I'm so sorry you're going through this.
Maddison had fibrosarcoma in his hind left leg/hip area. He was only a very young dog (4) but wasn't a candidate for amputation due to the location of the tumour. We managed his pain and we took the risk of letting him run and play as he wanted to because he was so young and energetic.
One night, he changed position on the settee and he really hurt his leg. It didn't fracture but whatever he'd done, it really hurt him and we knew that the battle was over and we let him go to The Bridge that night.
With regard to your question about it spreading to other organs - in my very honest opinion, I don't think there will be time for that to occur. (I'm sorry to say that).