Grade Characteristics I Origin of the osteoma posterior and inferior in the frontal recess; localization of osteoma medial to a virtual sagittal plane passing through the orbital lamina; Anteroposterior (AP) diameter of the tumour is < 75% of the AP diameter of the frontal sinus II As grade I; AP diameter of the tumour> 75% of the AP diameter of the frontal sinus. III Origin of the osteoma anterior and/or superior in the frontal sinus and/or osteoma extending lateral to a virtual sagittal plane passing through the orbital lamina. IV Osteoma fills the entire frontal sinus.

Table 1: Grading of osteomas as devised by Chiu et al. [15].