S1646
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Material/Methods: Four clinically acceptable CSI plans were created on Varian Eclipse TPS using Acuros 15.6 and the auto- feathering tool. The PTV and junction length ranged from 70-83 cm and 4.3-12 cm respectively. To verify the plan deliverability, Delta4 phantom verifications were used to assess the dose distribution per isocentre, while the junction dosimetry was evaluated by point dose measurements on the Cheese (TomoTherapy) phantom and EBT4 gafchromic film plane dose measurements using an in-house RW3 solid water (PTW) phantom. Junction deliveries were repeated with ±3 mm and ±5 mm deliberate shifts to assess robustness and TPS accuracy in predicting hot/cold spots. An end-to-end test was conducted on the Alderson RANDO phantom with TLDs at multiple anatomical sites.
dose measurements, while the 5-point PSD provided enhanced spatial resolution in high-gradient regions.
Conclusion: Multi-point PSDs enabled precise dose measurements offering extended potential to enhance the spatial resolution and efficiency of patient-specific QA. References: Mif-ten, M., Olch, A., Mihailidis, D., Moran, J., Pawlicki, T., Molineu, A., Li, H., Wijesooriya, K., Shi, J., Xia, P., Papanikolaou, N., & Low, D. (2018). Tolerance limits and methodologies for IMRT measurement ‑ based verification quality assurance: Recommendations of AAPM Task Group No. 218 (AAPM Report No. 218). Medical Physics, 45(4), e53 ‑ e83. Keywords: scintillation, multi-points, patient-specific QA
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Figure 1 Clinical CSI plan copied to RANDO for end-to- end delivery. TLDs were inserted in RANDO and placed on gafchromic film, which was used to confirm the TLD position. Multi-CBCT imaging verification used to match the phantom to the reference CT scan. Results: Delta4 measurements met 3%/3mm local γ (> 95%) for 83% of the cases. All Cheese point dose measurements were within ±3%, while 94% of the Cheese verifications with introduced set-up errors failed this tolerance (dose range –13.9% to 10.2%). EBT4 film analysis showed 38% pass at 3%/3mm local γ and 75% at 5%/3mm local γ (> 95%). Eclipse could simulate set-up errors within ±3% accuracy (mean – 0.8% ± 2.3%). Repeated tests confirmed reduced γ pass rates with 3–5 mm shifts and improved robustness with auto-feathering. The end to end test was successful in accurate measurement of 3/5 locations as shown in Table 1, with Isocentres 1, 2 and Junction 23 within ±10% (clinical tolerance). Isocentre 3 and Junction 12 exceeded this limit due to ant/post uncertainty (match to head) and systematic set-up
Dosimetric measurements for the clinical implementation of multi-isocentre VMAT craniospinal irradiation (CSI) on a TrueBeam linac Theodora Skopidou 1 , Katharine Lane 1 , Michael J Pearson 1 , Rubina Begum 1 , Saima Naz 2 , Meera Patel 2 , George N Mikhaeel 3 , Jessica Brady 3 1 Radiotherapy Physics, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom. 2 Radiotherapy, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom. 3 Clinical Oncology, Guy's and St Thomas' NHS Foundation Trust, London, United Kingdom Purpose/Objective: Helical tomotherapy had previously been used in our centre for CSI using a single continuous field. Following decommissioning of this system, a multi- isocentre VMAT solution on a Varian TrueBeam linac was developed and validated for clinical implementation. The aim was to assess delivery accuracy, junction robustness to set-up errors, and establish verification tolerances for PSQA.
error (head disconnected from torso) respectively.Table 1 End-to-end delivery results
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