Is it not possible that the CT showed a mass that the Vet suspected could be cancer. The cat test negative for cancer via some blood test but still has the mass? Maybe the mass is non-cancerous but still life threatening (like by causing an obstruction) or is it possible to not rule out the cancer until you have an actual bioposy of the mass? I am no vet but have worked in the medical field for 22 years and know that with humans not all cancers are DX with a blood test, alot of them you have to actually do a bioposy of the tissue to make a 100% DX.