Evaluation of the geometric accuracy of surrogate‐based gated VMAT using intrafraction kilovoltage x‐ray images

Abstract

Purpose: To evaluate the geometric accuracy of beam targeting in external surrogate-based gated volumetric modulated arc therapy (VMAT) using kilovoltage (kV) x-ray images acquired during dose delivery. Methods: Gated VMAT treatments were delivered using a Varian TrueBeam STx Linac for both physical phantoms and patients. Multiple gold fiducial markers were implanted near the target. The reference position was created for each implanted marker, representing its correct position at the gating threshold. The gating signal was generated from the RPM system. During the treatment, kV images were acquired immediately before MV beam-on at every breathing cycle, using the onboard imaging system. All implanted markers were detected and their 3D positions were estimated using in-house developed software. The positioning error of a marker is defined as the distance of the marker from its reference position for each frame of the images. The overall error of the system is defined as the average over all markers. For the phantom study, both sinusoidal motion (1D and 3D) and real human respiratory motion was simulated for the target and surrogate. In the baseline case, the two motions were synchronized for the first treatment fraction. To assess the effects of surrogate-target correlation on the geometric accuracy, a phase shift of 5% and 10% between the two motions was introduced. For the patient study, intrafraction kV images of five stereotactic body radiotherapy (SBRT) patients were acquired for one or two fractions. Results: For the phantom study, a high geometric accuracy was achieved in the baseline case (average error: 0.8 mm in the superior-inferior or SI direction). However, the treatment delivery is prone to geometric errors if changes in the target-surrogate relation occur during the treatment: the average error was increased to 2.3 and 4.7 mm for the phase shift of 5% and 10%, respectively. Results obtained with real human respiratory curves show a similar trend. For a target with 3D motion, the technique is able to detect geometric errors in the left-right (LR) and anterior-posterior (AP) directions. For the patient study, the average intrafraction positioning errors are 0.8, 0.9, and 1.4 mm and 95th percentile errors are 1.7, 2.1, and 2.7 mm in the LR, AP, and SI directions, respectively. Conclusions: The correlation between external surrogate and internal target motion is crucial to ensure the geometric accuracy of surrogate-based gating. Real-time guidance based on kV x-ray images overcomes the potential issues in surrogate-based gating and can achieve accurate beam targeting in gated VMAT.

Evaluation of reproducibility of tumor repositioning during multiple breathing cycles for liver stereotactic body radiotherapy treatment

“…16 Recent report evaluated the geometric accuracy of the surrogate-based gated VMAT with a respiratory phantom and also on real patients cases including liver tumors. 17 Li et al showed in a phantom study a high geometric accuracy (average error of 0.8 mm in the CC direction) when no targetsurrogate relation changes occurred during the treatment. However, including a phase shift of 5% and 10% increased the average errors to 2.3 and 4.7 mm, respectively.…”

Robust optimization of VMAT for lung cancer: Dosimetric implications of motion compensation techniques

“…Recent improvements in the technology of 4‐Dimensional Computed Tomography (4DCT) image binning,15, 16, 17, 18 respiratory motion monitoring,18, 19, 20, 21, 22, 23 functional imaging correlation,24, 25, 26 and faster imaging techniques have resulted in several delivery methodologies to decrease the impact of lung motion. Another approach to reducing the impact is to minimize the motion itself by incorporating compression belts27 to restrict diaphragm contraction and expansion, implementing breath hold techniques23, 28, 29, 30 or gating the treatment by restricting delivery to particular components of the breathing cycle 14, 20, 25, 31, 32.…”

“…Another approach to reducing the impact is to minimize the motion itself by incorporating compression belts27 to restrict diaphragm contraction and expansion, implementing breath hold techniques23, 28, 29, 30 or gating the treatment by restricting delivery to particular components of the breathing cycle 14, 20, 25, 31, 32. Recent works from several research groups have also investigated the tracking of tumor by dynamic correction of MLC positions 33, 34, 35, 36, 37.…”

Amplitude gating for a coached breathing approach in respiratory gated 10 MV flattening filter‐free VMAT delivery

“…Doing so necessitates many interruptions of the beam, which results in significantly increased treatment time. To compensate for this limitation, the idea of coupling respiratory gating with a fast delivery technique, such as volumetric‐modulated arc therapy (VMAT), has increasingly been adopted as the treatment of choice for moving tumors in the liver and the pancreas 5 , 6 , 7 , 8 . In addition, the new advancements in flattening filter‐free mode (FFF) for modulated treatments allow for an increased dose rate, thus significantly reducing the treatment time for treatments delivering high doses per fraction, such as stereotactic ablative radiation therapy (SABR) (9) .…”

“…Li et al (5) and Choi et al (6) investigated the geometric accuracy of gated VMAT using kV imaging and a dynamic phantom. Respiratory motion during intensity modulation, such as IMRT or VMAT, presents an additional challenge over 3D conformal therapy as dose gradients are no longer limited to the edges of the field and vary with time.…”