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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
I'm looking through the pictures and working on a response. It shouldn't take more than a few minutes.
This is one of two things- an anal gland abscess or an anal gland tumor (anal sac gland adenocarcinoma).
I would lean more toward an anal gland tumor. The reason is because of the size and location. An anal gland abscess will usually be smaller and eventually rupture through the skin. When they get this big without rupturing, I think of a tumor as the most likely cause.
The diagnosis is made on rectal exam. They have a firm feel when a tumor or a fluid feel when an abscess. The treatment is surgical removal of a tumor and biopsy. This will tell you if it's malignant or benign. It also tells you the biologic behavior and how aggressive it is. If it's an abscess, we surgically drain and flush it and that treatment is curative.
I would see your vet in the next 1-2 days to get this diagnosed and treated.
Here's a nice write up on these tumors in dogs:
I hope this helps, let me know if you have any other questions.