Would I need surgery to determine if a mass that is in my knee is a tumor or just a calcified lesion?

I am 20 year old female with no prior injuries. A marble sized mass was recently discovered in my knee, I have noticed it before because it will sometimes move around and get trapped behind my knee cap, making it so I cannot bend my knee until I move it again. My ortho ordered an MRI to get a better look at it, he briefly explained that it was growing but he was not sure what it was. He mentioned the possibility of it being a tumor (also mentioned cancer) or a calcified lesion. Will he be able to determine what this mass is by looking at the MRI results or will I possibly have to have knee surgery to have it removed to determine what it actually is? I am just a little freaked out that he mentioned cancer, and I have a month before I am able to see him again. Any answers or anyone's own experiences would be greatly appreciated!