ESTRO 2020 Abstract book

S788 ESTRO 2020

CL and AL of Octavius1500 and Octavius729 were assessed on an initial set of measurements (n=20) for the three sites separately and compared. Performance were compared and statistically significance (p<.05) was assessed with paired t-Test.

Germany ; 3 Universitat Politècnica de Catalunya, Institut de Tècniques Energètiques, Barcelona, Spain Purpose or Objective This work is devoted to investigate the feasibility of using the unmodified geometrical description of a Varian Clinac 2100 (CL21) linac for the Monte Carlo simulation of flattening-filtered (FF) beams of a Varian TrueBeam (TB) linac. Material and Methods Feasibility of simulating FF beams of a TB as a CL21 was approached in this work by: (i) comparing the energy and angle distributions of the Varian's published phase spaces for the TB with those simulated in this work with the CL21 geometry for the 6, 8, 10 and 15 MV FF photon beams; (ii) by comparing the dose distributions in water estimated by Monte Carlo simulations of those phase spaces for a range of field sizes. All the simulations and analysis accomplished in this work were made using the PRIMO system version 0.3.1.1774 (www.primoproject.net). Varian's TB phase spaces (version 2) for the 6, 8, 10 and 15 MV FF photon beams were obtained from the web site (www.myvarian.com/montecarlo). The phase spaces for the CL21 linac were simulated with PRIMO using PENELOPE as the Monte Carlo engine. The parameters of the initial electron beam employed in simulation were the same as those used for the simulation of the Varian's phase spaces. Energy and angular distributions of photons were generated for both set of phase-space files. Dose distributions were estimated in a water phantom for the CL21 and the TB at a SSD=100 cm and for square fields of side 2, 6, 10, 15, 20, 30 and 40 cm. Absorbed dose distributions were compared via gamma analysis. Results A good match for the energy and angular distributions was observed in all regions for the 6, 8 and 10 MV beams. The small differences found (<5%) for the energy distributions were attributed mainly to inaccuracies of the Geant4 standard EM model employed in the Monte Carlo simulations conducted by Varian to produce the phase spaces. Very large discrepancies were found for the energy distributions of the 15 MV beam in the 0.05 to 5 MeV energy interval. These differences indicate variations in the flattening filter design between the two accelerators. Dose distributions showed a very good agreement with gamma pass rates for criteria of 2%, 2 mm above 97% in all cases, except for the 15 MV beam for which large relative differences (>5%) were found at shallow depths [0-2.5 cm] as a consequence of the dissimilar energy distributions of the source phase spaces. Conclusion Discrepancies between the TB and the CL21 photon energy and angular distributions found for the beams studied in this work are not indicative of significant variations in the design of the targets or flattening filters between the two machines except for the 15 MV beam. This feasibility study can therefore conclude that the TB 6, 8 and 10 MV flattening filtered beams can be simulated in the geometry of the CL21 without loss of accuracy. PO‐1394 A revisited recipe for targeted linac QA: precise, efficient, and tailored to current needs C. Clermont 1 , A. Van Esch 2 1 CHU UCL NAMUR - SAINTE ELISABETH, Radiotherapy, Namur, Belgium ; 2 7Sigma, QA-team in radiotherapy physics, Kasteeldreef 2- 3150 Tildonk, Belgium Purpose or Objective With the evolution of radiotherapy treatment techniques, physicists have gradually added more tests to their conventional machine QA protocol to assure correct delivery. In many centers, QA protocols are a time consuming patchwork of ancient and new tests with a variety of phantoms and tools.

Results Octavius1500 always allowed to obtain better concordance between calculated and measured dose distributions with respect to Octavius729 (p<.001 for all sites). This led SPC analysis to define stricter CL and AL for Octavius1500 detectors array. CL and AL were significantly different among treatment sites, showing for both detectors the strictest CL for Breast treatment and the widest ones for Head&Neck. Process mean for γ-mean evaluation resulted lower than 0.5 for Octavius1500 while always higher for Octavius 729. In example, a comparison between γ(2%/3mm) passing rate i-charts is shown in Fig. 1.

Conclusion Locally based CL and AL limits are generally lower than TG218 universal tolerance limits. Nevertless, the SPC analysis indicated that the process is under control. CL and AL must be set specifically for technology and treatment site. PO‐1393 Monte Carlo simulation of a Varian's TrueBeam linac as a Clinac 2100: A feasibility study. M.L. Rodriguez Castillo 1 , L. Brualla 2 , J. Sempau 3 1 Centro Médico Paitilla - Hospiten, Oncology, Panama, Panama ; 2 University Medicine Essen- Germany, West German Proton Therapy Centre Essen WPE, Essen,