In vivo dosimetry using an electronic portal imaging device for pediatric myeloablative Total body irradiation with volumetric modulated arc therapy

Background and Purpose\r\nVolumetric Modulated Arc Therapy (VMAT) has enabled highly conformal Total Body Irradiation (TBI), improving dose homogeneity and sparing of organs at risk. However, the increased complexity of VMAT-based TBI requires robust quality assurance strategies. This study evaluated the feasibility, accuracy, and reproducibility of Electronic Portal Imaging Device (EPID)-based in-vivo dosimetry (IVD) for VMAT TBI treatments.\r\nMaterials and Methods\r\nForty pediatric patients treated with VMAT-based TBI were retrospectively analyzed with two prescription schemes: 12 Gy/six fractions and 9.99 Gy/three fractions. EPID transmission images were acquired during each VMAT arc and analyzed using a gamma passing rate (Pγ) of 5%/2 mm, 10% threshold. IVD was evaluated across different anatomical regions. The impact of inter-fraction anatomical variations (weight loss, abdominal swelling, gastrointestinal air) was assessed through dose recalculations on modified CT datasets. Phantom measurements were performed to validate the sensitivity of EPID-based IVD to clinically relevant perturbations.\r\nResults\r\nIVD measurements showed median Pγ of 97.3 ± 3.2%. Across the forty patients, a mean Pγ of 99.3 ± 0.8% and 99.0 ± 0.4% was observed in the head and thoracic regions respectively, lower values (96.5 ± 3.4% and 94.3 ± 3.6%) were found in the lumbar and pelvic regions respectively. Simulated abdominal swelling ≥1.5 cm reduced target coverage, while weight loss and gastrointestinal air had limited dosimetric impact. Phantom tests confirmed IVD sensitivity to significant anatomical changes.\r\nConclusions\r\nEPID-based IVD was a feasible tool for verifying VMAT-based TBI, enabling detection of clinically relevant inter-fraction anatomical variations and supporting safe treatment delivery.