Precision, high dose radiotherapy. II. Helium ion treatment of tumors adjacent to critical central nervous system structures.

In this paper we present a technique for treating relatively small, low grade tumors located very close to critical, radiation sensitive central nervous system structures such as the spinal cord and the brain stem. A beam of helium ions is used to irradiate the tumor. The nearby normal tissues are protected by exploiting the superb dose localization properties of this beam, particularly its well defined and controllable range in tissue, the increased dose deposited near the end of this range (i.e., the Bragg peak), the sharp decrease in dose beyond the Bragg peak, and the sharp penumbra of the beam. To execute this type of treatment, extreme care must be taken in localization of the tumor and normal tissues, as well as in treatment planning and dosimetry, patient immobilization, and verification of treatment delivery. To illustrate the technique, we present a group of 19 patients treated for chordomas, meningiomas and low grade chondrosarcomas in the base of the skull or spinal column. We have been able to deliver high, uniform doses to the target volumes (doses equivalent to 60 to 80 Gy of cobalt-60) while keeping the doses to the nearby critical tissues below the threshold for radiation damage. Follow-up on this group of patients is short, averaging 22 months (2 to 75 months). Currently, 15 patients have local control of their tumor. Two major complications, a spinal cord transection and optic tract damage, are discussed in detail. Our treatment policies have been modified to minimize the risk of these complications in the future, and we are continuing to use this method to treat such patients. We are enthusiastic about this technique, since we believe there is no other potentially curative treatment for these patients.