[Megora]

It is a tumor. Look up histiocytoma - which is basically a benign tumor. There's scarier tumors out there, but I'd leave that to your vet to discuss with you and rule out. As well, there are a lot of warty growths which middle aged to old dogs get - which owners keep an eye on, but don't need to do anything about unless they get irritated/infected and are bothering the dog.

I'm a little concerned looking at the pictures, because it's unclear how big it is. First reaction was basically holy crow that's huge.

I suggest you feel around the area - can you move the skin around fairly easily?

If you can and it's fairly clear that the tumor is not deeper than the top layer or so of skin, that is probably a good thing. It is more likely to be benign... but also easier to remove it needs to go. But get it checked by your vet.

 

[Megora]

If it seems rooted/attached, be prepared to remove it.

 

[Megora]

Yes, it’s definitely rooted and attached. In your experience is that cancer? Or is there a possibility it’s not .

I very vaguely remember one of my dogs had a growth on his chest which had been biopsied because the vet said it felt more hard and fixed than the typical fatty tumor. That turned out to be benign - so there is some hope yet, but you absolutely need to get this checked out.

Basically you are looking at the vet first doing an aspiration to check for cancer cells. If they find them, then you are looking at the vet doing chest xrays and bloodwork to see if the cancer has spread.

Based on appearance, size, feel, etc... of the tumor - your vet may recommend removing the tumor (where Robin above says remove with wide margins to ensure that all of it is removed - and this is thankfully possible when removing from the dog's back or side where there is typically plenty of skin). But to reiterate, your vet will likely not take this step if dog does not clear xrays and other tests which show that the cancer has spread.

But before you get too far ahead, there is a likelihood that this may be a benign tumor based on the aspiration/needle test. And then if it's cancer, then there is a chance that it has not spread yet. Take it one step at a time.

I only experienced a possible cancer with one dog who had a very bad lesion on one of his toes. This combined with lesions on his nose had our vet strongly discouraging any treatment because she felt it was likely an aggressive type of cancer which had already spread. My personal take four years later is wishing we'd at least done the biopsy, because there is a possibility that was not cancer but a serious fungal infection which I lost my other dog (his buddy) to almost two years later.

I've seen fatty tumors and other growths on dogs - but I will say that I've never had one that did not pass the "wiggle" test. Other than the one mentioned above.