Can a tumor in the maxilliary sinus cavity be seen when performing an endoscopy?

No: When performing endoscopy, the scope is passed in the nasal cavity and one can define any lesion here. The nasal cavity is separated from maxilla, not by the septum but a party wall thru which a lesion must perforate to be accessible for biopsy. A scan of the head will reveal a lesion in maxilla which is not a common one. To expose one needs to open anterior wall of maxilla by raising a flap.