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

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.