Limping Dog With Lumps On The Leg

This is a question from one of my friends, looking for insights and experiences:

Hello, I am hoping that someone can help shed some light on what may be going on with my dog. My dog is an (almost) 5 year old mastiff, on the large side at 220lbs. He is not overweight.

About 10 days ago I noticed him limping.

I thought maybe it was a minor "tweak" of something. His knees seemed fine, no clicking, decent movement, etc. An injury seemed sort of unlikely, because he had recently been neutered and became much lazier than before, so no running or jumping or playing with the other dogs. There also doesn't seem to be any inflammation.

I gave him pain meds, and waited to see if it would get better, and it hasn't. In fact, it appears worse as the days go by.

Today I noticed two weird lumps on his ankle area. 

In feeling them, the one is hard and immovable.. almost like it is just bone, and the one right next to it is able to be pushed in, as though whatever it is attached to is movable (but not the lump itself, if that makes sense).

They kind of feel like the knuckles on your hand.

They are on the inside ankle, around where a dewclaw would be.

I looked back at pictures I took of his ankles last week, looking for signs of swelling, and turns out that I somehow missed the lumps last week, as they are visible in the photos taken then.

My biggest concern is osteosarcoma, but the lumps don't SEEM sarcomaesque, in my semi-limited experience with it. I'm used to seeing larger singular swellings instead of two smaller lumps.

He is going to the vet tomorrow, but I am wondering if anyone has anything that jumps out at them?

Thanks in advance... this is how I cope with the wait to see the vet.

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