Abstract book - ESTRO meets Asia

S116 ESTRO meets Asia 2018

Conclusion Conclusion: R/B method would be useful for accurate dose response curve in pixel value and netOD evaluation. PO-281 An investigation of dosimetry evaluation on hip prosthesis with 6MV and 10MV in IMRT treatment J. Jayamani 1 , M.Z. Abdul Aziz 1 , N.D. Osman 1 , A.A. Tajuddin 2 1 Advanced Medical and Dental Institute- Universiti Sains Malaysia, Oncological and Radiological Sciences Cluster, Kepala Batas, Malaysia 2 Universiti Sains Malaysia, School of Physics, Gelugor, Malaysia Purpose or Objective Radiation therapy patient with hip prosthesis causes treatment planning inaccuracies when the beam passes through the prosthesis metal. Modern treatment planning system applies algorithm with improved dose calculation accuracies however this algorithm may not calculate dose precisely in the presence of high-density materials. This study main aim is to improve and maximise target coverage and reduce dose to the organ at risk (OAR) with the hip prosthesis Material and Methods An inhomogeneity pelvis phantom with bilateral hip prosthesis was scanned in Toshiba Aquilion (16-bit) scanner (120 kV/350 mAs) with 2mm slice thickness. The scanned images transferred to Monaco V.5.11.02 treatment planning system and 9 beams (0°, 40°, 80°, 120°, 160°, 200°, 240°, 280°, 320°) IMRT plan was calculated in Monte Carlo algorithm with 78Gy/39 fraction. A reference phantom (homogeneity) used to optimise the calculation on 6MV and 10MV photon beam. The same plan implemented on the phantom with a bilateral hip prosthesis and the streak artifacts on images were overridden by electron density of 1. Plan evaluation was based on the OAR dose constraint categories provided in Radiation Therapy Oncology Group (RTOG) protocol 0126. Results Bilateral hip prosthesis phantom shows PTV95% volume coverage in 6MV is 49% and 47% in 10 MV beam. Override electron density function increased the PTV95% dose volume by 50% in 6MV and 58% in 10MV. 10 MV photon beam shows higher volume (85.42%) coverage in PTV95% with the lowest dose in OAR compared to 6MV photon beam. Heterogeneity index (HI) was 1.18 before correction and 1.1 after override function implemented in both 6MV and 10MV photon beam.

Conclusion Several dosimetric benefits were observed when using 10 MV instead of 6 MV for IMRT based treatment plans. Benefits include sparing more dose from the OAR while increasing the dose coverage to the PTV. PO-282 Error detection sensitivity test using complex errors on three patient-specific VMAT QA systems S. Thongsawad 1 , T. Fuangrod 1 , S. Srisatit 2 1 Chulabhorn Hospital, Academic Physics, Laksi-Bangkok, Thailand 2 Chulalongkorn university, Nuclear engineer, Bangkok, Thailand Purpose or Objective The purpose of this study was to investigate error detection sensitivity for three patient-specific head and neck (HN) VMAT QA systems; Delta4, EPID-based Ten patient-specific HN VMAT QA were randomly selected to test their error detection sensitivities. Artificial complex errors were introduced to the original plans then repeated the QA tests. These errors were simulated into three possible scenarios: uncertainty, miss–calibration, and worst-case scenario. For uncertainty scenario, the random errors (σ) of MLC at ± 2.0 mm and gantry angle at ± 2.0 o were introduced. A systematic error of +2MU, and random errors of MLC and gantry angle at ± 2.0 mm and ± 2.0 o were applied as a miss-calibration scenario. For worst case scenario, errors were integrated between systematic and random variation of MLC and gantry angle at 2±0.5 mm and 2±0.5 o , respectively. The dosimetric agreements between QA tests on original versus artificial error plans were determined to investigate error detection sensitivity used gamma analysis with 3%, 3 mm criteria. Results Table 1 presents the average gamma pass-rate (3%, 3mm) ±1SD for all scenarios. EPID had the most sensitivity to detect the uncertainty, miss-calibration, and worst-case scenarios (largest gamma pass-rate drop from original plan QA tests). Delta4 and Log file QA had similar sensitivity on error detection in worst-case scenario. However, Log file had more error detection sensitivity than Delta4 on miss- calibration scenario. Table 1. Percent average gamma pass-rate ±1SD (3%,3mm) for all scenarios dosimetry, and Log file. Material and Methods