ESTRO 2025 - Abstract Book

S2679

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

The results are presented in Table1 and Figure1. In general, CL and AL were found to be within the initially set thresholds. Variability between sites was observed for both detectors. Notably, the most significant case involved breast VMAT plans verified with EPID. Here, the analysis showed that AL would lead to a broader threshold compared to the initial. These plans are characterized by a relatively high modulation and significant spatial extent (12 out of 20 included irradiation of axillary levels III and IV), two factors that may explain the lower GPR with EPID. Conclusion Establishing custom control and action levels enhances the understanding of PSQA processes and facilitates the monitoring of potential out-of-control behavior over time. Segmenting data by anatomical sites enables more detailed process characterization and helped identify suboptimal performance in one of the detectors, which requires monitoring and further investigation. The findings presented may be of interest to centers using similar PSQA setups.

Keywords: Statistical process control, PSQA, Action Limits

References [1] Miften M, Olch A, Mihailidis D, et al. Tolerance limits and methodologies for IMRT measurement-based verification QA: recommendations of AAPM task group No. 218. Med Phys.2018; 45:e53-e83.

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Digital Poster Ultrafast dosimetric commissioning on a MR-linac Carlos Ferrer 1 , Concepción Huertas 1 , Marcos Feijoo 2 , Moisés Sáez 1 1 Medical physics, La Paz University Hospital, Madrid, Spain. 2 R&D, Blue Physics LLC, Lutz, USA

Purpose/Objective The installation of a new linear accelerator (linac) in a radiation oncology department always involves its acceptation and commissioning. During the commissioning, the beam data are collected for reference and treatment planning system (TPS) modelling. This task should be performed carefully, with the proper knowledge and can take up to four or more weeks. Beam data commissioning is usually is accomplished with ionization chambers or diode or diamond detectors and at a minimum, for photon beams, percent depth dose (PDD) and profiles at various depths and field sizes should be collected in a water phantom (WP). Although less common, other detectors could be used, as plastic scintillation detectors (PSD). Among the properties of PSDs, they show a rapid response to radiation, with about 10 9 seconds to reach their maximum response and 10 -8 to 10 -10 seconds signal fall-off. This work studies the suitability of using a PSD to collect the beam data much faster than with ionization chambers or diode/diamond detectors, without losing quality that may lead to radiation misleading, reducing significantly the time required for commissioning. Material/Methods Measures were performed on a 7 MV flattening filter-free (FFF) and 1.5 T Elekta Unity MR-linac with a PTW BEAMSCAN MR WP, which has the capability of continuous measurement disabled and its measures are made point-by-point. The user must select distance between points and measurement time in each one. The ultrafast measurements were acquired in water with the Bluephysics Model 10 PSD mounted in the TRUFIX detector holder for the PTW semiflex 3D ionization chamber. 10x10 cm field size and 10 mm/s position speed were analyzed. The PSD position was controlled with PTW BEAMSCAN software, selecting the movement speed and setting only the start and end detector positions, while the radiation lecture was collected with the Blue Physics electrometer and software. This way, the PSD moves all range without stopping at the selected speed. PDD and profiles at 1.3, 5, 10 and 13 cm depth were measured. 3%/3mm gamma analysis criteria with PTW diamond detector measurements were done, with Blue Physics software .