Gamma knife irradiation - induced histopathological changes in the trigeminal nerves of Rhesus monkeys

Objective The authors sought to observe histopathological changes in the trigeminal nerve(TN)after Gamma knife treatment in rhesus monkeys and to investigate the radiobiological mechanism of Gamma knife treatment for primary trigeminal neuralgia. The nerve length dose effect of irradiation was discussed. Method One of five rhesus monkeys randomly served as control and the other four monkeys were randomly administered a target dose of 60,70,80, and 100 Gy respectively. The size of the collimator was 4 mm and the target point was the TN root. In each experimental monkey, one side was exposed to single - target point irradiation and the contralateral side to two - target point irradiation. After 6 months, the TN root was examined by light microscopy, transmission electron microscopy, and immunohistochemistry.Results At the same dose of radiation, the damage to the nervous tissue by single - target point irradiation was identical to that caused by two - target point irradiation. In the trigeminal nerve tissues of the monkeys irradiated with 60 and 70 Gy,there was scant nerve demyelination and degeneration, fragmentation, or loss of axons. In the trigeminal nerve tissue of the monkey irradiated with 80 Gy, the nervous tissue showed a disordered structure. In the trigeminal nerve tissue of the monkey irradiated with 100 Gy, there were severe derangement in the structure of the nervous tissue and extensive demyelination, fragmentation, and loss of axons. Conclusions The target doses of 60 and 70 Gy have very little impact on the structure of the TN. Irradiation at 80 Gy can cause partial degeneration and loss of axons and demyelination. A 100 Gy dose can cause some necrosis of neurons. Comparing the single - target irradiation with the two - target point irradiation,the extent of damage to the nervous tissue is identical and no difference in the nerve length dose effect was found. Key words: Gamma knife;  Macaque;  Trigeminal neuralgia;  Pathology;  Stereotactic radiosurgery