Technical Aspects of a Proton Stereotactic Radiosurgery Program

Abstract

Our institution’s Proton Stereotactic Radiosurgery (PSRS) program has significantly evolved since its inception in 1965. A large patient volume and requirements for stringent patient setup and accurate dose delivery have posed numerous technical and logistical challenges. With the increasing availability of proton facilities, the feasibility of using protons for cranial radiosurgery is of growing interest. We provide an overview and insights of technical aspects of our proton radiosurgery program. We have reviewed our system of PSRS that has significantly evolved over decades of use and modification and now provide a summary of the technical workflow, highlighting the PSRS-specific adaptations of the common proton therapy techniques designed for fractionated treatments, including immobilization, setup verification and dose delivery. Our PSRS system is a dedicated, in-house designed stereotactic beamline that is based upon a fixed horizontal, single scattering proton beam with a large SAD to minimize penumbra. Patients can be rotated to 90 degrees around the cranio-caudal axis with a 5-degree-of-freedom positioner. Approximately 80% of PSRS cases are treated in the dedicated beamline while the remainder is triaged to a commercial fully rotational gantry system. Patient immobilization is achieved by either a non-invasive cranial ring via custom dental and occipital molds or thermoplastic mask-based systems including a bite block. All immobilization devices are adapted to minimize impact on the proton beam. Commercial planning software is employed for treatment planning with an average use of 3 beams per lesion. Custom aperture and range compensator are fabricated for each field. The system is commissioned and routinely verified to eliminate the need for patient specific dosimetry. Setup verification for treatment utilizes fiducial markers implanted into the outer table of the skull. Alternatively to the implants, anatomical landmarks on x-rays compared to DRRs can be used for setup. Approximately 200 patients are treated annually with this PSRS system, diagnoses covering malignant (e.g. brain metastases) and benign (e.g. pituitary adenomas, schwannomas, arteriovenous malformations). Efficient procedures have been developed to ensure optimal workflow between the various involved professional groups (immobilization staff, radiation therapy technicians, physicians, neurosurgeons, physicists) to achieve safe and accurate dose delivery. Our institution has developed a unique, efficient, and highly accurate system for delivering PSRS which may serve as a model for other centers.