A phase I trial of neutron brachytherapy for the treatment of malignant gliomas

We performed a phase I trial to test the feasibility of neutron brachytherapy using californium-252 (²⁵²Cf) as the sole source of radiation, and to determine the maximum tolerable dose (MTD), for the treatment of malignant gliomas. Previous studies using external beam neutron radiation have shown that neutrons are capable of totally eradicating malignant gliomas. However, in most cases, fatal widespread radiation necrosis resulted. Radioactive implants are a logical method of increasing the dose to the tumour and decreasing the dose to normal brain. ²⁵²Cf is a relatively stable neutron-emitting isotope suitable for implant therapy. The study was an open ended dose-escalation study. All radiation was delivered by using only ²⁵²Cf implants, without external beam therapy of any type. The first dose step was 900 neutron cGy (ncGy); each subsequent step was increased by 100 ncGy. Three patients with newly diagnosed malignant gliomas were entered at each dose step, and the number was increased to six patients in dose steps at which necrosis of brain occurred. The study ended when two patients at any dose step developed radiation necrosis of brain outside the prescribed radiation field. 33 patients were entered into the study. 10 patients developed scalp necrosis associated with scalp doses above 900 ncGy. The study ended when two patients at the 1300 ncGy dose step developed radiation necrosis of brain. We conclude: (1) neutron brachytherapy using ²⁵²Cf as the sole source of radiation is a feasible treatment for malignant gliomas; (2) the scalp tolerates less neutron radiation than the brain; (3) the MTD (and the recommended dose for a phase II trial) of interstitial neutron brachytherapy is 1200 ncGy.