Dosimetric characteristics of cardiorespiratory motion during cardiac stereotactic body radiotherapy and dose gain from respiratory gating

Introduction

Cardiovascular disease is a major contributor to human mortality, and accounted for 31% of global deaths in 2020 (1). Among cardiovascular disorders, arrhythmias are the most prevalent and clinically significant (2), and are typically managed through antiarrhythmic drugs, radiofrequency catheter ablation, and implantable cardiac defibrillators (3). Nonetheless, these methods face challenges such as low overall efficacy (4), potential complications (3), and limited patient tolerance (5).

Cardiac stereotactic body radiotherapy (CSBRT) represents a novel and promising treatment approach for patients with refractory arrhythmias (6-8). CSBRT uses high-energy X-rays at 6 or 10 megavoltage (MV) in a flattening filter-free (FFF) high-dose-rate mode to irradiate a target. Compared to existing therapies, CSBRT provides a noninvasive approach, characterized by shorter treatment times, and an ability to treat multiple targets concurrently. Currently, approximately 30 radiotherapy facilities offer CSBRT worldwide (9-12). However, the absence of standardized protocols for treatment planning and motion management remains a barrier to its broader clinical adoption and cross-institutional consistency. To address this issue, recent efforts, such as the multicenter STOPSTORM benchmark study (13) and the ventricular tachycardia ablation through radiation therapy (VT-ART) consortium initiative (14), have aimed to promote harmonized CSBRT workflows and reproducible dosimetric evaluation frameworks across international centers. Nonetheless, the effects of complex cardiorespiratory motion on actual dose delivery remain an open and pressing issue that need to be addressed.

Motion is a crucial consideration in precision radiotherapy, as neglecting it may result in suboptimal target irradiation and damage to surrounding healthy tissues. The complexities of CSBRT stem from the interplay between cardiac pulsation and respiratory motion. Notably, the intricate rotational motions of the heart during systole and diastole, resulting from the helical structure of the myocardial bands, present unique challenges for motion management compared to conventional thoracic and abdominal tumor treatments (15). Therefore, conventional motion management techniques may not entirely mitigate dosimetric errors in CBSRT (16). A recent literature review (17) emphasized that target motion induced by systole-diastole of the heart during CSBRT can reach up to 8, 6, and 6.5 mm in the superior-inferior (SI), anterior-posterior (AP), and left-right (LR) directions, respectively. Another study by Stevens et al. (18) employed a four-dimensional (4D) extended cardiac-torso digital phantom to create an envelope capturing only diastolic and respiratory motion, simulating scenarios where respiratory 4D computed tomography (4DCT) scans inadequately captured cardiac motion. Their findings indicated that the cardiorespiratory envelope, limited to the diastolic phase of the cardiac cycle, covered 63–86% of cardiorespiratory motion. Further, Bellec et al. (16) demonstrated that cardiac motion alone could increase the target volume by up to 75%. Collectively, these results highlight the significant effects of cardiac motion on CSBRT outcomes.

While innovative strategies for administering radiation exclusively during specific cardiac cycle phases are currently being developed (19) and ongoing research exploring the integration of cardiac gating with real-time respiratory motion tracking is being conducted (20), these innovative techniques are in the early stages of development and have not yet been implemented in clinical settings. Reported clinical trials of arrhythmic CSBRT have predominantly used envelopes encompassing cardiorespiratory motion and respiratory-gating techniques for comprehensive cardiorespiratory motion management (1,6,21). However, it is not yet known whether these respiratory motion management techniques can ensure the accuracy of the irradiated dose in CSBRT target areas and the safety of the surrounding normal tissue.

Currently, research on the potential effects of cardiac motion on the radiation dose in patients undergoing CSBRT is limited (22). Moreover, the dosimetric uncertainty induced by complex cardiorespiratory motion in CSBRT remains unclear, and might have contributed to differences in the efficacy and complications of CSBRT for arrhythmias observed in clinical trials conducted by various medical institutions (21). Thus, further research needs to be conducted to understand the dose advantages produced by respiratory motion management techniques in CSBRT, as understanding the dosimetric effects of cardiorespiratory motion and motion management measures in CSBRT is crucial for conducting large-scale CSBRT trials. The use of a physical phantom for dose assessment represents a reasonable approach. Therefore, this study employed a dynamic cardiac motion phantom to simulate cardiac pulsation and respiratory motion to facilitate a systematic investigation of the dependence of CSBRT dosimetry on various motion parameters, including the cardiac motion amplitude, heart rate, respiratory amplitude, breath cycle, and motion onset phase. Further, the study aimed to evaluate the dosimetric advantages of the respiratory-gating technique employed for arrhythmic CSBRT. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1443/rc).

Methods

CSBRT plan design

Before CSBRT, each patient underwent both electrocardiography (ECG)-gated 4D cardiac computed tomography (4DcCT) and 4DCT scans using a Revolution energy spectrum (ES) computed tomography (CT) scanner (GE Healthcare, Chicago, IL, USA) to separately characterize cardiac and respiratory motion. The 4DcCT and 4DCT datasets each comprised 10 temporal phases (0–90%) across the cardiac cycle and respiratory cycle, respectively.

The substrate clinical target volume (CTV) and organs at risk (OARs) were delineated in the 0% phase of contrast-enhanced 4DcCT by a multidisciplinary team of cardiologists and radiation oncologists. To account for cardiac motion, the CTV and OARs were deformably registered and propagated across all 10 cardiac phases using MIM software (version 5.2, MIM Software Inc., Cleveland, OH, USA), followed by structure fusion on the average intensity projection (AIP) image to generate the internal target volume (ITV_4DcCT) and internal organ-at-risk volumes (IRV_4DcCT). The 4DcCT AIP image was then rigidly registered to the 10-phase 4DCT image set. The 4DCT phase with a lung volume and diaphragmatic position closest to the 4DcCT AIP image was selected as the reference phase. ITV_4DcCT and IRV_4DcCT were mapped to this reference phase, and renamed CTV_4DCT and OAR_4DCT, respectively. These structures were then propagated across all 10 4DCT phases to obtain ITV_4DCT and IRV_4DCT, representing the combined effect of respiratory and cardiac motion (Figure 1).

Figure 1 Workflow for generating the ITV_4DCT and IRV_4DCT from the 4DcCT and 4DCT datasets. The left panel shows the image datasets, and the right panel shows the propagation and merging of regions of interest. For 4DcCT, the CTV and OARs were manually delineated in the reference (0%) phase and propagated across the remaining cardiac phases via deformable image registration, followed by merging to form the ITV_4DcCT and IRV_4DcCT. The AIP image was then rigidly registered to the 4DCT image set, and the closest respiratory phase was selected as the 4DCT reference. The ITV_4DcCT and IRV_4DcCT were transferred to this phase and propagated across all the respiratory phases to generate the ITV_4DCT and IRV_4DCT, which incorporated both cardiac and respiratory motion. 4DCT, four-dimensional computed tomography; 4DcCT, four-dimensional cardiac computed tomography; AIP, average intensity projection; CTV, clinical target volume; IRV, internal organ-at-risk volume; ITV, internal target volume; OARs, organs at risk; ROI, region of interest.

The CSBRT plan was generated on the reference phase of the 4DCT using the Eclipse treatment planning system (version 13.5, Varian Medical Systems Inc., Palo Alto, CA, USA). Treatment planning employed volumetric modulated arc therapy (VMAT) with two arcs of 6 MV FFF X-rays, delivered at a maximum dose rate of 1,400 MU/min. The dose was calculated using the anisotropic analytical algorithm with a 2-mm grid size. To further characterize the plan quality, we also calculated the plan complexity metrics; the calculation method and results are detailed in Appendix 1 (Section 1.1) and Table S1. The prescribed dose was 25 Gy in a single fraction. In addition, the detailed dose constraints for the target and OARs are provided in Table S2. The dose constraints to cardiac substructures were minimized as much as reasonably possible, given the absence of well-defined tolerance thresholds for the single-fraction irradiation of these critical structures.

Simulation of cardiorespiratory motion

This study employed a Dynamic Cardiac 008C phantom (CIRS Inc., Norfolk, VA, USA) to simulate both respiratory motion in the SI direction and three-dimensional (3D) cardiac pulsation. The 3D cardiac motion was created by merging a one-dimensional (1D) vector in the SI direction with a rotation vector perpendicular to SI aligned with the spiral motion features of the human heart (15). To mimic respiratory motion in the SI direction, the “cos⁴” function was used (23). Figure 2 shows the simulation scenarios, including cardiac pulsation alone (the C group), respiratory motion alone (the R group), and combined cardiorespiratory motion (the CR group) (for further information, see Figure 3 and Table S3).

Figure 2 Setup of cardiac, respiratory, and cardiorespiratory motion simulations using the Dynamic Cardiac 008C phantom. (A) Components of the Dynamic Cardiac 008C phantom, including the actuator unit, control system, and thoracic insert. (B) Setup for CT simulation using the 008C phantom with the motion actuator attached. (C) Cardiac motion insert and EBT-XD film setup. The film plane corresponds to the coronal plane of the heart, with the SI and AP directions indicated. (D) Irradiation setup on a Varian Edge linear accelerator. The phantom was programmed to generate combined cardiac and respiratory motion during dose delivery. AP, anterior-posterior; CT, computed tomography; SI, superior-inferior.

Figure 3 Motion parameter settings for the C, R, and CR groups. In the CR group, combinations of motion parameters in the 90 beats per minute (bpm)/120 bpm states were transmitted only through the red line. C group, cardiac motion only; R group, respiratory motion only; CR group, combined cardiac and respiratory motion. AP, anterior-posterior; LR, left-right; SI, superior-inferior.

Dose measurement

Dose measurements were performed using a medical electron linear accelerator (Edge; Varian Medical Systems). For dosimetry in this study, External Beam Therapy-eXtended Dose (EBT-XD) film (Ashland Inc., Bridgewater, NJ, USA) with a measurement range of 0–40 Gy was used. All the irradiated films were scanned 24 hours post-irradiation using the Expression 11000XL scanner (Epson, Suwa, Japan) with a one-scan protocol (24), and analyzed in the red channel by Sun Nuclear Corporation (SNC) patient software (version 8.4.1; SNC, Melbourne, FL, USA). The scanner setup employed the transmission mode with 48-bit color depth, a spatial resolution of 75 dpi, and no color correction. The film pieces were clamped into the cardiac motion component, securely positioned as illustrated in Figure 2C, and set at the resting zero position. Subsequently, the Cardiac 008C phantom was activated (Figure S1), and dosimetric measurements for CSBRT were performed according to the motion parameters specified for the C group (6 groups), R group (8 groups), and CR group (20 groups). Reference doses for each motion parameter group were measured under static phantom conditions.

Two specific groups (Table S3, rows 3 and 4 from the bottom) in the CR group underwent dosimetry using the respiratory-gating technique. The respiratory gating for scanners and cameras (RGSC) system (Varian Medical Systems) was employed, and the irradiation was performed with end-exhalation gating (Figure 4).

Figure 4 Experimental setup for respiratory gating-based dose measurement using the RGSC system. (A) Experimental setup of the respiratory gating-based system using three infrared cameras (Cameras 1–3) to track a surrogate marker placed on the phantom. (B) Motion curve showing the programmed surrogate respiratory waveform (“cos⁴” waveform) used to generate the respiratory motion signal for the gating measurement. ECG, electrocardiogram; RGSC, respiratory gating for scanners and cameras; ROI, region of interest.

Given that CSBRT may commence at any phase of cardiorespiratory motion in patients, we investigated the effect of the onset phase on CSBRT dosage. Three repetitions were performed for each of the three motion pattern groups—small amplitude, medium amplitude, and large amplitude—in the CR group. The onset phase of the cardiorespiratory motion was randomized for each measurement.

To ensure the reliability of the dose measurements, six repeated dose measurements were conducted using a static phantom, with each measurement separated by an interval of more than 1 week. Additionally, to validate the accuracy of the film-based dose measurements, the differences between the calculated doses extracted from the film plane and the actual measured doses were compared.

Dose extraction from the film plane

The CSBRT plan, as detailed in the plan design section, was transferred to the CT images obtained by scanning the static cardiac phantom on a Revolution ES CT simulator (GE Healthcare) to recalculate the 3D dose with a dose grid of 2 mm. Subsequently, the Digital Imaging and Communications in Medicine files of the CT images, plan, and dose were imported into the SNC patient dose validation system. The 3D dose extraction function module of the system was used to extract the 2D dose distribution of the film plane (Figure 5).

Figure 5 Workflow of CSBRT plan recalculation on static phantom CT and extraction of film plane dose using the SNC patient system. The static cardiac phantom was scanned using a CT simulator, and the CSBRT plan was recalculated on the acquired images. The plan and dose were then imported into the SNC patient system, where the 3D dose distribution was used to extract the 2D dose on the film plane for validation. The red arrows indicate the plane where the film was inserted into the moving phantom to measure the dose during the simulation of different motion patterns. 2D, two-dimensional; 3D, three-dimensional; CSBRT, cardiac stereotactic body radiotherapy; CT, computed tomography; SNC, Sun Nuclear Corporation.

Dose analyses

The dose distributions measured under different motion parameters were compared with the static reference dose. The gamma passing rate (GPR) was calculated using the 3%/2 mm criterion to evaluate the dosimetric agreement. 1D dose profiles were extracted along the SI and AP directions at the isocenter (Figure 2C). Based on the SI-direction profiles, we calculated the 80% and 90% isodose widths (IDWs) and the penumbra widths, defined as the distance between the 80% and 20% dose levels on the superior and inferior sides (Figure S2). Additional gamma criteria (3%/3 mm, 2%/2 mm), the 50% IDW (SI direction), and the AP-direction indices were calculated and are presented in Appendix 1 (Section 2).

Results

Accuracy and reproducibility of film-based dose measurement

To validate the accuracy of the film-based dosimetry, we compared the calculated dose distribution in the film plane with the measured static dose. At the 3%/2 mm gamma criterion, the GPR was 97.8%. Additional validation results, including GPR values at the 3%/3 mm and 2%/2 mm criteria, are provided in Appendix 1 (Section 2.2).

Stability testing over five repeated static measurements revealed a GPR of 99.3%±0.6% at the 3%/2 mm criterion (Figure S7), confirming high reproducibility.

Dosimetric effects of cardiac pulsation in CSBRT

The GPR (3%/2 mm) decreased as the amplitude of cardiac pulsation increased (Table S4); for example, increasing the amplitude from group 2 (SI: 2.5 mm) to group 3 (SI: 3.75 mm) decreased the GPR from 97.0% to 71.6%. Dose deviations were concentrated in the dose fall‑off region. As the pulsation amplitudes increased, the 90% and 80% IDWs narrowed and the penumbra in the SI direction widened, indicating edge blurring at the target (Figure 6). The results for the APdirection indices and the minor dependence on heart rate are provided in Appendix 1 (see Sections 2.1 and 2.4).

Figure 6 Dosimetric analysis under different cardiac pulsation amplitudes. (A) Dose difference maps across four cardiac motion groups: Group 1–4, representing small to large amplitudes in the SI, AP, and LR directions, respectively. (B) Variations in the IDW at the 50%, 80%, and 90% levels in the SI (Y) direction, and at the 80% and 90% levels in the AP (X) direction. (C) Penumbra width variation in the SI direction, separately assessed at the inferior (I-side) and superior (S-side) edges. AP, anterior-posterior; IDW, isodose width; LR, left-right; SI, superior-inferior.

Dosimetric effects of respiratory motion in CSBRT

Consistent with the cardiac motion group, the GPR (3%/2 mm) decreased as the respiratory motion amplitude increased (Table S4). For example, compared to the 2.5-mm amplitude, the 10-mm amplitude led to a GPR decrease of over 40% under both the 3‑ and 5‑s respiratory cycles.

Similarly, the 90% and 80% IDWs in the SI direction narrowed substantially as the amplitude increased, reflecting compromised target coverage (Figure 7). Conversely, the effect of the respiratory cycle duration was minimal, with ≤1.1 mm deviations in the IDWs when the cycle was extended from 3 to 5 s.

Figure 7 Variations in the key dosimetric indices with increasing respiratory amplitude and different respiratory periods under respiratory motion alone. (A) 90% IDW variation along the SI (Y) and AP (X) directions. (B) 80% IDW variation along the SI (Y) and AP (X) directions. (C) Penumbra width variation in the SI direction, evaluated separately at the inferior (I-side) and superior (S-side) edges. AP, anterior-posterior; IDW, isodose width; SI, superior-inferior.

Detailed regression slopes and additional results, including the results of the 50% IDW analysis, are provided in Appendix 1 (Sections 2.3 and 2.5).

Dosimetric effects of cardiorespiratory motion in CSBRT

When both the cardiac and respiratory motion amplitudes were high, the mean GPR (3%/2 mm) decreased to 42.6%, indicating a significant degradation in dose conformity. Conversely, changes in the respiratory cycle duration and heart rate had minimal effects on the GPR, with deviations of only 0.2% and 5.1%, respectively (Table S4).

The penumbra widths in the SI direction increased as the respiratory amplitudes increased, reflecting worsened dose sharpness at the target edge. The cardiac pulsation amplitude had a relatively smaller effect on the penumbra width in this direction, with mean deviations around 1.8 mm (Figure 8). The effect of the respiratory cycle was more pronounced in the AP-direction dose metrics than the SI-direction dose metrics; for further details and the slope fitting results, see Appendix 1 (Sections 2.4 and 2.5).

Figure 8 Dose metric variations under different cardiorespiratory motion amplitudes and respiratory cycles. (A) Variation in the 90% IDW along the SI direction. (B) Variation in the 80% IDW along the SI direction. (C) Penumbra width variation on the inferior-side. (D) Penumbra width variation on the superior-side. IDW, isodose width; SI, superior-inferior.

Comparison of motion-induced dose degradation patterns in CSBRT

As shown in Figure 9, all types of motion—cardiac, respiratory, and cardiorespiratory—introduced widespread dose degradation compared to the static reference. These included alternating cold and hot spots in the dose fall-off region and reduced dose homogeneity in the target, consistent with pronounced dose-blurring and interplay effects.

Figure 9 Dose difference for (A) cardiac pulsation alone, (B) respiratory motion alone, and (C) cardiorespiratory motion compared with the resting state.

Dosimetric improvement from respiratory gating in CSBRT

Compared to free-breathing delivery, respiratory gating improved dose precision, increasing the GPR (3%/2 mm) by an average of 19.4% under the same motion conditions, it also reduced deviations in the 1D dose profiles (Table 1). However, as the cardiac pulsation amplitude increased, the GPR continued to decrease, and dose variations—especially those in the AP direction—persisted. These results highlight the partial but significant benefit of gating. Further dosimetric evaluations are detailed in Appendix 1, including the GPR results under alternative gamma criteria (Section 2.2) and the results of the 50% IDW analysis in the SI direction (Section 2.3).

Minimal dosimetric effects of the motion onset phase in CSBRT

Across repeated measurements of small, medium, and large motion scenarios, the GPRs (3%/2 mm) remained above 99.7%, with no significant variation across different onset phases (Table S5). The spatial distribution of the dose deviations was also consistent across repetitions (Figure S6), suggesting that the onset phase of cardiorespiratory motion had a minimal effect on CSBRT dosimetry.

Discussion

In this study, we systematically evaluated the dosimetric effects of complex cardiorespiratory motion in CSBRT using a dynamic cardiac phantom. Our results demonstrated that increased motion amplitude, particularly respiratory amplitude, led to significant degradation in the GPR and isodose coverage, while heart rate, respiratory cycle, and onset phase only had minimal effects. Further, we confirmed that respiratory gating can partially mitigate these dosimetric degradations.

Previous studies on motion effects in CSBRT have been limited in scope (22). For example, Harms et al. found that cardiac motion alone reduced target coverage from 27.0 to 20.5 Gy. Our study extended these findings by modeling compound motion patterns and generating extensive dosimetric datasets, revealing that combined cardiorespiratory motion induces more severe dose-blurring and interplay effects than either component alone. While no dosimetric studies have directly simulated cardiorespiratory motion, previous work based on respiratory motion alone has consistently reported dose-blurring effects along the motion direction, including target undercoverage, penumbra widening, and alternating cold and hot spots at the target periphery (25-27). Using a 1D respiratory motion lung phantom, Palmer et al. (27) found that a 7-mm amplitude reduced the GPR from 96.9% (static) to 82.5% at the 2%/2 mm criterion—a result comparable to our findings under respiratory motion alone. Similarly, Fernandez et al. (25) used a respiratory motion phantom to simulate lung radiotherapy with FFF-VMAT, and observed a linear decrease in the 95% IDW as the amplitude increased; at the 2-cm amplitude, dose errors near the target edge reached up to 20%, particularly under complex plans. Consistent with these studies, our results also demonstrated that respiratory motion in CSBRT reduced target dose coverage and increases peripheral irradiation, reflecting similar dose degradation patterns.

Mechanistically, we observed dose ambiguity characterized by a narrowing of the 90% and 80% IDWs, and an expansion of the penumbra regions, especially along the SI axis. A decrease in the overall uniformity of the target area can be conceptualized as an interaction effect. Dose ambiguity represents a convolution-like averaging effect of motion within the target (28-32). Conversely, interplay effects are the result of the dynamic interaction between target motion and a modulated irradiation field (33-35). Unlike conventional fractionated treatments where such interplay effects are averaged out (36-38), single-fraction high-dose CSBRT may amplify interplay-related dosimetric uncertainties (39-41). Notably, the treatment plans in this study were generated using motion-informed approaches rather than static CT. Cardiac and respiratory motion were captured through ECG-gated 4DcCT and respiratory 4DCT, respectively, and target volumes were delineated using structure propagation across 10 cardiac phases and fusion on AIP images, analogous to ITV generation in thoracic radiotherapy. Therefore, the interplay and dose-blurring effects observed in this study should be interpreted as residual uncertainties that persist despite the use of motion-informed planning, underscoring the need for effective motion management, such as respiratory gating.

In this study, heart rate, respiratory cycle, and motion onset phase had minimal effects on CSBRT dosimetry. For example, the difference in the GPR between the 3‑ and 5s respiratory cycles was only 0.2%. Conversely, previous studies have shown that in conventional fractionated VMAT based on respiratory motion simulations, respiratory cycle variability enhances interplay effects and reduces target dose homogeneity (31). This discrepancy may be explained by the use of single-fraction CSBRT with a high prescribed dose (of up to 25 Gy) and prolonged beam-on time, which increases temporal averaging and reduces the dosimetric impact of the interplay effects (39).

Controlling the amplitude of cardiorespiratory motion during dose delivery may be key to reducing associated dose uncertainties in CSBRT. Our findings confirmed that respiratory gating improved the GPR and reduced dose blurring in the SI direction, supporting its clinical utility. However, gating alone was insufficient to fully mitigate cardiac-induced uncertainties, particularly in the AP direction, where residual errors persisted. Until cardiac-gated delivery becomes clinically available, respiratory gating remains the most viable mitigation method. Notably, international clinical trials on CSBRT have already incorporated respiratory gating to manage complex cardiorespiratory motion patterns. Future technologies should aim to address multi-dimensional motion compensation more holistically.

The importance of standardized workflows and multi-institutional reproducibility has been highlighted in recent international CSBRT planning efforts, such as the STOPSTORM (13) and VT-ART (14) studies. These initiatives emphasize the need to better understand motion-induced uncertainties, reinforcing the relevance of our dosimetric analysis in refining such protocols. While this study provides valuable insights into the dosimetric effects of cardiorespiratory motion, several limitations should be acknowledged. First, treatment plan robustness against motion variability was not quantitatively assessed, limiting our ability to evaluate performance under diverse motion scenarios. Second, this study compared the dose differences in measured CSBRT to account for the changes in CSBRT dosimetric characteristics due to different cardiorespiratory motions; however, it only focused on a single CSBRT plan and the findings were not extended to multiple CSBRT plans. Third, this study focused exclusively on VMAT-based delivery. The dosimetric effects of cardiorespiratory motion under other techniques, such as intensity-modulated proton therapy (IMPT), have yet to be investigated. Although IMPT may offer superior dose conformity, it could also be more sensitive to motion-related uncertainties and warrants further study. In addition, the present analysis was limited to two-dimensional film measurements, which precluded volumetric dose evaluation. Future work will include incorporating volumetric dose difference metrics using advanced 3D dosimetric platforms such as stacked film arrays, electronic portal imaging device-based reconstructions, or gel dosimeters, and performing end-to-end analyses across multiple CSBRT cases to comprehensively characterize dose-blurring and interplay effects under real-world conditions.

Conclusions

This study systematically quantified the dosimetric effects of complex cardiorespiratory motion in CSBRT. Our findings indicate that motion amplitude—rather than heart rate, respiratory cycle, or onset phase—is the dominant factor contributing to dose degradation. Increased amplitude resulted in underdosage at target edges and the emergence of cold and hot dose spots across all spatial directions. The application of respiratory gating significantly improved dosimetric outcomes, particularly in the SI direction, although residual uncertainties remained in the AP direction due to cardiac pulsation. Based on these results, we recommend that patients undergoing CSBRT be individually evaluated for cardiorespiratory motion characteristics. For those with large-amplitude motion, more stringent motion management strategies should be considered to ensure adequate dose coverage and treatment precision.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1443/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1443/dss

Funding: This work was supported by the National Natural Science Foundation of China (Nos. 12475348, 12205209 and 62401636).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1443/coif). G.W. reports that this work was supported by the National Natural Science Foundation of China (No. 62401636). Q.X. reports that this work was supported by the National Natural Science Foundation of China (No. 12205209). G.L. reports that this work was supported by the National Natural Science Foundation of China (No. 12475348). The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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