2753

Abstract

To determine the feasibility of using a commercially available helical tomotherapy unit (Tomotherapy HiArt, Tomotherapy Inc., Madison, WI) for frame-based intracranial stereotactic radiosurgery. Tomotherapy radiosurgery treatment plans using 2.5cm and 1.0 cm long tomotherapy fan beams were generated using image sets from patients receiving conventional linear accelerator radiosurgery. Plans were generated using a variety of tomotherapy beam widths, pitches, and modulation factors. Planned radiosurgery dose distributions were evaluated on the basis of conformity, dose gradient, and homogeneity. Phantoms were used to evaluate the accuracy of localizing targets in MVCT images, and of the spatial accuracy of the HiArt system’s MVCT image guidance capability. The only hardware modifications necessary (for frame-based radiosurgery) are to add docking hardware for a stereotactic headring to attach to the treatment couch. The HiArt system was capable of producing radiosurgery plans meeting the RTOG acceptability criteria for dose conformity and homogeneity. The HiArt system image guidance consistency (multiple phantom alignments based on MVCT imaging) was 0.35mm. MVCT 3D target localization accuracy was 0.63 mm, or approximately the size of one MVCT image pixel. Tomotherapy beam on times increased with target prescription dose. Beam on times were shortened with larger tomotherapy fan beam lengths and for larger helical pitches. For simulated solitary metastasis targets, beam on times were on the order of 60–70 minutes to deliver 24 Gy to the periphery of the target volume using pitches of 0.1–0.3. Narrow (small) pitches were necessary to successfully treat small target volumes. Due to limitations on minimum gantry rotational speed, it is necessary to deliver a tomotherapy radiosurgery treatment in several passes, if a typical radiosurgery dose of 12–20+ Gy is to be delivered. The tests described here demonstrate the ability of the HiArt system to plan and deliver a radiosurgical dose distribution to a rigid body in phantom. The same process could be used to treat an intracranial radiosurgery patient using a minimally-invasive headframe for immobilization. Frame based intracranial radiosurgery can be feasibly accomplished with a HiArt treatment unit with minimal hardware modification. However, tomotherapy treatment planning and delivery procedures must be modified from typical fractionated radiotherapy practice to accommodate the unique requirements of stereotactic radiosurgery.