Comparing phase- and amplitude-gated volumetric modulated arc therapy for stereotactic body radiation therapy using 3D printed lung phantom.

PURPOSE
To compare the dosimetric impact and treatment delivery efficacy of phase-gated volumetric modulated arc therapy (VMAT) vs amplitude-gated VMAT for stereotactic body radiation therapy (SBRT) for lung cancer by using realistic three-dimensional-printed phantoms.


METHODS
Four patient-specific moving lung phantoms that closely simulate the heterogeneity of lung tissue and breathing patterns were fabricated with four planning computed tomography (CT) images for lung SBRT cases. The phantoms were designed to be bisected for the measurement of two-dimensional dose distributions by using EBT3 dosimetry film. The dosimetric accuracy of treatment under respiratory motion was analyzed with the gamma index (2%/1 mm) between the plan dose and film dose measured under phase- and amplitude-gated VMAT. For the validation of the direct usage of the real-time position management (RPM) data for respiratory motion, the relationship between the RPM signal and the diaphragm position was measured by four-dimensional CT. By using data recorded during the beam delivery of both phase- and amplitude-gated VMAT, the total time intervals were compared for each treatment mode.


RESULTS
Film dosimetry showed a 5.2 ± 4.2% difference of gamma passing rate (2%/1 mm) on average between the phase- vs amplitude-gated VMAT [77.7% (72.7%-85.9%) for the phase mode and 82.9% (81.4%-86.2%) for the amplitude mode]. For delivery efficiency, frequent interruptions were observed during the phase-gated VMAT, which stopped the beam delivery and required a certain amount of time before resuming the beam. This abnormality in phase-gated VMAT caused a prolonged treatment delivery time of 366 s compared with 183 s for amplitude-gated VMAT.


CONCLUSIONS
Considering the dosimetric accuracy and delivery efficacy between the gating methods, amplitude mode is superior to phase mode for gated VMAT treatment.


| INTRODUCTION
Respiratory-induced movement is an important consideration during radiotherapy, particularly for volumetric modulated arc therapy (VMAT) for lung stereotactic body radiotherapy (SBRT). SBRT is a technique that is commonly used for treating early stage non-small cell lung cancer and metastatic lung tumors. 1,2 Accordingly, the accuracy of dose delivery, conformity of dose distribution, and accurate target volume localization using motion management techniques are important for delivering safe and effective VMAT-based SBRT.
There are various techniques for managing the respiratoryinduced motion of organs and tumors. 3 Respiratory control approaches, including active breathing control and deep-inspiration breath holding, not only increase the discomfort of a patient for an extended time but also alter the position of anatomical structures in the lung and diaphragm region. 4 In gating approaches, the patient can breathe freely during the computed tomography (CT) scan and treatment. These techniques can be divided into two categories: phase gating and amplitude gating. In phase gating, the radiation beam is activated in a certain phase of the respiration cycle. In amplitude gating, the radiation beam is activated whenever a certain amplitude value is reached regardless of the phase in the patient's respiratory cycle. Amplitude gating is better at suppressing respiratory motion artifacts compared with phase gating. 5 One study has shown that an irregular breathing pattern could be the reason for poor dosimetric results in phasegated studies, and it could be a more significant issue with phase gating compared with amplitude gating. 6 In addition to the reasons mentioned above, amplitude gating may be more beneficial because amplitude gating has a shorter treatment time than phase gating. Short treatment times are highly recommended for dose-delivery accuracy and delivery efficacy. Hoogeman et al. 7  Nonetheless, most of the clinically used gating techniques are time-based phase-gating methods. 4,10,11 Furthermore, several studies have compared the phase-and amplitude-gating methods by acquiring the four-dimensional (4D) CT image. To the best of our knowledge, no study has compared the dosimetric accuracy and delivery efficacy by generating a realistic patient-specific phantom model. [12][13][14] In this study, individualized lung phantoms were generated via a three-dimensional printer (3D EDISON, Lokit, Korea) to achieve realistic simulation, and VMAT lung SBRT treatment was delivered.
Gamma comparison was performed to evaluate the dosimetric accuracy of phase and amplitude gating, and the treatment time was compared by analyzing the trajectory log file to evaluate the treatment efficacy. 2.A | Patient-specific 3D-printed respiratory lung phantoms Four patients who had been treated by lung SBRT-gated VMAT were selected for this study. We attempted to reproduce various lung and tumor conditions, and Table 1 shows the detailed information of each patient. was produced with a mesh grid structure that was filled with 0.3 mm strips of 2 mm air gaps rather than an empty space to obtain a density similar to that of a real lung. 15 Fused deposition modeling with a 3D printer was used for phantom generation, and polylactic acid with 1.25 g/cm 3 density was used as the printing material.

2.B | Data acquisition
Respiratory motion data for approximately 4 min were obtained via Varian's RPM system during a planned CT scan. It was continually repeated via QUASAR ™ phantom for the beam delivery. The patientspecific lung phantom was scanned using 4D CT with 1.25 mm slice thickness, and the RPM block was placed on the phantom (Fig. 3).
The 4D CT images of each phantom with its respiratory pattern were reconstructed and sorted by phase-and amplitude-gating methods. The same CT was used for both gating methods.
For phase gating, the breathing cycle was evenly divided into 10 phases, and a 30% to 70% gating window was selected as patient treatment. For amplitude gating, the lower and upper thresholds of the selected amplitude range were defined from the 40% to 50% gating window in the 10 phases used in phase gating of each patient. 16 An element of visual and/or verbal coaching could be used

2.C | Analysis
To assess the dosimetric accuracy of phase-and amplitude-gated VMAT treatment, gamma analysis was performed using EBT3 film under the 2%/1 mm criterion with 80% passing rate. Many studies recommended a stricter gamma criterion of 2%/1 mm or 2%/2 mm rather than 3%/3 mm, particularly for VMAT quality assurance (QA).
However, 3%/3 mm was still an acceptable standard for evaluating IMRT and other plans in a clinical setting. [18][19][20][21] The film images were scanned in 48-bit RGB (red, green, and blue) mode with 72 dpi resolution (pixel size: 0.35 mm) and were measured and analyzed using FilmQA pro 2012 software (Ashland Inc., Bridgewater, NJ, USA).
Delivery efficacy was also evaluated by analyzing the trajectory log file from the treatment records. The TrueBeam ™ control system generated a trajectory log file, which recorded the beam status data and various information about the expected and actual values of the treatment. By using this information, the following were evaluated: total treatment time, total time interval for the treatment, gate-on time, time interval when the gating system allowed the beam on, beam-on time, and time interval when the beam was delivered.

3.B | Delivery efficacy
In the phase-gating method shown in Fig. 4, gating-and beam-on intervals could not be synchronized owing to an irregular breathing pattern with an unexpected sudden change (marked as beam interruption). When this phenomenon occurred, the system automatically shut down until the periodicity of the normal breathing pattern was reestablished. There were also moments when the beam instantly turned off even without a sudden irregular change of breathing pattern. In the four patient-specific phantoms, this type of beam interruption occurred 39 times on average in phase gating (49, 63, 29, and 17) but rarely occurred in amplitude gating (1, 0, 1, and 0). As the beam-off time and frequency increased, the treatment efficacy decreased, and the overall treatment time was extended. Figure 5 shows the total delivery time, gate-on time, and beamon time according to the gating method. As expected, the total F I G . 3. Installation of the 4DCT scan and the 4D lung phantom. The 4D lung phantom was composed of the QUASAR ™ phantom and 3D-printed lung phantom. The EBT3 film was inserted inside the lung phantom.
T A B L E 2 Gamma evaluation results of the phase-and amplitudegating methods with 2%/1 mm criterion.
Phantom no.  Table 3 shows the summary of phase-and amplitude-gated VMAT deliveries, including the number of beam interruptions. According to this table, the average numbers of beam interruptions for the phase-and amplitude-gating methods were 39.5 and 0.5, respectively.

| DISCUSSION
The QA of gating systems should include an analysis of both the time delay and dosimetric characteristics of the gated delivery.
Thus, we compared the dosimetric impact and treatment delivery efficacy of phase-and amplitude-gated VMAT for stereotactic lung cancer treatment by using realistic patient-specific lung phantoms fabricated with a 3D printer. According to the results, there are noticeable differences between the two gating methods in terms of dosimetric accuracy. The average gamma passing rate of the amplitude gating was 5.2 ± 4.2% higher than the phase gating and in all cases exceeded 80%, which was an acceptable passing rate F I G . 4. Schematic illustration of the phase-gated beam delivery with an irregular breathing pattern (beam interruption). The shaded area shows the moment that the beam was off when the gating was on.
F I G . 5. Graph to compare the treatment time between phase-and amplitude-gating methods.
T A B L E 3 Summary of phase-and amplitude-gated VMAT deliveries. at the 2%/1 mm criterion. Similar to the other studies, these results support that amplitude gating is better in terms of dosimetric impact. 5,6 Amplitude gating was also better in terms of delivery efficacy because phase gating takes double the total delivery time compared with amplitude gating on average. We think that not increasing in total treatment time in amplitude gating is a major clinical benefit.
The longer the treatment time, the possibility for the patient motion will be increased and the accuracy of treatment will be decreased accordingly. It is assumed that this time difference is mainly due to the moment that the beam instantly turns off even without a sudden irregular change of breathing pattern (i.e., beam interruption). This phenomenon occurs more frequently in phase gating (~39.5 times) than in amplitude gating (~0.5 times). Gated VMAT radiotherapy is periodically interrupted as a response to a gating signal from the RPM system.

| CONCLUSION S
We investigated the dosimetric impact and treatment delivery efficacy of phase-gated VMAT vs amplitude-gated VMAT for stereotactic lung cancer treatment by using realistic lung phantoms fabricated with a 3D printer. Patient-specific lung phantoms that closely simulated the actual lung tissue were generated using 3D printing techniques, and the respiratory patterns of each patient were demonstrated with the QUASAR ™ breathing equipment.
Considering the two aspects (dosimetric impact and treatment delivery efficacy) of the phase-and amplitude-gating strategies, amplitude mode would be superior to phase mode for gated VMAT treatment. The amplitude method shows a 5.2 ± 4.2% higher average gamma passing rate than the phase method under the 2%/1 mm

CONFLI CT OF INTEREST
No conflicts of interest.