Optic nerve motion and gaze direction: Their impact on intraorbital tumor radiotherapy.

Management of inter- and intra-fraction movements of target volumes and organs at risk (OARs) during radiotherapy is essential. While there is little OAR or target volume movement, the movements and orientation of the eyes can be significant during radiotherapy and they can affect the position of the optic nerve. The objective of the present study was to assess the variations of the optic nerve position due to gaze direction and to discuss their clinical consequences on the radiation treatment of intraorbital tumors. Three patients without a history of oculomotor nerve palsy underwent six CT acquisitions with a thermoplastic mask: eyes open with different gaze directions (straight ahead, left, right, up, down) and eyes closed. The acquisition with the straight-ahead gaze was chosen as the reference position. Left and right optic nerves were segmented on the six acquisitions, and total volumes and maximum amplitude motions were calculated in three dimensions. Maximum differences were observed while looking left and up, with a median maximum amplitude of 5 and 6mm [range: 2-7mm], respectively. These motions induced a position variation of more than 50% of the volume of the optic nerve (compared to the reference position). Greater variations of motion were observed for the anterior portion of the nerve. The gaze position with the fewest variations compared to the reference position was eyes closed. Optic nerve positions vary significantly due to the gaze direction, especially for the anterior portion of the nerve. These variations should be taken into account for the treatment of small intraorbital tumors involving the anterior third of the optic nerve.

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