ESTRO 2024 - Abstract Book

S4896

Physics - Quality assurance and auditing

ESTRO 2024

Material/Methods:

The RTsafe Prime head phantom, equipped with Gafchromic EBT3 film and PTW PinPoint 3D ion chamber inserts, was used for both CT simulation and dose measurements. To ensure accurate positioning of the phantom, a custom designed head support was employed. Within the brain region, three spherical targets volumes with diameters of 1.5cm, 1cm, and 0.5cm, were delineated. The first two targets shared the same coronal plane, while the third one aligned with the axial plane of the first target. Additionally, various critical organs were delineated as organs at risk. Subsequently, participating institutions were asked to generate a treatment plan, prescribing 6Gy to all designated targets, in accordance with their clinical protocols. Point dose and planar dose measurements were carried out using ion chambers and film at each institution, followed by a comparison of the measured doses against those calculated by their respective treatment planning systems. The agreement between the planned dose and the measured dose was assessed and gamma passing rates were calculated using various criteria including global and local assessments, employing gamma criteria of 5%/1mm, 3%/1mm, and 2%/1mm. A comprehensive evaluation encompassed 30 SRS treatment plans, conducted across 24 radiotherapy departments. These plans were implemented on various treatment platforms, including 21 Varian Truebeam machines equipped with the Eclipse Treatment Planning System (TPS), two Elekta Synergy machines employing the Monaco TPS, one Elekta machine utilizing the Raystation TPS, three Cyberknife systems with the Precision TPS, one Cyberknife system integrated with the Raystation TPS, one Radixat machine using the Precision TPS and one Radixact using the Raystation TPS. All TPSs employed type A dose calculation algorithms. Five of these platforms were not employed for clinical SRS treatment. Among the treatment plans, half utilized a single plan for all targets, while the other half employed two or more plans. A median difference of 1.94% and a range of 0.2% to 9% between the ion chamber measured dose and the dose calculated by the TPS was found. The ion chamber and film measurements exhibited excellent concordance, with a variation of less than 1% observed for 27 treatment platforms, and staying within 2% for the remaining 3 platforms. Figure 1 illustrates a representative example of the dose distribution comparisons, with thin lines representing film-measured doses and thick lines representing doses calculated by the treatment planning system for the two designated targets. Figure 2 shows the gamma passing rates (5%,1mm) related to both global and local normalization for these specific targets, considering various platforms. The larger variation for the local gamma pass rates for the inferior target compared to the superior target might be due to smaller target size. Among the five centers that do not currently perform SRS as part of their clinical procedures, two of these centers recognized the need to enhance dose accuracy before integrating SRS into their clinical practice. Two centers employing Varian technology and three centers utilizing Elekta equipment preferred to avoid the use of a couch kick in their treatment plans. Two data outliers are discernible in Figure 2. One of these outliers reflects a dose deviation of approximately 8% and is associated with a center that had not yet commissioned their linac for SRS treatment. The other outlier related to a shift of 1.5mm in both the lateral and longitudinal directions, with a discrepancy of about 7% in absolute dose. This discrepancy occurred within a machine actively utilized in clinical SRS practice. The measurement was repeated for three treatment platforms, and implementing a corrective action led to an improvement in the outcomes for all machines. In one platform, the enhancement was a direct result of rectifying a delivery error made during the initial measurement. Furthermore, the measurement will be repeated in three additional centers as per their specific requests. Results: