ESTRO 2021 Abstract Book

S1577

ESTRO 2021

Figure 2:

Conclusion Using the AXB m

algorithm and carefully reviewing the beam calibration procedure in H&N VMAT-FFF treatments ensures (1) dose accuracy increase by approximately 3%, (2) a consequent dose increase in targets; and (3) a dose reporting mode that is consistent with the trend of current algorithms

PO-1850 On the link between the aperture shape controller and plan complexity metrics L. Vieillevigne 1,2 , M. More 1 , F.X. Arnaud 1 , C. Khamphan 3 , J. Saez 4 , V. Hernandez 5

1 Institut Claudius Regaud, Institut Universitaire du Cancer, Medical Physics, TOULOUSE, France; 2 Centre de Recherches en Cancérologie de Toulouse, INSERM, TOULOUSE, France; 3 Institut Sainte Catherine, Medical Physics, AVIGNON, France; 4 Hospital Clinic de Barcelona, Medical Physics, BARCELONE, Spain; 5 Hospital Sant Joan de Reus, Medical Physics, TARRAGONA, Spain Purpose or Objective The Aperture Shape Controller (ASC) tool was recently released with the Photon Optimizer (PO) within Eclipse TPS version 15. This tool was developed in order to control MLC modulation during plan optimization. The purpose of this study is to parse the impact of ASC in terms of complexity metrics. Materials and Methods 18 cases were investigated: 3 head and neck (HN), 5 prostate/pelvic (GU), 10 brain with different target sizes: 2 large (BL), 3 medium (BM) and 5 SRS (BSRS). All plans were first generated with POv13.7 and re-optimized with POv15.6 keeping the same objectives with ASC set to off and to its five available levels (very low, low, moderate, high, very high). The complexity metrics ‘average leaf pair opening’ (meanGap), ‘Tongue-and-Groove index’ (TGi) and the TGi/meanGap ratio were calculated for each plan with an in-house Eclipse script. All plans were measured on a TrueBeamSTx with the 4D Octavius (PTW) and Gamma Passing Rates (GPRs) were reported. Results Moving from POv13.7 to POv15.6 (ASCoff) involved higher TGi/meanGap ratios: from 1.2 to 1.8 (HN), 0.5 to 0.8 (GU), 0.9 to 1.0 (BL), 1.8 to 2.0 (BM) and 2.0 to 2.5 (BSRS). The higher the ASC level, the larger the meanGap and the lower the TGi. Compared to ASCoff, a reduction of 60% (HN), 78% (GU), 22% (BL), 11% (BM) and 16% (BSRS) was found for the median TGi/meanGap ratio with the ‘very high’ level. The 2%/2mm GPR improved around 5% (HN), 2.4% (GU) and 2.5% (BL). No clear GPR improvement was found for BM and BSRS. Conclusion POv15.6 without ASC leads to more complex plans than POv13.7. ASC offers the capability to minimize complexity by increasing the average MLC opening, minimizing tongue-and-groove effects and thereby improving plan robustness and