ESTRO 2020 Abstract Book

S90 ESTRO 2020

Studi di Firenze, Specialization School in Medical Physics, Firenze, Italy ; 3 Sant Joan de Reus Hospital, Department of Medical Physics, Tarragona, Spain ; 4 Hospital Clınic de Barcelona, Department of Radiation Oncology, Barcelona, Spain ; 5 Zhianawa Cancer Center, Zhianawa Cancer Center, Sulaymaniyha, Iraq Purpose or Objective Different clinical treatment sites can have inherent differences in the degree of plan complexity required to achieve a clinically acceptable plan. The delivery accuracy of a VMAT plan can be predicted by the score of plan modulation complexity. The aim of this work is to evaluate the influence of the detector resolution on the ability of various complexity metrics to predict delivery accuracy in VMAT treatments. Material and Methods 50 VMAT plans for three anatomical sites (17 Head&Neck, 19 Lung, 14 Breast) were considered. Pre-treatment QA was performed with both the PTW Octavius1500 and Octavius729 detectors array inserted in the PTW Octavius4D phantom. Measurements were compared with TPS Elekta Monaco5.1 computed doses via 3D absolute dose γ-analysis by using PTW VeriSoft 6.1. Global and Local 3D volumetric γ passing rate and γ mean evaluations with normalization at the 90% of the maximum dose and different criteria (3%/3mm, 3%/2mm, 2%/2mm) were performed. The complexity metrics (CM) evaluated from RT plans were: Modulation Complexity Score (MCS), Total Modulation Index (MITotal), the ratio of MLC side edge length and aperture area (EdgeMetric), the ratio between Leaf Travel and the Arc Length (LT/AL), meanMLCSpeed, MLCSpeedModulation, gantry speed modulation (GSmodulation), average leaf pair opening (mean MLC gap), plan averaged amounts of T&G effect (MeanTGi) and the ratio meanTGi/meanMLCgap. Spearman’s correlation coefficients between CM and γ evaluations were computed and statistical significance was assessed (Spearman’s rank test, p<0.01 and p<0.05). Results Spearman’s correlation coefficient showed high correlation between CM and γ valuations for some of the analysed CM (Tab. 1). When found, correlations were statistically significant on both γ passing rate and γ mean for both Local and Global Normalization. Octavius1500 allowed to highlight higher correlations with respect to Octavius729 for almost all the evaluated indices. The strongest correlations for Octavius1500 were found with MCS (>0.6 for all γ evaluations) and mean MLC gap (up to 0.799 for γ passing rate, Local Normalization). Octavius 729 results with local normalization showed lower correlation than Octavius1500 (fig.1). High correlations were found for LT/AL, meanMLCSpeed and MLCSpeedModulation indices with Octavius 1500 measurements for all γ evaluations. The same metrics showed weaker correlation, often not statistically significant, with Octavius 729.

Conclusion CM showed correlation with γ evaluations: as a general behaviour, higher plan complexities were associated with lower concordances between computed and measured doses. Different characteristics of the optimized plans (beam aperture and shapes, leaf travel, variations in speed and acceleration of the MLC, as well as variations of the gantry speed) have a significant influence on the treatment delivery accuracy. Correlations between QA results and CM depend on the technical solution adopted for QA measurements and the detector resolution is a key parameter. PD-0184 Proton dosimetric comparison of CT metal artifact reduction techniques for head and neck patients D. Branco 1 , P. Taylor 1 , S. Kry 1 , X. Zhang 1 , J. Rong 2 , S. Frank 3 , D. Followill 1 1 MD Anderson Cancer Center, Radiation Physics, HOUSTON, USA ; 2 MD Anderson Cancer Center, Imaging Physics, Houston, USA ; 3 MD Anderson Cancer Center, Radiation Oncology, Houston, USA Purpose or Objective Dental metal amalgam filling artifacts obscure the visualization of tissues in the oral cavity and oropharyngeal region leading to poor visualization of the anatomy, potential erroneous patient diagnoses, in addition to negatively impacting the quality of radiation therapy treatments, specifically in proton therapy. The purpose of this study is to compare the proton dosimetric impact of the use of an in-house metal artifact management for proton planning (AMPP) technique and Phillip’s commercial solution (OMAR). Material and Methods A head and neck anthropomorphic phantom composed of proton tissue equivalent materials with a human skull, air cavities and a removable jaw was used. The removable jaw allowed for the exchange of bone equivalent and metal filled teeth inserts for the creation of baseline and artifact filled scans. A clinically relevant proton treatment plan was designed on the baseline scan using two anterior oblique beams and one posterior beam. The target mimics