The maximum tolerated dose of gamma radiation to the optic nerve during γ knife radiosurgery in an animal study.

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 region was 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.