2310 - Impact of Surface-Guided Radiation Therapy (SGRT) on Inter- and Intra-Fractional Motion Errors and Dose Delivery in Pediatric Stereotactic Body Radiotherapy

Purpose/Objective(s): Stereotactic body radiotherapy (SBRT) amplifies sensitivity to motion errors due to its steep dose gradients, particularly in pediatric patients who are prone to intra-fractional motion, risking target underdosage. The clinical use of surface-guided radiation therapy (SGRT) in pediatric SBRT remains limited. This study aims to evaluate the impact of SGRT on inter- and intra-fractional motion errors and to quantify the dosimetric effects of residual intra-fractional motion on target coverage in pediatric SBRT.

Materials/Methods: This retrospective study included 11 pediatric patients (median age: 6 years, range: 3-11 years) with 19 lesions treated with SBRT (30 Gy in 5 fractions). Two cohorts were analyzed: nine treatment plans utilized SGRT with motion tracking system for pre-treatment patient setup and real-time monitoring during beam delivery, while ten non-SGRT plans relied on conventional positioning protocols. All patients underwent daily cone-beam computed tomography (CBCT) for inter-fractional 6 degrees of freedom (6DOF) error correction. Intra-fractional motion data were extracted from SGRT log files. Mean absolute intra-fractional translational displacements for each patient in the SGRT group were incorporated into the treatment planning system to simulate dose distribution deviations. Target coverage was recalculated and compared against baseline plans.

Results: A total of 95 CBCT datasets and 45 motion tracking system log files were analyzed. Inter-fractional 6DOF errors in the non-SGRT group significantly exceeded those in the SGRT cohort for translational displacements: lateral (3.3±2.4 mm vs. 1.3±1.1 mm, P<0.001), longitudinal (4.8±3.5 mm vs. 2.3±2.0 mm, P<0.001), and vertical (2.3±2.0 mm vs. 2.0±1.7 mm, P=0.785). Rotational errors showed comparable trends, with yaw deviations reduced in the SGRT group (1.3±0.8° vs. 1.0±0.8°, P=0.023), while differences in pitch (0.9±0.7° vs. 0.8±1.1°, P=0.085) and roll (1.2±0.8° vs. 0.9±0.8°, P=0.065) were non-significant.

Intra-fractional motion analysis in the SGRT cohort revealed maximum translational and rotational displacements within ±5 mm/5°, with mean absolute errors of 1.1±2.3 mm (lateral), 1.3±2.4 mm (longitudinal), 1.2±1.8 mm (vertical), and =1.1° across rotations. Frequency analysis indicated that 98.86% of translational errors were within ±1 mm, and at least 88% of rotational errors were within ±2°. In pediatric SBRT plans, dose recalculation incorporating intra-fractional translational errors demonstrated a median reduction of 3.51% (interquartile range: 1.1% to 7.13%) in target coverage.

Conclusion: SGRT effectively mitigates inter-fractional motion errors in pediatric SBRT. However, residual intra-fractional displacements =1 mm persist, potentially compromising target dose coverage, emphasizing the need for stringent motion thresholds in pediatric applications. These findings advocate for the integration of SGRT into pediatric SBRT workflows to optimize precision.