Frame versus Frameless Leksell Stereotactic Radiosurgery

Abstract

For more than 65 years localization of brain targets suitable for stereotactic radiosurgery has been performed after application of an intracranial guiding device to the cranial vault. After imaging and dose planning the same frame is used to secure the target at the focus of the intersection of the ionizing radiation beams that create the radiobiological effect. Non-invasive immobilization systems first proposed for linear accelerator or proton radiation technologies have now been developed for the Leksell Gamma Knife ICON radiosurgical system. The ICON technology adds a cone-beam computed tomography (CBCT) scan to the original Perfexion radiosurgical device in order to define the brain stereotactic space. Marketed since 2015, the ICON can be used for standard radiosurgical procedures, most of which remain frame based, but also coupled with a non-invasive thermoplastic mask for carefully selected patients who undergo standard single-session radiosurgical procedures, as well as multisession procedures using repeat mask fixation. Both at UPMC as well as worldwide, mask immobilization has to date been used for approximately 10% of patients with specific characteristics: relatively simple dose plans, short radiation delivery times, and non-anxious patients, most of whom have metastatic or primary brain cancers. In certain cases, multisession radiosurgery is also performed using the mask. The workflow of frame versus frameless procedures is often altered, and is reliant on high-definition imaging, mostly MRI, done prior to dose planning. Since each CBCT takes 10-12 min to set up and acquire, co-register, and review with the treatment plan, and two CBCT scans are necessary to initiate the treatment plan, this workflow must be added to the beam on time. Although frame-based immobilization remains the predominant method to secure target fixation for problems suitable for single-session radiosurgery, the advent of a mask immobilization technique has proven valuable for a select group of patients. It also provides a non-invasive method to perform multisession or fractionated radiation in patients for whom traditional single-session radiosurgery is not feasible.