A Dosimetric Study of an Advanced SRS/SRT System for Intra/Extracranial Tumors

Purpose/Objective(s)
Several specialty machines are available clinically for stereotactic radiosurgery/radiation therapy (SRS/SRT). Gamma Knife provides superior delivery accuracy for intracranial lesions and CyberKnife allows accurate SRS/SRT treatments for extracranial tumors with real-time image guidance and respiratory tracking. This work investigates a new SRS/SRT system that provides both superior dose conformity/gradient and accurate stereotaxy for intra/extracranial treatments.
Materials/Methods
The CybeRay system (Cyber Medical Corp., China) consists of a ring gantry with a treatment head containing 16 gamma sources with a span of 33° in the superior-inferior direction, each focusing at the isocenter with 4 collimators measuring 4, 8, 16 and 32mm diameter. The treatment head can also swing 35° in the superior direction, allowing a total of 68° non-coplanar beam incident. The treatment couch provides 6-degrees-of-freedom motion compensation and the kV cone-beam CT system has a spatial resolution of 0.4mm for target localization. Monte Carlo simulations were used to compute dose distributions and compare with measurements. A retrospective study of 125 previously treated SBRT patients was performed to evaluate the dosimetric characteristics of the CybeRay system in comparison with existing SBRT systems.
Results
Monte Carlo results confirmed the CybeRay design parameters including output factors and 3D dose distributions. Its beam penumbra/dose gradient is similar to that of Gamma Knife and its stereotaxy/isocenter accuracy is 0.3mm. Compared to the 6 MV beams of the CyberKnife, Co-60 beams produce lower-energy secondary electrons that exhibit better dose properties in low-density lung tissues. Because of their rapid depth dose falloff, Co-60 beams are ideal for peripheral lung tumors with half-arc arrangements to spare the opposite lung and other critical structures. The table below shows the patient lateral size, body weight and target volume (mean±1SD) for the 125 patients investigated. Superior dose distributions have been obtained for brain, head and neck, breast, spine and lung tumors with half/full arc arrangements.