Abstract
We assessed long‐term stability of tracking accuracy using the Vero4DRT system. This metric was observed between September 2012 and March 2015. A programmable respiratory motion phantom, designed to move phantoms synchronously with respiratory surrogates, was used. The infrared (IR) markers moved in the anterior–posterior (AP) direction as respiratory surrogates, while a cube phantom with a steel ball at the center, representing the tumor, and with radiopaque markers around it moved in the superior–inferior (SI) direction with one‐dimensional (1D) sinusoidal patterns. A correlation model between the tumor and IR marker motion (4D model) was created from the training data obtained for 20 s just before beam delivery. The irradiation field was set to 3×3 cm2 and 300 monitor units (MUs) of desired MV X‐ray beam were delivered. The gantry and ring angles were set to 0° and 45°, respectively. During beam delivery, the system recorded approximately 60 electronic portal imaging device (EPID) images. We analyzed: 1) the predictive accuracy of the 4D model (EP), defined as the difference between the detected and predicted target positions during 4D model creation, and 2) the tracking accuracy (ET), defined as the difference between the center of the steel ball and the MV X‐ray field on the EPID image. The median values of mean plus two standard deviations (SDs) for EP were 0.06, 0.35, and 0.06 mm in the left–right (LR), SI, and AP directions, respectively. The mean values of maximum deviation for ET were 0.38, 0.49, and 0.53 mm and the coefficients of variance (CV) were 0.16, 0.10, and 0.05 in lateral, longitudinal, and 2D directions, respectively. Consequently, the IR Tracking accuracy was consistent over a period of two years. Our proposed method assessed the overall tracking accuracy readily using real‐time EPID images, and proved to be a useful QA tool for dynamic tumor tracking with the Vero4DRT system.PACS number: 87.59.‐e, 88.10.gc, 87.55.Qr
Highlights
When treating thoracic and abdominal cancer patients, one of the most important concerns is intrafractional motion
Intrafractional motion affects the accuracy of beam delivery in high-precision radiotherapy,(1-3) which may increase the rate of complications in the normal tissue surrounding the target
Various methods have been proposed to reduce the impact of intrafractional motion during beam delivery including forced shallow-breathing, breath holding, respiratory gating, and dynamic tumor tracking (DTT).(4,5) Recent interest has focused on DTT methods because of the ability to reduce the internal margin without an extension of the treatment time or burdening patients with breath holding
Summary
When treating thoracic and abdominal cancer patients, one of the most important concerns is intrafractional motion. Various methods have been proposed to reduce the impact of intrafractional motion during beam delivery including forced shallow-breathing, breath holding, respiratory gating, and dynamic tumor tracking (DTT).(4,5) Recent interest has focused on DTT methods because of the ability to reduce the internal margin without an extension of the treatment time or burdening patients with breath holding. There can be gradual changes as a result of aging of the machine components. These patterns of failure must be considered when establishing a periodic QA program. Several investigators have reported fundamental performance assessments of the Vero4DRT,(10,11,15-20) there are no reports regarding general QA tests of IR Tracking, and no ‘gold standard’ methods for QA of the Vero4DRT system
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