We showed our vet the lump. She seemed calm… she always seems calm even when she is being bitten or covered in cat poo, as is usually the case with Castiel. He always pees and poos when he is nervous — a real nightmare at vet visits. Our vet’s office takes these nervous movements in stride, even when he smells up the whole office so bad that we’re left gagging. No amount of fasting solves the problem, we’re convinced he saves up poo for car rides and vet visits. We come prepared with pads and cat wipes. We often joke that we should get our vet a full raincoat that she can reserve for his visits. Our vet is super nice and gentle — and calm. And she was calm while she felt the lump.
She mentioned that lumps could be lots of things. I think she was hoping it was something else too. We clung to that hope — even though, I didn’t think the result would be good. She did a Fine Needle Aspiration. She took X-rays of his leg and thoracic area (lungs). The X-rays didn’t show anything, just a little arthritis in that leg. We had to wait three days for the FNA. They were painstaking. We wanted it to be a lipoma or a cyst but it wasn’t rubbery and it didn’t hurt but still we wanted to believe it wasn’t malignant.
Three days later, we received a call from our vet with the results — it showed inflammatory cells and spindle cell proliferation. The report said there was “concern for sarcoma” but more testing was needed. The pathologist couldn’t say for certain. We clung to hope. Spindle cells don’t necessarily mean cancer — but they often do — they often mean some kind of sarcoma.
Our vet suggested we do a surgery to remove it. We scheduled the surgery for the first available date –early the following week. We wanted to test sooner but there were no appointments. We debated if we should have the surgery at a specialty vet hospital but our vet was confident she could do the surgery. I wonder if maybe she thought it might be something else too. We still had hope it might be something else. In retrospect, we probably should have gone to the specialty hospital when we suspected sarcoma, but we didn’t know it was sarcoma and we were still in diagnostic mode.
In less than a week, the lump grew. Cancer means rapidly dividing cells. Benign things don’t grow that fast, I told myself. 
Oh yikes. When you know too much, it’s not always a good thing. But you know what? I hope you don’t have any regrets about the way things worked out. You did the best you could with the information you had at the time. And your vet sounds wonderful! I would have taken the same path myself.