Intracranial application of IMRT based radiosurgery to treat multiple or large irregular lesions and verification of infra-red frameless localization system.

ABSTRACT

We have employed a frameless localization system for intracranial radiosurgery, utilizing a custom biteblock with fiducial markers and an infra-red camera for set-up and monitoring patient position. For multiple brain metastases or large irregular lesions, we use a single-isocenter intensity-modulated approach. We report our quality assurance measurements and our experience using Intensity Modulated Radiosurgery (IMRS) to treat such intracranial lesions. A phantom with integrated targets and fiducial markers was utilized to test the positional accuracy of the system. The frameless localization system was used for patient setup and target localization as well as for motion monitoring during treatment. Inverse optimization planning gave satisfactory dose coverage and critical organ sparing. Patient setup was guided by the infrared camera through fine adjustment in three translational and three rotational degrees for isocenter localization and verified by orthogonal kilovoltage (kV) images, taken before treatment to ensure the accuracy of treatment. The relative localization of the camera based system was verified to be highly accurate along three translational directions of couch motion and couch rotation. After verification, we began treating patients with this technique. About 8-12 properly selected fixed beams with a single isocenter were sufficient to achieve good dose coverage and organ sparing. Portal dosimetry with an Electronic Portal Imaging Device (EPID) and kV images provided excellent quality assurance for the IMRS plan and patient setup. The treatment time was less than 60 min to deliver doses of 16-20 Gy in a single fraction. The camera-based system was verified for positional accuracy and was deemed sufficiently accurate for stereotactic treatments. Single isocenter IMRS treatment of multiple brain metastases or large irregular lesions can be done within an acceptable treatment time and gives the benefits of dose-conformity and organ-sparing, easy plan QA, and patient setup verification.