I'm so sorry to hear your old girl has a tumor...I'm afraid I can relate to your predicament. We had a wonderful buff Orp who of course was my favorite 'cause she followed me around. She was just over a year old when I discovered a tumor, but it was at the top of her leg, near the joint. It didn't bother her at first, but after while she began to pick at it--I tried to keep it clean but it must have irritated her quite a bit because she continued to peck at herself until it was quite nasty. The other girls didn't bother her, she continued to eat and act herself, until one Sunday morning when I went out before church, she was laying on the floor of the coop, I held her as she died. I know I should have put her out of her misery--I just kept hoping she'd improve. Selfish, I know, but true. I sometimes forget that God made creatures so they don't focus on their pain, discomfort...they aren't going to gripe like we do...
Best of luck with your girl.