I just noticed this post and thought I would comment.
I am not a doctor, but I do know that polyps in the sinuses are very common. I am sure they can happen in the maxillary as well as the other cavities. There are also benign cysts. Cancer in the cavities is very, very rare.
Or, as the doctor indicated, it may just be a big infection in there. Treatment would be antibiotics, nasal irrigation using a machine, and follow this up by a nasal steroid to reduce the polyp (if there is one).
Let us know how it goes