The Maximum Tolerated Dose of Gamma Radiation to the Optic Nerve during Gamma Knife Radiosurgery in an Animal Study

Abstract

Background: The safety of gamma knife radiosurgery should be considered when treating pituitary adenomas. Objectives: To determine the maximum tolerated dose of radiation delivered by gamma knife radiosurgery to optic nerves. Methods: An animal model designed to establish prolonged balloon compression of the optic chiasm and parasellar regionwas developed to mimic the optic nerve compression caused by pituitary adenomas. Twenty cats underwent surgery to place a balloon for compression effect and 20 cats in a sham operation group received microsurgery without any treatment. The effects of gamma knife irradiation at 10–13 Gy on normal (sham operation group) and compressed (optic nerve compression group) optic nerves were investigated by pattern visual evoked potential examination and histopathology. Results: Gamma knife radiosurgery at 10 Gy had almost no effect. At 11 Gy, P100 latency was significantly prolonged and P100 amplitude was significantly decreased in compressed optic nerves, but there was little change in the normal optic nerves. Doses of 11 Gy and higher induced significant electrophysiological variations and degeneration of the myelin sheath and axons in both normal and compressed optic nerves. Conclusions: Compressed optic nerves are more sensitive to gamma knife radiosurgery than normal optic nerves. The minimum dose of gamma knife radiosurgery that causes radiation injury in normal optic nerves is 12 Gy; however, the minimum dose is 11 Gy in compressed optic nerves.

Risk factors for radiation‐induced optic neuropathy: a case–control study

“…Maximum radiation dose to the anterior visual pathway has been established as a significant determinant for the development of RION . The anterior visual pathway has been shown to tolerate no greater than 50 Gy of cumulative radiation in fractions less than 2 Gy, with the incidence of RION increasing markedly at higher levels . The incidence of RION following radiation treatment of head and neck or skull‐base tumours has been quoted as ranging from 0% for <50Gy to 16% for >70Gy .…”

“…Demizu et al found a significant association for diabetes mellitus . Deng et al suggested that compression of the optic nerve by tumour may sensitize the visual pathway to radiation injury . Guy et al reported adjuvant chemotherapy as a potential risk factor .…”

“…1,3,4 The anterior visual pathway has been shown to tolerate no greater than 50 Gy of cumulative radiation in fractions less than 2 Gy, with the incidence of RION increasing markedly at higher levels. [5][6][7][8][9] The incidence of RION following radiation treatment of head and neck or skull-base tumours has been quoted as ranging from 0% for <50Gy to 16% for >70Gy. 10 Current EBRT treatment planning protocols limit maximum point-doses to the optic pathway to 54-55 Gy in 1.8-2 Gy fractions to minimize this complication.…”

“…15 Deng et al suggested that compression of the optic nerve by tumour may sensitize the visual pathway to radiation injury. 16 Guy et al reported adjuvant chemotherapy as a potential risk factor. 17 Gragoudas et al argue that risk may be enhanced by underlying vascular disorders, 18 and therefore that hypertension, hyperlipidaemia and smoking status may prove significant.…”

Radiation-Induced Optic Neuropathy: Literature Review

“…In addition, it is reported that the increase in the incidence of RION is proportional to the increase of these doses. 2,23,[26][27][28][29][30] However, more recent data indicate that a single dose less than 12 Gy will induce RION in 1% of patients. 31,32 Data also suggest that no patient receiving less than 8 Gy to the anterior optic pathways will develop RION, but the frequency rises steeply to 78% when the dose is 15 Gy or more following stereotactic radiosurgery, whose the security dose is less than 8-10 Gy.…”