Or possibly I am being paranoid and she is just getting a bit heavier and has a fat deposit, due to less activity after surgery (though it is only on one side)?

As a side note, the skin moves freely around it, I cannot tell fully if it is connected to anything below.

Any experience or shared advice would be much appreciated.

Thank you!