A radiation fraction size greater than 2 Gy and/or a total radiation dose greater than 50 Gy are suggested to be risk factors for RON in general5,65. As shown in Table 2, it appears that in 16 of 32 cases (50%) radiation fraction size was greater than 2 Gy and/or total radiation dose was greater than 50 Gy. Information with respect to minimum and maximum radiation doses according to ICRU 50/62 recommendations80 within the treated target volume, that also encompass the optic system, was not available in most reports.