ESTRO 2020 Abstract book

S781 ESTRO 2020

Conclusion Experimental results are in excellent agreement with the dose distributions given by the PRIMO software for a set of SRS plans designed in clinical conditions. Therefore, the PRIMO software may be used for independent verification of SRS plans for 6 MV beams from a Varian Clinac 2100 CD and a Millennium 120 MLC. PO‐1381 Performance in clinical conditions of the dual‐layer multileaf collimator from a Halcyon 2.0 linac M. Hermida Lopez 1 , D. Sánchez-Artuñedo 1 , F. Cereijo- Graña 1 1 Hospital Universitario Vall d'Hebron, Servei de Fisica i Protecció Radiològica, Barcelona, Spain Purpose or Objective The Halcyon platform (Varian Medical Systems) introduced a multileaf collimator (MLC) with a novel dual-layer design. In the Halcyon 1.0, only the lower (distal) layer was used for beam modulation. In the recently introduced Halcyon 2.0, both distal and proximal layers modulate the beam, thus allowing more complex fluences to be delivered. This works aims to characterize the performance in clinical conditions of the MLC of one Halcyon 2.0 unit, by analyzing the treatment logs of delivered clinical plans, and the results produced in the daily machine performance checks (MPC). Material and Methods Trajectory logs of 5043 treatment fractions were analyzed, corresponding to 271 IMRT and VMAT plans. Treatments were delivered from Mar. 6, 2019 to Oct. 2, 2019. Logs were analyzed with home-made Python scripts, based on the package pylinac [https://pylinac.readthedocs.io/en/stable/]. MPC analysis was based on the results of 160 MPC checks. The analysis was done with home-made R scripts. MLC RMS and 95 th percentile values were obtained from the logs and were grouped by treatment site and modality (VMAT or IMRT). For the MPC checks, we report accuracy and repeatability grouped by MLC layer and bank. Time of unscheduled machine maintenance due to MLC malfunctions was recorded. Results The maximum RMS error (0.164 mm) and the 95 th percentile RMS error (0.110 mm) for all analyzed logs (both specified at isocenter distance) are far below the tolerance of 3.5 mm proposed by the TG-142 report. Boxplots and point clouds in figure 1 show the distribution of maximum RMS error values for each treatment session, grouped by treatment site and modality. The number of analyzed logs by site ranged from 65 for bladder to 1391 for prostate. Although a one-way ANOVA test found statistically significant differences of mean RMS error among sites, these differences probably do not have practical relevance, as the mean values of RMS maximum errors for all sites ranged from 0.055 mm to 0.076 mm.

Quirónsalud Barcelona, Servicio de Oncología Radioterápica, Barcelona, Spain

Purpose or Objective Accuracy of the PRIMO Monte Carlo software was previously studied [Radiat Oncol. 2018 7;13(1):144], including only single static beams on a water phantom. This work aims to validate PRIMO for the calculation of stereotactic radiosurgery (SRS) plans. Material and Methods Planning : a 16×16×14 cm 3 RW3 slab phantom (EasyCube, Scanditronix) enabled to support a film on its middle plane (film plane) was used. The phantom was CT scanned (CT Optima CT660, G. E.) with 1 mm-distance slices, and the images were imported into the Eclipse v. 13.7 treatment planning system (Varian Medical Systems). Several targets were outlined: spheres of 0.5, 1, 2 and 3 cm-diameter mimicking small lesions, and 3 spheres of 1 cm-diameter. Also one brain metastasis (1 cm 3 ), and one vestibular schwannoma target (1 cm 3 ) which were mapped from clinical cases to the phantom. All targets were placed at the EasyCube with their centers at the film plane. For each target, an SRS plan with 11-14 non-coplanar IMRT fields ( sliding window ) was designed, using 6 MV photon beams from a Varian Clinac 2100 CD equipped with a Millennium 120 MLC. Measurements : each plan was delivered onto the EasyCube containing a Gafchromic EBT3 film (Ashland). Multichannel film dosimetry was done using radiocromic.com v. 3.0.15. In addition, film measurements at 10 cm-depth on a water phantom were performed for the static 1x1 cm 2 and 2x2 cm 2 field sizes defined by MLC. Three irradiations were performed in all cases. Simulations: each plan was exported from Eclipse to PRIMO v. 0.3.1.1681 to be simulated. Simulation conditions: PRIMO default transport parameters for 6 MV from Clinac 2100; 850 million histories; DPM algorithm; particle splitting ×170; CT scanner calibration curve of the CT Optima CT660; assignment of RW3 density for the EasyCube; and calibration factor of 0.0174 Gy/MU g/eV. Analysis : A MATLAB script was written to extract the film dose plane from the 3D dose distribution computed by PRIMO on the EasyCube, with the same scanning resolution of the film (0.35 mm/bin). Using radiochromic.com, the PRIMO dose plane was registered with the corresponding measured film dose plane, and the agreement between both dose planes was evaluated with the 2D gamma tool using 5% (global)/1 mm criteria and a 10% dose threshold. Results Table shows that gamma passing rates were in all cases > 97%, and target dose differences were within ±3.6%. Mean uncertainties (k=2) were: 4.1% (experimental) and 1.3% (simulations).