Accessory Sinuses

T-C31 denotes the accessory sinuses, the four paired paranasal cavities - maxillary, frontal, ethmoid and sphenoid - that lie adjacent to the nasal cavity and orbit. In the ICD-O-3 they serve as the topography for neoplasms arising in any of these sinuses. This code is used to record the primary site of cancers of the accessory sinuses.