ESTRO 2025 - Abstract Book

S2663

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

Results: Motions with higher velocities and accelerations (2-second period cos⁴ and trapezoid waves) did not show a drop in GPR compared to slower, more clinically relevant motions (6-second period cos⁴ and cough wave). The periodic quick jerk in the cough motion had no negative effect on the GPR, suggesting that the RGSC system’s operational accuracy is unaffected by rapidly changing motions. The trapezoid wave showed the smallest drop in GPR, attributed to periods where the target remained static within the isocentre, closely mimicking a static treatment plan. In contrast, the triangle wave showed the largest drop in GPR due to its immediate acceleration to maximum velocity, reducing the effective time at the isocentre (Table 1).

Conclusion: The primary cause of inaccuracies in 4DRT treatments appears to arise from the target being within the gating window but outside the isocentre. This inherent limitation of respiratory gating methods, where the target is continuously irradiated within the gating window, was evident. However, the acceleration and velocity of target motion do not necessarily compromise the targeting and dose accuracy of 4DRT with this gating method. The method demonstrated high accuracy in detecting and managing respiratory motion, which is crucial for optimizing treatment delivery.

Keywords: Respiratory motion management, Gating, 4DRT

3999

Digital Poster Simulation or Measurement? A Comparative Study of Sensitivity Methods for PSQA Devices Riccardo Baldoni 1 , Andrea Bruschi 2 , Alessandro Ghirelli 2 , Silvia Pini 2 , Serenella Russo 2 , Marco Esposito 2,3 1 UOC Fisica Medica e Alte Tecnologie, AST Pesaro Urbino, Pesaro, Italy. 2 Medical Physics Unit, Azienda Sanitaria USL Toscana Centro, Firenze, Italy. 3 Medical Physics, Abdus Salam International Centre for Theoretical Physics, Trieste, Italy Purpose/Objective: Patient-specific quality assurance (PSQA) devices are essential for verifying the accuracy of dose delivery in radiotherapy. However, many PSQA devices exhibit limited sensitivity in detecting clinically significant errors, underscoring the need for comprehensive preclinical evaluations of their sensitivity and specificity. To address this, several studies have assessed these parameters by simulating inverse errors (IE) within the treatment planning system (TPS), rather than introducing errors directly during measurements (DE), to minimize measurement time. Although this approach has gained widespread adoption, the impact of direct versus inverse error methods on sensitivity and specificity remains insufficiently understood. This study aims to evaluate and compare the sensitivity and specificity of IE and DE methods in the context of back-projection electronic portal imaging device (EPID) algorithms. Material/Methods: Three types of errors—setup errors, anatomical variations, and delivery errors—were introduced into nine treatment plans using a CIRS lung phantom. The dosimetric impact of each error was assessed (Figure 1) through