Investigation of 3-dimensional Optical Image Guided Frameless Stereotactic Radiosurgery Using Homegrown Volumetric Modulated Arc Therapy

Abstract

Recently, powerful on-board imaging technologies have propelled SRS into the frameless era. Yet, all the mainstream on-board imaging systems resort to ionizing X-rays as imaging sources. Moreover, most SRS procedures are planned with IMRT, a modality that suffers from an elevated MU, a reduced MU-to-cGy coefficient, and a prolonged treatment time. To tackle these issues, we commissioned a novel frameless stereotactic system for intracranial SRS immobilization, a 3D optical surface imaging system for target localization and real-time patient monitoring, and a homegrown VMAT system for SRS planning. Here, we present our initial results of a feasibility study. The frameless stereotactic system includes carbon fiber base boards, a cranial stereotactic localizer frame, a support bridge, patient-specific mouthpiece assemblies, and a portable vacuum pump. The portable vacuum pump applies a negative pressure to the patient mouthpiece. The resulting vacuum suction seals the mouthpiece precisely into the palate of the mouth. VMAT-SRS plans were computed with a single 360° arc containing 180 equally-spaced beams, optimized incrementally using a differentiated optimization scheme. The target localization was accomplished with a video-based 3D optical surface imaging system, consisting of three 3D camera units and fast 3D image registration software. After the patient had been initially setup, the 3D optical surface imaging system acquired a verification skin surface of the patient. This verification image was registered to a reference skin surface created from the planning 3DCT in a user-defined region of interest. The registration software calculated three translational shifts (ΔVRT, ΔLNG, and ΔLAT), and three rotational shifts (ΔPITCH, ΔROLL, and ΔYAW) for patient re-positioning. The frameless system was found to be able to secure a sustained immobilization accuracy of ∼ ±1.0 mm and 1.0° for intracranial SRS as measured by CBCT and 3D real-time optical surface imaging. With this system, the patient could be simulated, planned, and treated on different days. VMAT-SRS plan optimization converged in 10 minutes for a PTV of ∼ 35 cm3 and 6 other critical structures. VMAT-SRS plans exhibited superior dose distribution to IMRT-SRS plans as measured by conformality and homogeneity indices. The target dose inhomogeneity was less than 17% compared to about 30% for conventional SRS plans. The doses to the critical structures for VMAT-SRS plans were less than or comparable to those for conventional SRS plans. With this new approach, the treatment time was reduced by more than one hour. Our initial data revealed that the synergistic combination of the frameless system, VMAT-SRS, and the 3D real-time optical surface imaging could be an optimal intracranial SRS sub-modality.