**3.1 Visual complications**

During the irradiation of tumors, with close anatomical relation to the optic chiasm and nerves, a certain degree of collateral irradiation of these intact but sensitive structures occurs [58, 59]. In therapy protocols, the optic nerves, chiasm, and tracts are usually outlined and defined as organs at risk (OAR) [57]. Radiation-induced optic neuropathy (RION) is defined as painless rapid visual loss and is attributed to radiation necrosis of the anterior optic pathways [60]. It often has a delayed onset and can result in either visual acuity or visual field loss. The risk of radiationinduced optic neuropathy is dependent on both the total cumulated radiation dose and the fraction dose [60]. The risk is markedly increased at cumulated optic chiasm radiation doses of ≥60 Gy in the case of fractionated stereotactic radiation therapy and at a single dose of >12 Gy in the case of radiosurgery [60]. The risk is greater with increasing age, preexisting compression of the optic nerves/chiasm, and previous radiation therapy. Percentages of 3–7 and 7–20% of RION in the dose ranges 55–60 and above 60 Gy, respectively, have been reported, as presented in the review by the QUANTEC initiative [60].

Fractionated stereotactic radiation therapy combines the advantage of a high accuracy of stereotactic technique and the biological advantage of fractionation [1, 48]. For stereotactic radiosurgery (SRS) of tumors in the vicinity of the optic structures, there is a dose-limiting factor, meaning that the minimal effective tumor dose may be equal to or greater than the dose tolerated by the optic structures. For example, the treatment of tumors of the cavernous sinus, with single-dose SRS, has been shown not to affect the optic pathways at a single dose of <10, whereas the incidence of optic neuropathy has been shown to be 27% after a single dose of 10 Gy–15 Gy and 78% after a single dose of >15 Gy [58]. Other SRS studies of perioptic tumors have reported variable results [4–6, 61–64].
