ESTRO 37 Abstract book

S1222

ESTRO 37

6 Kyoto University, Radiation Oncology, Kyoto, Japan 7 Catharina Hospital, Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective Beam output audits involve an independent measurement of an institution’s machine calibration. Such an audit is conducted remotely using mailed dosimeters by several members of the Global Harmonisation Group. This work reviews the results and trends of these audit results. Material and Methods Remote beam output audit results from the Australian Clinical Dosimetry Services (ACDS), the International Atomic Energy Agency (IAEA), the Imaging and Radiation Oncology Core (IROC), and Radiation Dosimetry Services (RDS) were compiled and evaluated from 2010 to the present. The failure rate (i.e., beam output was outside the +/-5% tolerance) was evaluated for photon and electron beams as a function of the year of irradiation as well as the nominal beam energy. Finally, examples of confirmed calibration errors were compiled to provide guidance to clinical physicists as well as auditing bodies. Results Of the 211,003 measured output audit results, 1326 (0.6%) were outside of tolerance. As a function of date, for both photon and electron beams, there was an overwhelming trend towards improved audit performance for more recent dates (highly significant for most evaluations). As a function of nominal beam energy, all photon beam energies generally showed uniform rates of audit failure. However, low (6 MeV) and high (≥18 MeV) electron beams showed significantly elevated rates of audit failure. Confirmed beam calibration errors were attributed to a wide range of causes, including equipment failures, setup errors, errors implementing the calibration protocol, and calculation errors. It was also found that a substantial number of these errors were detected during ongoing annual monitoring, and not simply during the first audit or when the machine was first calibrated. Conclusion Remote beam output audit results were outside of tolerance a small percent of the time, and the frequency of such failing results is decreasing globally. Nevertheless, these audits have identified, and continue to identify numerous, often substantial, beam calibration errors. EP-2207 Novel volume-based algorithm rapidly optimizes lung dose in VMAT & tomotherapy for esophageal cancer C.X. Hsu 1,2 , K.H. Lin 1,2 , C.H. Chang 1 , H.J. Tien 1,2 , T.H. Wu* 2 , P.W. Shueng* 1,3 1 Far Eastern Memorial Hospital, Department of Radiation Oncology, New Taipei City, Taiwan 2 National Yang-Ming University, Department of Biomedical Imaging and Radiological Sciences, Taipei, Taiwan 3 National Yang-Ming University, Department of Medicine- School of Medicine, Taipei, Taiwan Purpose or Objective Novel arc-based radiotherapy including volumetric modulated arc therapy (VMAT) and tomotherapy provides highly conformal dose distribution by rotating gantry arcs and angles. However, VMAT and tomotherapy may contribute to extensive low-dose distribution in both lungs and result in potential complications such as radiation pneumonitis and radiation pericarditis. It is time-consuming to perform the traditional inverse planning of arc-based radiotherapy by repeated trial-and- error tests. In this study, we aim to optimize rotating gantry arcs/angles by a novel automatic volume-based algorithm (VBA) and to rapidly reduce radiation dose to lungs prior to the inverse planning in the VMAT and

tomotherapy treatment plannings (TPs) for esophageal cancer (EC). Material and Methods This study included 236 TPs (tomotherapy n=146; VMAT n=90) of EC patients who received curative radiotherapy at our institute from January 2007 to June 2017. The prescription radiation dose of planning target volumes (PTV) was normalized to 50 Gy in 25 fractions. Prior to inverse planning in TPs, we developed a VBA which can automatically calculate the arc angles, related restricted angles, predicted lung V5 and the related lung dose in the VMAT and tomotherapy [Figure 1]. The restricted angle was designed as a fan-shaped virtual restricted area contoured in both lungs to avoid primary radiation beams/arcs. The restricted angles could further constitute restricted volumes as shown. The partial arc/gantry angles, related restricted angles/volumes, dosimetric parameters for organs at risk including the spinal cord, heart, lungs, and homogeneous index of planning target volumes were analyzed. Accuracy of predicted lung V5 and actual lung V5 was assessed.

Results Tomotherapy and VMAT TPs using arc angles/related restricted angles by VBA significantly reduced the mean lung dose (24.62%, p=0.002), lung V5 (39.78%, p<0.001) and lung V10 (46.35%, p<0.001) as compared with non- restricted arc angle plans. There was a trend toward reduced mean lung dose and increased mean heart dose when the restricted angle increased. The restricted angle and restricted volume showed strong correlation with mean lung dose (r= -0.998) and lung V5 (r= -0.988). R 2 value between actual lung V5 and predicted lung V5 was 0.977. The predicted lung V5 using VBA was accurately and rapidly achieved within 20 iterations by 5 minutes in each TPs. Conclusion Optimal arc/gantry angles created by this novel VBA successfully achieved expected lung V5 and significantly reduced lung dose in VMAT and tomotherapy. VBA enables to provide accurate predicted lung V5 rapidly in the initial setting of arc angles/gantry angles before the traditional time-consuming inverse planning of arc-based radiotherapy. Further validation of VBA predicted models in different cancer sites would be considered. EP-2208 Implementing Modulated Electron Radiotherapy using customised 3D printed bolus C. Malone 1 1 St.Luke's Radiation Oncology Network, Physics, Dublin, Ireland Purpose or Objective Electron radiotherapy (ERT) is particularly useful in treating superficial tumours where the maximum sparing of underlying normal tissue is required. ERT treatment