Purpose: Radiation induced optic neuropathy or loss of vision is associated with high radiation exposure to the anterior optic track, including optic nerves (ON) during radiotherapy. Existing studies reported that the ON moves along with eye movement. In this study, using MRI, we investigate the pattern of the movement of ON in response to globe movement, and evaluate the effect of this movement on the dose distribution of a representative treatment plan.
Methods: Three healthy volunteers (S1-3, Male, age: 28-33) underwent multi-slice T2-weighted FLAIR-HASTE imaging acquisition (duration: 74 seconds, resolution: 0.7x0.7x1.3 mmÂ³, TE/TR=90/3060ms). All subjects were requested to gaze toward three directions during the time of MRI acquisitions, including straight ahead, left and right. A whole-body 3T MR scanner (Siemens Magnetom Prisma, Erlangen, Germany) and 64-channel head coil was used for imaging. The motion of the ON was captured and the corresponding radiation dose with different ON locations was evaluated.
Results: The movement of ON followed a consistent pattern. The largest displacement is at the head (attached to the eye bulb) ranging from 8.1 to 11.1 mm for the left ON, and 7.8 to 11.3 mm for the right ON. While the tail (attached to optic chiasm) has negligible displacement. Furthermore, the range of motion of ON can be well estimated based on the size of the eye opening. A representative case was evaluated with a previous dose to ON getting 52 Gy, but with the consideration of potential motion of ON, it could exceed the radiation tolerance at 56 Gy.
Conclusion: Optic nerve motion can be well estimated from anatomical images of the brain. The motion effect of eye movement should be evaluated on an individual case basis, particularly for targets receiving large doses adjacent to the optic nerves.
Not Applicable / None Entered.