Application of a Newly Developed High-Accuracy Image Guided Radiation Therapy System to Stereotactic Radiosurgery

Abstract

Our newly developed Image-Guided Radiation Therapy (IGRT) system has a high degree of isocenter accuracy compared to conventional linac systems. This accuracy is obtained because beam directions are actively controlled by gimbals mechanism. The system also has a kilovoltage (kV) X-ray imaging subsystem and an on-line image registration tool to provide easy and accurate image-guided setup. The aim of this study was to examine the operational performance when the IGRT system is applied to stereotactic radiosurgery for intracranial lesion. In the IGRT system, a megavoltage (MV) X-ray head and an imaging subsystem are installed on a rigid round gantry called O-ring. The O-ring gantry rotates around a vertical axis instead of couch rotation, so that a patient geometry does not change throughout a treatment after setup completion even if non-coplanar beams are applied. The gimbals mechanism supports the X-ray head with a multileaf collimator and controls beam directions to focus onto the isocenter in any gantry position with an accuracy of 0.1 mm. Semi-automatic isocenter accuracy testing is routinely carried out every morning which takes less than 5 minutes. The imaging subsystem consists of two sets of a kV X-ray tube combined with a flat panel detector. It provides two orthogonal X-ray images simultaneously, and also provides cone beam computed tomography (CBCT) images. The images are compared with reference images in an on-line fashion, and the difference can be measured manually on a display or automatically using an image registration software. In this study, a treatment simulation using a head phantom was performed according to a virtual stereotactic radiosurgery plan for an intracranial round tumor which was preparatively made with a radiation treatment planning system. The procedure consisted of the phantom setup, kV X-ray images acquisition, CBCT reconstruction, image registration, and treatment beam delivery. Before the simulation, beam arrangements were confirmed to be deliverable without any collision of the gantry with the couch. Operation time excluding the time for human interaction was measured for every process as well as the total process for the purpose of assessing the system operational performance. Orthogonal X-ray imaging setup including image registration took 1 minute 15 seconds in the first operation and 40 seconds from the 2nd. The time for CBCT setup including image registration was 2 minutes 40 seconds when the images were reconstructed with 1 mm thickness and 1 mm pitch. Beam-on time and total delivery time was 7.5 and 11 minutes, respectively, to deliver 25 Gy (3700 MU) to the isocenter with 4 conformal arcs that had 480 degrees of gantry rotation in total. The total treatment time was estimated from 22 minutes to 25 which process included patient entrance, patient immobilization, orthogonal X-ray imaging setup, CBCT setup, setup verification by an operator, beam delivery, and release of the patient. The operation was performed smoothly without entering of the operator to the treatment room after patient setup. The IGRT system showed to have suitable operational performance maintaining high geometrical accuracy in the application to stereotactic radiosurgery for intracranial lesion.