Surface-Guided Radiation Therapy (SGRT) for Pediatric Oncology: An Analysis of Inter- and Intra-fraction Motion Variability and PTV Margins

Abstract

Introduction

To quantify inter- and intra-fraction motion variability in pediatric radiotherapy using surface-guided radiation therapy (SGRT) and evaluate its impact on personalized planning target volume (PTV) margins.

Methods

A retrospective analysis included 27 pediatric cancer patients (≤15 years old). Fourteen patients received conventional laser alignment based on skin markings (LAS group), while thirteen underwent SGRT for initial setup and continuous real-time intra-fraction monitoring. Inter-fraction errors were assessed using pre-treatment cone-beam CT (CBCT), and intra-fraction motion was captured via SGRT log files. Population systematic (Σ) and random (σ) errors were calculated. PTV margins compensating for setup uncertainties were derived using the van Herk formula. Dosimetric impact was evaluated by recalculating plans using SGRT-informed anisotropic margins compared to the applied 5 mm isotropic margins.

Results

SGRT improved inter-fraction setup accuracy over LAS, reducing median translational errors to 1.1 mm (lateral), 1.5 mm (longitudinal), and 1.7 mm (vertical), compared with 2.6 mm, 3.6 mm, and 1.9 mm (lateral/longitudinal, P < 0.001). Rotational errors were reduced to ≤ 0.7° (YAW/Roll, P < 0.05). Population systematic errors for inter-fraction setup with SGRT were 0.6-1.1 mm versus 1.6-2.2 mm for LAS, and random errors were 1.0-1.7 mm versus 1.4-2.4 mm. SGRT quantified intra-fraction motion (medians: ≤ 0.65 mm for translations, ≤ 0.62° for rotations), with more than 86% of displacements within ±2 mm and more than 91.5% of rotations within ±2°. When intra-fraction uncertainties dominated, anisotropic margins derived from SGRT data reduced mean PTV volume by 11% (P < 0.001) while maintaining CTV coverage, achieving significant dose reductions to the rectum, bladder, and small intestine.

Conclusions

SGRT is an effective modality for significantly improving inter-fraction setup accuracy and enabling real-time intra-fraction motion monitoring in pediatric radiotherapy. It may allow individualization of PTV margins, reducing irradiated volumes and sparing critical organs.

Keywords

surface-guided radiation therapy pediatric radiotherapy inter-fraction motion intra-fraction motion PTV margin

Introduction

Pediatric cancer has emerged as a significant health challenge globally, with approximately 429,000 children and adolescents aged 0 to 19 diagnosed annually worldwide.¹ In China, the incidence rate of childhood cancer among children aged 0 to 14 years was approximately 122.86 cases per million between 2018 and 2020.² Radiation therapy (RT) serves as a critical component in the multidisciplinary management of pediatric malignancies, playing an indispensable role in treating various childhood tumors.^3,4 However, pediatric patients typically exhibit lower compliance than adults, which increases the sensitivity of dose delivery accuracy to motion-related errors during treatment.

Motion errors during radiotherapy encompass both inter-fraction and intra-fraction variations. Inter-fraction errors are typically corrected using daily cone-beam computed tomography (CBCT),⁵ a cornerstone of image-guided radiation therapy (IGRT) that provides three-dimensional visualization of tumors and surrounding anatomical structures, enabling accurate pre-treatment positional adjustments.⁶ However, CBCT is limited in its ability to provide continuous real-time monitoring during treatment delivery, thus failing to capture detailed intra-fraction motion. Furthermore, each CBCT scan contributes additional ionizing radiation exposure to the patient.^7,8

Surface-guided radiation therapy (SGRT) utilizes non-invasive optical imaging to achieve real-time 3D reconstruction of the patient’s surface. By eliminating the need for external markers and avoiding additional ionizing radiation exposure, SGRT is primarily used to assist in pre-treatment positioning and continuous intra-fraction motion monitoring.⁹ In recent years, clinical applications of SGRT in adult oncology have expanded significantly, particularly for deep inspiration breath-hold treatments in breast cancer,¹⁰ non-coplanar cranial therapies,¹¹ and RT for tumors located at various anatomical sites including extremities.¹² SGRT has demonstrated benefits in improving treatment efficiency, reducing patient displacement, and shortening treatment times. Guo et al. further demonstrated that SGRT can support personalized reduction of planning target volume (PTV) margins for adult lung cancer patients undergoing stereotactic body radiotherapy (SBRT), thereby sparing normal tissues.¹³

Although magnetic resonance-guided radiotherapy (MRgRT) similarly avoids additional radiation exposure¹⁴ and permits reduced PTV margins,¹⁵ its clinical implementation in pediatrics faces significant limitations including prolonged anesthesia requirements, large-field irradiation constraints, and extended treatment sessions (averaging >45 minutes per fraction).¹⁶ These factors restrict its applicability for many pediatric patients. In such scenarios, SGRT emerges as a practical alternative that balances precision with clinical feasibility.

Despite these advancements, the adoption of SGRT in pediatric radiotherapy remains limited. A survey conducted by the European Society for Paediatric Oncology reported that only 33 out of 246 affiliated radiotherapy centers utilized SGRT for pediatric patients, underscoring a critical implementation gap.¹⁷ Notably, emerging evidence supports SGRT’s value in high-risk pediatric scenarios, such as anesthesia-contraindicated cases or patients with severe positional intolerance, by enabling automatic beam hold. For instance, a Wilms tumor case with mediastinal compression achieved safe palliation using SGRT despite the inability to maintain supine positioning.¹⁸ To ensure radiotherapy precision, a comprehensive assessment of inter- and intra-fraction motion uncertainties is essential for determining appropriate setup margins. However, published data guiding appropriate SGRT-based specific PTV margins for pediatric patients remain scarce. This study aims to quantitatively assess inter- and intra-fraction motion errors during radiotherapy in pediatric patients managed with SGRT, and to further investigate the impact of SGRT on personalized PTV margin optimization and the associated potential dosimetric consequences.

Materials and Methods

Patient Selection

A retrospective analysis was conducted on 27 pediatric oncology patients who received radiotherapy in the Department of Radiation Oncology between November 2023 and September 2024. Patient informed consent was not required for this retrospective study. Inclusion criteria were: (1) age ≤18 years; (2) histopathologically confirmed pediatric malignancy; (3) completed the full course of radiotherapy. Exclusion criteria: (1) patients who failed to complete the scheduled radiotherapy due to intolerance, disease progression or other unplanned reasons; (2) patients with incomplete setup data, CBCT images or SGRT logfiles that could not support quantitative motion variability analysis. Among them, 14 patients used only the traditional protocol positioning method (laser alignment based on skin markings, LAS group), and 13 children underwent full-process management with SGRT (SGRT group). Prior to treatment, the guardians were informed of potential risks and the clinical procedures. The reporting of this study conforms to the STROBE guidelines.¹⁹ We have de-identified all patient details. The median age of the patients was 7 years (range: 3–15 years). Patient and treatment-related characteristics are summarized in Table 1.

Table 1.

Patient and Treatment Characteristics (27 Patients, 38 Lesions)

Characteristics, by patient	LAS	SGRT
Number of Patients	14	13
Median age(range, years)	8(3-15)	7(3-12)
Gender
Female	7	6
Male	7	7
Number of target volumes	20	18
Number of CBCT evaluations	103	200
SGRT logfiles	-	207
Diagnosis(number of patients)
Neuroblastoma	7	6
Rhabdomyosarcoma	2	4
Hepatoblastoma	1	-
Sarcoma	4	3
Treated site(number of target volumes)
Head and Neck	2	-
Thorax	3	2
Abdomen	2	2
Pelvis	6	5
Extremity	7	9
Fractionation regimen
Conventional fractionation	7	9
SBRT	13	9

Note: Fractionation regimen numbers are based on target volumes (treatment courses). Some patients contributed multiple target volumes.

CT Simulation and Radiation Therapy

All patients were positioned in a supine posture on vacuum cushions under free-breathing conditions. Images were acquired using a Philips Spectral CT with a slice thickness of 3 or 5 mm. Once the scanning was completed, the obtained images were transferred in DICOM format to the Eclipse TPS workstation. Following this, radiation oncologists meticulously outlined the clinical target volume (CTV) as well as the organs at risk (OARs). The planning target volume (PTV) was obtained by expanding the CTV isotropically by 5 mm. After the delineation was completed, the images and structure sets were imported into the Monaco 5.11 system to design volumetric modulated arc therapy (VMAT) or intensity-modulated radiation therapy (IMRT) plans. All plans, including those for conventional fractionation, were delivered solely through IMRT or VMAT, and no 3D conformal plans were utilized. All plan designs were based on a Versa HD (Elekta AB, Stockholm, Sweden) accelerator, which was equipped with an Agility multileaf collimator (MLC) with a total of 160 leaves and a spatial resolution of 5 mm. The Monte Carlo algorithm was used for 3D dose calculation, and the statistical uncertainty for each calculation was 0.5%.

SGRT System and Clinical Workflow

The SGRT workflow utilized the AlignRT system (Version 5.1.2; VisionRT Ltd., London), which consisted of three ceiling-mounted stereoscopic cameras that reconstructed patient surfaces using optical speckle projection.²⁰ Six-degree-of-freedom (6DOF) displacements (translational and rotational) were calculated via rigid registration with the reference CT-derived region of interest (ROI). ROI delineation prioritized comprehensive utilization of all three camera pods to minimize occlusion-induced positional inaccuracies during gantry rotation, covering the PTV’s anatomical surface projection including adjacent rigid structures such as joints.

Before each treatment session, the therapist initially performed patient positioning using the conventional skin-marking technique in conjunction with laser alignment, followed by fine-tuning based on real-time surface imaging and 6DOF data obtained from the SGRT system. A tolerance threshold of 3 mm/3°was applied. When the 6DOF deviation approached zero, a CBCT scan was conducted for image registration. If the registration result was within 1 mm/1°, the setup was deemed acceptable and no further correction was necessary. Otherwise, the treatment couch was automatically adjusted to correct the positional errors. Following the correction, the SGRT system captured a new reference surface to continuously monitor patient motion throughout the treatment. The 6DOF deviations derived from CBCT scans prior to treatment were classified as inter-fraction setup errors, while the real-time displacements recorded by the SGRT system during treatment were defined as intra-fraction motion errors. All positional data were exported for offline analysis.

PTV Margin Analysis

The mean and standard deviation of inter-fraction and intra-fraction errors were calculated for all treatment fractions of each patient, representing the individual systematic error and individual random error, respectively. The population systematic error (Σ) was determined as the standard deviation of the individual systematic errors, while the population random error (σ) was computed as the root-mean-square of the individual random errors. PTV margins were calculated using the van Herk formula: M_PTV = 2.5Σ+0.7σ, ensuring that 95% of the prescribed dose covers 90% of the volume of the CTV.²¹

Statistical Analysis

GraphPad Prism 8 was employed for data visualization. Data normality was assessed using the Shapiro-Wilk test. For variables that were not normally distributed, descriptive statistics were presented as median and interquartile range (IQR). Intergroup comparisons were conducted using the Mann-Whitney U test, while the Wilcoxon signed-rank test was applied for comparing dosimetric parameters within groups. A P value < 0.05 indicates statistically significant differences.

Results

Comparison of SGRT to LAS Group in Inter-fraction Positioning

A comparative analysis of inter-fraction positioning accuracy revealed statistically significant differences in median errors between the LAS and SGRT cohorts. Figure 1 shows the boxplot of the inter-fraction errors. For translational displacements, the LAS group exhibited significantly larger median deviations compared to the SGRT group in lateral (2.6 mm [IQR:1.0-3.6] vs. 1.1 mm [IQR: 0.5-2.0]; P < 0.0001) and longitudinal directions (3.6 mm [IQR: 1.8-5.7] vs. 1.5 mm [IQR: 0.7-3.3]; P < 0.0001). No statistically significant difference was observed for vertical displacements (1.9 mm [IQR: 0.8-3.6] vs. 1.7 mm [IQR: 0.9-3.0]; P = 0.2196). Rotational errors followed a similar trend, with the LAS cohort showing greater median YAW deviations (1.0° [IQR: 0.5-1.6] vs. 0.55° [IQR: 0.3-0.8]; P < 0.0001) and Roll deviations (0.9° [IQR: 0.5-1.5] vs. 0.7° [IQR: 0.3-1.3]; P = 0.0244). In contrast, Pitch deviations were comparable between the two groups (0.8° [IQR: 0.4-1.4] vs. 0.7° [IQR: 0.3-1.1]; P = 0.083).

Figure 1.

Boxplot of inter-fraction positioning accuracy in the LAS group and the SGRT group in pediatric radiotherapy. (A) Translational errors; (B) Rotational errors. VRT: vertical; LNG: longitudinal; LAT: lateral. Bold lines represent the medians. Box limits indicate the 25th and 75th percentiles. The “+” symbol denotes the mean. Statistical significance is indicated as follows: **** P < 0.0001; * P < 0.05; n.s.: not significant

Monitoring and Quantification of Intra-fraction Motion Variation

The SGRT system demonstrated high sensitivity in detecting and quantifying intra-fraction motion during pediatric radiotherapy. Intra-fraction motion analysis in the SGRT cohort revealed median absolute deviations of 0.64 mm (vertical), 0.65 mm (longitudinal), and 0.48 mm (lateral) for translational displacements, alongside rotational deviations of 0.45° (YAW), 0.62° (Roll), and 0.59° (Pitch). Frequency analysis indicated that the majority of translational and rotational errors were effectively controlled within clinical thresholds, as illustrated in Figure 2. Specifically, over 86% of translational errors across all directions remained within ±2 mm, while at least 91.5% of rotational errors were confined to ±2°. When extending the range to ±3 mm for translations and ±3° for rotations, the respective percentages rose to over 92.4% and 97.6%.

Figure 2.

Frequency distribution histogram of intra-fraction motion errors in translational(A) and rotational(B) dimensions in SGRT-guided pediatric radiotherapy. VRT: vertical; LNG: longitudinal; LAT: lateral

Exploratory Analysis of the Relationship Between Motion Errors and Patient Age

To investigate the relationship between motion magnitude and patient age, Spearman’s rank correlation analysis was conducted between the age (range: 3–12 years) and the median absolute errors in each of the six degrees of freedom for the 13 patients in the SGRT group.

For inter-fraction errors, significant positive correlations were observed for VRT (r = 0.63, 95% CI: 0.11 to 0.88, P = 0.02) and LAT (r = 0.58, 95% CI: 0.02 to 0.86, P = 0.04), whereas a significant negative correlation was found for YAW (r = −0.62, 95% CI: −0.88 to −0.09, P = 0.03). No significant correlations were detected for LNG (r = −0.33, P = 0.28), Roll (r = −0.35, P = 0.24), or Pitch (r = 0.29, P = 0.34).

For intra-fraction errors, a significant negative correlation was observed only for YAW (r = −0.56, 95% CI: −0.86 to −0.002, P = 0.05). No significant correlations were found for VRT (r = 0.06, P = 0.86), LNG (r = −0.08, P = 0.79), LAT (r = −0.05, P = 0.88), Roll (r = −0.12, P = 0.70), or Pitch (r = −0.41, P = 0.16).

These results indicate that certain directional components (particularly YAW) show significant correlations with age. However, given the modest sample size (n = 13), these findings should be interpreted with caution. The scatter plots are presented in Figure 3.

Figure 3.

Scatter plots of motion errors versus patient age in the SGRT group. Each point represents the median absolute error calculated from all fractions for a single patient. (A) Inter-fraction errors; (B) Intra-fraction errors. VRT: vertical; LNG: longitudinal; LAT: lateral

PTV Margin Analysis Based on Inter- and Intra-fraction Errors

The SGRT group demonstrated reduced population systematic (Σ) and random (σ) inter-fraction errors compared to the LAS group in all translational directions (Table 2). In the SGRT group, the recommended PTV margin calculated based on the Van Herk formula was significantly reduced. Specifically, the VRT margin decreased from 5.6 mm with LAS (Σ = 1.7 mm, σ = 1.7 mm) to 3.4 mm with SGRT (Σ = 0.9 mm, σ = 1.5 mm). Similarly, the LNG margin was reduced from 7.1 mm (LAS: Σ = 2.2 mm, σ = 2.4 mm) to 4.0 mm (SGRT: Σ = 1.1 mm, σ = 1.7 mm), and the LAT margin from 5.1 mm (LAS: Σ = 1.6 mm, σ = 1.4 mm) to 2.2 mm (SGRT: Σ = 0.6 mm, σ = 1.0 mm).

Table 2.

Inter-fraction Errors-Driven PTV Margins for LAS and SGRT Groups

Direction(mm)	LAS group			SGRT group
Direction(mm)	Σ	σ	PTV margin	Σ	σ	PTV margin
VRT	1.7	1.7	5.6	0.9	1.5	3.4
LNG	2.2	2.4	7.1	1.1	1.7	4
LAT	1.6	1.4	5.1	0.6	1	2.2

VRT: vertical; LNG: longitudinal; LAT: lateral.

Table 3 details the intra-fraction errors and the combined PTV margins for the SGRT group. Further analysis of the intra-fraction errors in the SGRT group indicated that the systematic errors in the VRT, LNG, and LAT directions were 1.0 mm, 0.9 mm, and 0.7 mm, respectively, while the random errors were 2.2 mm, 2.3 mm, and 2.2 mm, respectively. The PTV margins calculated based exclusively on intra-fraction errors were determined to be 4.1 mm, 4.0 mm, and 3.3 mm, respectively. When both inter- and intra-fraction errors were taken into account, the PTV margins increased slightly to 5.2 mm, 5.6 mm, and 4.0 mm, respectively.

Table 3.

SGRT-specific Intra-fraction Errors and Total PTV Margins

Direction(mm)	Intra-fraction error			Combined inter- and intra-fraction error
Direction(mm)	Σ	σ	PTV margin	PTV margin
VRT	1	2.2	4.1	5.2
LNG	0.9	2.3	4	5.6
LAT	0.7	2.2	3.3	4

VRT: vertical; LNG: longitudinal; LAT: lateral.

Given that inter-fraction errors were effectively mitigated through daily CBCT verification, intra-fraction errors have become the dominant factor affecting the PTV margin. The PTV margins were therefore personalized based solely on intra-fraction uncertainties (VRT: 4.1 mm; LNG: 4.0 mm; LAT: 3.3 mm), replacing the conventional 5 mm isotropic margin. For 18 pediatric target volumes, dose distributions were recalculated using identical beam parameters while maintaining CTV coverage objectives (V_100% > 98%). The dosimetric parameter changes in the target and organs at risk are presented in Figure 4. Implementation of SGRT-derived anisotropic margins significantly reduced the mean PTV volume from 203.2 to 180.9 cm³ (P < 0.001). Statistically significant dose reductions were observed in the rectum (mean dose decreased from 962.9 to 857.3 cGy, P = 0.02), bladder (mean dose decreased from 1466.2 to 1436.0 cGy, P = 0.04), and small intestine (D_2cc decreased from 2718.5 to 2600.4 cGy, P = 0.01). Dose reductions in other organs such as the heart, lungs, liver, and spinal cord were also observed, although these changes were not statistically significant.

Figure 4.

Dosimetric changes following implementation of SGRT-derived anisotropic margins. The vertical axis represents the percentage dose change relative to the original plan (positive values indicate dose reduction compared to the original plan, and negative values indicate dose increase). V_100% denotes the volume percentage covered by 100% of the prescription dose and D_xcc represents the dose received by x cm³. Bold lines represent the medians. Box limits indicate the 25th and 75th percentiles. The “+” symbol denotes the mean. Whiskers extend from the minimum to the maximum values

Discussion

This study provides the comprehensive quantification of inter- and intra-fraction motion management using SGRT in pediatric radiotherapy, demonstrating its dual role in enhancing motion management and enabling personalized margin optimization.

SGRT has been extensively studied for its ability to measure inter- and intra-fraction setup errors in adult patients, with several studies reporting excellent setup accuracy. For instance, Lee et al. observed minimal intra-fraction motion during reverse semi-decubitus breast radiotherapy, with systematic errors of 1.8-3.3 mm and random errors of 1.8-2.1 mm in translation, contributing to PTV margins of 6.5-10.2 mm pre-correction and 4.7-6.3 mm post-correction.²² Similarly, Mankinen et al. evaluated the inter-fraction variability of whole-breast VMAT irradiation and found that after initial patient setup using both CBCT and SGRT, a 5 mm CTV-PTV margin could completely cover the CTV in 85% of cases.²³ Studies on extremity sarcomas have reported translational errors of less than 2 mm and rotational errors of less than 1° with SGRT, enabling PTV margin reductions of 30-50% (e.g., 4.4-4.7 mm vs. 6.6-10.9 mm anisotropically) compared to laser-based setups.²⁴ Heinzerling et al. demonstrated that the inter-fraction deviation was within 5 mm after the application of SGRT, and that deviations exceeding 2 mm were detected in 73.5% of the treatment fractions.²⁵ In comparison, our findings revealed that SGRT significantly reduced both systematic (Σ) and random (σ) errors compared to conventional laser alignment, with median translational errors reduced by 1.5–2.1 mm (lateral/longitudinal, P < 0.001) and rotational errors by 0.2°–0.45° (YAW/Roll, P < 0.05) in the pediatric cohort aged 3 to 15 years. These findings suggest that the movement amplitudes of children under surface monitoring can reach levels comparable to those observed in adults.

In addition, SGRT captured intra-fraction motion (medians: ≤ 0.65 mm, ≤ 0.62°). More than 86% of the displacements were limited within ±2 mm, and 92.4% were within 3 mm—comparable to Guo et al.’s lung SBRT data.¹³ These findings suggest that a stricter gating threshold of 2 mm/2° could be clinically feasible for pediatric patients, potentially replacing the commonly used 3 mm/3° threshold. Meanwhile, in the era of IGRT, quantified intra-fraction motion variability directly informs PTV margin individualization because intra-fraction motion is more difficult to manage than the inter-fraction errors. The PTV margins for children calculated using the van Herk formula (3.3-4.1 mm) in our study were significantly smaller than the conventionally recommended 4-6 mm for adults.²⁶ However, these values align with margins of 2-4 mm reported in adult SBRT or DIBH scenarios with SGRT.¹³ Dickie et al. also reported small intra-fraction motion and recommended a 5mm PTV margin.²⁷ These findings were consistent with previous reports, indicating that children may tolerate smaller margins than adults due to their typically reduced respiratory organ motion amplitudes (e.g., pediatric kidney motion: 1.6 mm vs. 8.7 mm in adults in the craniocaudal direction).²⁸ It should be noted that our study did not directly compare the motion of pediatric and adult patients. High-quality immobilization, rather than age per se, is crucial for reproducibility. In our study, the observed difference in inter-fraction errors between the LAS and SGRT groups was related to the method, not to age. With SGRT providing continuous feedback, pediatric motion was effectively managed. Therefore, when SGRT is implemented in pediatric patients, it provides both a theoretical and empirical basis for applying personalized, potentially reduced margins, which is also consistent with the reports of Meijer et al.²⁸

SGRT’s real-time monitoring and beam-hold capabilities mitigate two critical pediatric concerns: anesthesia exposure and treatment safety. The avoidance of anesthesia is particularly valuable, as pharmacological interventions carry inherent risks—including neurotoxicity, cardiorespiratory depression, and drug resistance—while increasing treatment complexity and cost. Previous research results have demonstrated that non-pharmacological interventions—such as audiovisual distraction or sleep deprivation protocols—when integrated with SGRT, effectively eliminate anesthesia requirements. Ritchie et al. reported that 72.5% of children aged 3-6 years completed treatment without anesthesia, significantly reducing psychological and physiological burdens.²⁹ Liu et al. demonstrated that sleep deprivation combined with SGRT eliminated sedation needs in 96% of fractions for children aged 0-3 years, with intra-fraction motion comparable to sedated controls (0.30±0.45 mm vs. 0.42±0.46 mm, P = N.S).³⁰ Concurrently, SGRT enhances safety through its displacement-triggered beam-hold function (>3 mm threshold). This mechanism prevents geographic misses during positional drift—a vital safeguard for non-sedated or restless children. Our data substantiated this capability: All children were treated while awake. SGRT was used throughout the radiotherapy process. If the deviation exceeded the threshold, it could be adjusted back within the threshold within 2 seconds to allow continuous beam delivery; otherwise, the beam would be paused. Ultimately, more than 92.4% of the intra-fraction displacements remained within the tolerance range of ±3 mm/3°. The convergence of anesthesia avoidance and real-time motion control establishes SGRT as a transformative tool for pediatric radiotherapy, optimizing both therapeutic precision and children’s well-being.

While this study demonstrates SGRT’s effectiveness in managing motion during pediatric radiotherapy, several limitations should be noted. First, as a retrospective analysis, we did not systematically record overall setup times, precluding a quantitative assessment of SGRT’s workflow efficiency. Second, the diverse treatment sites may obscure differences in motion patterns across anatomical locations. Therefore, our findings should be considered as a general evaluation of SGRT across pediatric patients rather than site-specific recommendations. Future multi-center studies with larger sample sizes are needed to assess whether motion patterns and optimal PTV margins differ by tumor location. Third, the modest sample size (27 patients) limits statistical power, and the observational design precludes causal inference. Thus, our findings should be regarded as preliminary, and the lack of statistical significance for some dosimetric endpoints does not necessarily imply a lack of clinical effect.

Conclusion

Our study expands the application of SGRT to pediatric patients, demonstrating its association with improved inter- and intra-fraction setup accuracy. These capabilities suggest that SGRT may allow for reduction of PTV margins while maintaining treatment safety, although larger prospective studies are needed to confirm these observations.

Footnotes

ORCID iD

Zhen Zhou

Ethical Considerations

This study was approved by the Research Ethics Committee of Peking Union Medical College Hospital, Chinese Academy of Medical Sciences (approval number: S-K1355) on August 31, 2020 in Beijing, China. Patient informed consent was not required for this retrospective study.

Authors Contributions

ZZ conducted data analysis and wrote the manuscript. FJ and FZ assisted with part of the data acquisition. BY and WL were responsible for experimental design and supervision. JQ revised the manuscript and provided funding support. All authors contributed to the study and critically reviewed the manuscript.

Funding

This work was supported by the National Key Research and Development Program of China(Grant No.2022YFC2404606), the National High Level Hospital Clinical Research Funding (2022-PUMCH-B-116).

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Data Availability Statement

The data are available upon request.*

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