Development of a four‐axis moving phantom for patient‐specific QA of surrogate signal‐based tracking IMRT

Abstract

Purpose:The purposes of this study were two-fold: first, to develop a four-axis moving phantom for patient-specific quality assurance (QA) in surrogate signal-based dynamic tumor-tracking intensity-modulated radiotherapy (DTT-IMRT), and second, to evaluate the accuracy of the moving phantom and perform patient-specific dosimetric QA of the surrogate signal-based DTT-IMRT.Methods:The four-axis moving phantom comprised three orthogonal linear actuators for target motion and a fourth one for surrogate motion. The positional accuracy was verified using four laser displacement gauges under static conditions (±40 mm displacements along each axis) and moving conditions [eight regular sinusoidal and fourth-power-of-sinusoidal patterns with peak-to-peak motion ranges (H) of 10–80 mm and a breathing period (T) of 4 s, and three irregular respiratory patterns with H of 1.4–2.5 mm in the left–right, 7.7–11.6 mm in the superior-inferior, and 3.1–4.2 mm in the anterior–posterior directions for the target motion, and 4.8–14.5 mm in the anterior–posterior direction for the surrogate motion, and T of 3.9–4.9 s]. Furthermore, perpendicularity, defined as the vector angle between any two axes, was measured using an optical measurement system. The reproducibility of the uncertainties in DTT-IMRT was then evaluated. Respiratory motions from 20 patients acquired in advance were reproduced and compared three-dimensionally with the originals. Furthermore, patient-specific dosimetric QAs of DTT-IMRT were performed for ten pancreatic cancer patients. The doses delivered to Gafchromic films under tracking and moving conditions were compared with those delivered under static conditions without dose normalization.Results:Positional errors of the moving phantom under static and moving conditions were within 0.05 mm. The perpendicularity of the moving phantom was within 0.2° of 90°. The differences in prediction errors between the original and reproduced respiratory motions were −0.1 ± 0.1 mm for the lateral direction, −0.1 ± 0.2 mm for the superior-inferior direction, and −0.1 ± 0.1 mm for the anterior–posterior direction. The dosimetric accuracy showed significant improvements, of 92.9% ± 4.0% with tracking versus 69.8% ± 7.4% without tracking, in the passing rates of γ with the criterion of 3%/1 mm (p < 0.001). Although the dosimetric accuracy of IMRT without tracking showed a significant negative correlation with the 3D motion range of the target (r = − 0.59, p < 0.05), there was no significant correlation for DTT-IMRT (r = 0.03, p = 0.464).Conclusions:The developed four-axis moving phantom had sufficient accuracy to reproduce patient respiratory motions, allowing patient-specific QA of the surrogate signal-based DTT-IMRT under realistic conditions. Although IMRT without tracking decreased the dosimetric accuracy as the target motion increased, the DTT-IMRT achieved high dosimetric accuracy.

Impact of sampling interval in training data acquisition on intrafractional predictive accuracy of indirect dynamic tumor‐tracking radiotherapy

“…During treatment, a new data point is added to the training dataset every minute on average, with deletion of an older data point. 6,17,18 Poels et al 23 compared the accuracy of the correlation models derived for the CyberKnife and the Vero4DRT, and found no significant differences between the predictive accuracies when training was performed using high sampling intervals. In addition, the same authors in a different study developed an update method similar to that employed in the CyberKnife.…”

“…17 Next, a 4D model was constructed using the training data of target positions acquired over 20 s, employing sampling intervals of 80 or 320 ms. Each target position was calculated by reference to the centroid of fiducials embedded in a homogeneous cube placed on the moving phantom. The peak-to-peak amplitude of the detected target motions, and mean and standard deviation (SD) of the residual training error, were automatically calculated during the training process.…”

Verification of Dosimetric and Positional Accuracy of Dynamic Tumor Tracking Intensity Modulated Radiation Therapy

“…However, the accumulated dose from summation of all beams was within the tolerance. Mukumoto et al also reported that the peripheral dose to the planning target volume (PTV) was reduced under moving conditions [10].…”

“…However, simultaneous delivery of IMRT and DTT is expected to increase the burden on the multi-leaf collimator (MLC) and the gimbal mechanism, causing an interplay effect. Mukumoto et al [10] verified the dose distribution during DTT IMRT using GAFCHROMIC EBT3 films (Ashland, Kentucky, USA). Depuydt, T. et al and Garibaldi, C. et al reported on log file analysis for DTT IMRT [11] [12].…”

A novel dynamic robotic moving phantom system for patient‐specific quality assurance in real‐time tumor‐tracking radiotherapy

“…16 A four-axis moving phantom for patient specific QA, in which four high precision prismatic actuators are used to control the center-of-gravity of the phantom, was also proposed recently. 17 The proposed moving phantom provides sufficiently high accuracy but can be costly. Use of an industrial robotic manipulator for a dosimetric phantom has also been attempted, 18 and evaluation of its positioning accuracy was performed using the log of the joint actuators of the manipulator.…”