A Daily QA Phantom for Linear Accelerator with Image-Guided Radiosurgery Capability

Abstract

Image guided radiosurgery and stereotactic body radiotherapy (IG-SRS/SBRT) have been widely adopted on linear accelerators. Ideally, a daily quality assurance program on such machines should provide the targeting accuracy from image guidance quantitatively and efficiently. A suitable phantom for such test requires sufficient landmarks for robust online image registration and a high contrast ball bearing (BB) to provide clear images for Winston Lutz (WL) test. Such phantoms, however, are not widely available. In this study, we investigated a prototype daily QA phantom that allows all major components of the IG-SRS/SBRT process to be tested. The phantom is a 12 cm cube with 3 pairs of AL rods at 3 corners, and 4 embedded AL ceramic BBs. The BB in the center of phantom is 5/16 inch in diameter, while the others are ¼ inch in size. On the treatment table, the phantom sits atop a holder with build-in pitch, roll, and pitch. The holder can be index to the couch at a preset starting position. A high resolution planning CT was acquired of the phantom on a level surface. A plan was then generated with isocenter placed precisely in the central BB of the phantom. The plan was delivered on a medical linear accelerator and a radiosurgery system, using cone beam CT (CBCT) and stereoscopic x-ray system for image guidance, respectively. Both imaging systems provide 6DoF deviations that were subsequently applied to the treatment couch. Afterwards, verification imaging was acquired with an on-board imaging (OBI) KV/MV pair on a medical linear accelerator and a 2nd pair of stereotactic x-ray images on a radiosurgery system, and WL tests were performed to measure the agreement between central BB and machine isocenter. The plan was delivered 12 times each on the two machines. The initial correction, deviation from verification imaging, and WL test results (μ ± σ) are provided in the table below, where R represents 3D vector length.The verification imaging indicated that all 6DoF corrections were applied correctly, and WL test showed that both systems are able to localize well within 1 mm accuracy on a phantom. The phantom incorporates necessary features to enable daily QA with the goal of simulating IG-SRS/SBRT workflow and quantitatively evaluating localization accuracy. Accuracy determined from this daily QA procedure should represent the upper limit of targeting accuracy for ideal conditions.Abstract 3613; Table 1Medical Linear Accelerator (n=12)Radiosurgery System (n=12)CorrectionVerificationWL testCorrectionVerificationWL testVRT (mm)-8.0±0.10.0±0.10.02±0.04-8.6±0.30.0±0.10.30±0.15LNG (mm)6.8±0.10.2±0.20.25±0.087.0±0.40.0±0.10.18±0.10LAT (mm)6.3±0.2-0.1±0.2-0.20±0.146.0±0.40.1±0.10.20±0.13R (mm)12.2±0.20.4±0.10.34±0.0912.6±0.40.2±0.10.45±0.11Yaw (o)1.4±0.1-0.2±0.1-1.4±0.20.1±0.1-Pitch (o)-1.8±0.10.1±0.1--1.7±0.10.0±0.1-Roll (o)-1.0±0.10.1±0.1--0.9±0.10.0±0.1- Open table in a new tab