Yes, xrays are useful tools in diagnosing a lump or tumor. However, it can't end there, they will need to do
other tests such as a needle biopsy and/or a CAT
scan to really tell what's going on.
Luckily, you are obviously on top of your cats medical care. There are a variety of treatments available with these types of situations, just like with people. It's hard for me to speak on this specifically, since the only information I have is "tumor", not what kind, etc.