S1645
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
reference dose was determined to be 3.1 cGy. All calculated dose values consistently demonstrated an under-read relative to their reference doses, with the exception of the readings associated with the highest tested dose (69.6 cGy). Importantly, no significant time trend was observed in the resulting dose values across the measurement period. Conclusion: The implemented time-resolved calibration methodology successfully confirmed the system’s ability to deliver consistent and accurate point-dose measurements of manufacturers' declared accuracy, which is specified to be within 10 cGy for low doses (< 80 cGy). No significant time-dependent trend was evident in the dose values obtained during the 24- hour reading protocol. Keywords: in vivo dosimetry, Gafchromic film Enhancing Patient-Specific QA by combining a Multi-Point Plastic Scintillation Dosimeter with a clinical QA platform Emily Cloutier 1,2 , Laurie J.M. de Vries 3 , Luc Gingras 1 , Simon Lambert-Girard 4 , François Therriault-Proulx 4 , Benjamin Côté 4 , Martin F Fast 3 , Louis Archambault 5,6 1 Oncology division, CHU de Québec-Université Laval, Québec, Canada. 2 Science and engineering faculty, Université Laval, Québec, Canada. 3 Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands. 4 -, Medscint, Québec, Canada. 5 Oncology division, CHU de Québec-Université Laval Research Center, Québec, Canada. 6 Department of physics, Université Laval, Québec, Canada Digital Poster 1951 Purpose/Objective: Patient-specific quality assurance (QA) is important for ensuring safe and accurate radiotherapy delivery. This study investigates the use of multi-point plastic scintillation detector (PSD) to enhance a clinical QA system by enabling simultaneous, water-equivalent, and non-perturbing dose measurements at critical locations. Material/Methods: Multi-point PSDs (PRO-Series, 2-Point detector, and FLEX-Series, 5-Point detector, Medscint) with sensitive elements of 1mm lengths x 1mm diameter were inserted in custom PMMA plugs within a ArcCHECK (Sun Nuclear). The PRO-Series has a larger collecting fiber that increases SNR, whereas the FLEX-Series has a smaller form factor that improves spatial resolution. PSDs were read out using a HYPERSCINT RP-200 (Medscint). Four 10FFF plans, each having 2 arcs, from three patients were delivered. Patient cases included spinal SBRT (Patient 1), dual-target spinal and pelvic plans (Patient 2), and thoracic treatment (Patient 3).
For the 2-point detector, one detector was positioned at the clinically selected ion chamber (IC; Exradin A26, Standard Imaging) location corresponding to a high- dose, low-gradient region, and the second at another clinically relevant site. For Patient 2, both detectors corresponded to optimal IC positions for each plan (Figure 1). The 5-point detector provided additional measurements for Patient 2, with four FLEX detectors in a single PRO PMMA plug to capture dose gradients within position 2 and the fifth at position 1. PSD measurements were analyzed alongside IC, ArcCHECK, and TPS (RayStation, RaySearch) calculations.
Results: For all patients, the 2-point PSD agreed with TPS within 3.37% per beam and 1.07% per plan, and with the IC within 3.29% per beam and 1.13% per plan. PSD results for Patient 2 are summarized in Table 1. The combined detection and delivery precision of the 2- point PSDs was 0.0013±0.0009Gy (0.04±0.02%), while setup precision was 0.14±0.02Gy (5±3%). Differences were primarily due to detector placement in high- gradient regions (up to 25.86%) rather than detector performance or plan delivery. Swapping 2-point detector positions led to mean differences of 0.4±0.1%, attributed again to positioning precision. For the 5-point PSD, channels 2–5 showed standard deviations of 1.2, 2.24, 3.0, and 2.83%, reflecting local gradients. All arcs met the AAPM TG-218 recommended gamma passing criteria when measured with the ArcCHECK. The 2-point PSD improved clinical workflow by enabling simultaneous
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