You have my sympathy, tough position to be in. The hard part is that your girl is only 5 years young.
Had an old vet that I really trusted diagnose one of my previous dog with cancerous large mass cell tumors. His advise was to leave them alone. He explained that once you disturb them that they spread and grow faster.
Please keep in mind that this was an older dog and many years ago. There is probably advancements in managing them since then.