This study reported two unusual optic nerve toxicities (CTCAE grade 3 and 4) in patients with meningioma treated with IMPT. For these two unexplained toxicity cases, the investigators found that the optic chiasm and the left optic nerve (in one patient) were not associated with high dose levels nor with high LET values, but with a higher dose rate where less than 10% of the total dose was delivered at as high as 3.9 GyRBE/s and 4.3 GyRBE/s. The authors concluded that their observations cannot establish causality between exposure to high dose rates and the development of late optic apparatus toxicities.