S2757
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
translational and rotational setup deviations. In-vivo dosimetry (IVD) was performed with calibrated detectors placed at the head, thorax, abdomen, pelvis, and thigh. Endpoints included planned-to-delivered dose differences, setup variability, inter-isocenter alignment, and total treatment time. Results: Ten pediatric patients (median age: 10.9 years) were treated between April and July 2025 according to ESTRO–ACROP–SIOPE recommendations. IVD showed a mean absolute deviation of 3.2% (range: 1.1–4.9%) from planned dose across all patients and anatomical sites (Figure 1). Setup reproducibility was high, with average translational shifts <1 cm and rotational deviations <0.3°. The pelvic region exhibited the greatest variability due to bladder and bowel filling fluctuations, generating local deviations up to 6%, all within clinical acceptability.Dose recalculation on CBCT-derived anatomy confirmed preservation of coverage metrics (D95, D98). Immobilization and positioning remained stable in both standard and anesthesia-assisted treatments. Median treatment time was 61 minutes (55–68). No acute grade ≥ 3 toxicities were observed.
Figure 1.Reports the in-vivo dosimetry measurements across anatomical regions. All readings clustered around 98–100% of the planned dose, with regional mean values showing minimal variability. The largest spread was observed in the pelvic region, though all points remained within acceptable clinical limits. Conclusion: VMAT-TBI demonstrated excellent dosimetric precision, reproducibility, and robustness in pediatric allogeneic HSCT conditioning. The strong concordance between planned and delivered dose, confirmed by systematic IGRT and IVD, supports VMAT-TBI as a reliable and safe alternative to conventional TBI. The modified once-daily regimen for anesthetized patients achieved comparable performance, reinforcing the adaptability and clinical suitability of VMAT-TBI in complex pediatric settings. Keywords: VMAT-TBI; In-Vivo Dosimetry; Pediatric RT
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