This type of cancer, at this stage, is not a good responder to chemo or radiation, sadly. By stage, it's obvious with a palpably large mass, she is in later stage neoplastic development, and likely has metastases, as abdominal masses spread their lil cancer cells everywhere. She needs palliative care, aka Pain Management, in the form of buprenorphine, to administer in her cheek pouch or under her tongue 2 to 3 times daily, and possibly compounded gabapentin suspension you can give orally 3 times daily, both for Pain. She is in pain. Without a doubt.That's what I keep thinking...my old teacher went through chemo for lung cancer and it was so bad, she chose death when the cancer came backnot that I could afford chemo for Blackie if it came to that...so unfair...
I treat renal cats and cancer cats every day. Blackie is no exception. You don't need to afford chemo. You need to ask your vet to fill a prescription for buprenorphine (aka Buprenex) syringes, or a volumetric bottle from which you can pull volume needed from adapter tip, and call in a prescription for compounded gabapentin to RoadRunner Pharmacy, 877-518-4589, https://www.roadrunnerpharmacy.com/vets/about-us/ or Veterinary Pharmacies of America, 877.838.7979, http://www.vetrxrx.com/vparx/indexer.aspx. I prefer VPA as they'll fill the most volume for the minimum price, which was $43.00 minimum per compounded med. The suspension is easiest for gaba, but they make chews, dissolvable tongue strips, exact dosed tabs/caps, transdermal gels, pastes, otic meds, etc.
You are Blackie's palliative care advocate. That your vet's "face fell" and she wants a second opinion to verify her "suspicions" are moot, but she has to cover her ass, so to speak; request the pain meds anyway, without another exam, as money is best vested in pain therapy, and potentially an ultrasound to confirm, not just another palpating of the abdomen. That doesn't tell them anything except something big and weird is in the belly.
She needs to be fed whatever she will eat at this point. Any groceries are good groceries. Hold back on gross meds that will discourage appetite due to bitterness, like sucralfate (Carafate), metronidazole (Flagyl), no tramadol (Ultracet), and antihistamines. Food, pain meds, and B vitamin/iron/potassium supplements are your prioritized oral meds right now. Even the binders need to wait on the back burner, as they test horribly, and will reduce palatability of any food she does show interest in.
Good Luck with Blackie, and, please, keep us posted as to her progression, be it negative or positive. Euthanasia is not a wrong answer here. Keeping her comfortable until that point is your only option; no pain meds is not an option.