ESTRO 2020 Abstract Book

S194 ESTRO 2020

with metastases from NSCLC showed an 1-year OS of 0% vs 32% (P=0.02). Conclusion The HCMV-reactivation of brain cancer patients after the start of radio(chemo)therapy is a significant risk for cognitively detrimental, but treatable encephalopathy and premature death. Due to inconclusive routine diagnostic, the HCMV reactivation is mostly missed. We urge routinely performed HCMV-diagnostics in combination with basophil count assessment and study-based anti-viral regimens for brain cancer patients treated with radio(chemo)therapy.

Poster Highlights: Poster highlights 12: PH Daily challenges in routine RT

Figure 1. Average changes in calculated primary target mean doses after modifying parameter values to 2.5 th to 97.5 th percentiles of IROC survey and site visit measurement distributions. DLG = dosimetric leaf gap; MLC = multileaf collimator; PDD = percent depth dose. Conclusion This work demonstrates that significant variation exists in community practice, and the effects exhibited from this variation are of clinical concern. Greater attention is needed in the commissioning and validation of photon beam modeling, specifically in defining MLC characteristics and inputting valid PDD information. Because TPS deficiencies are both prevalent and difficult to identify, this work may assist institutions in identifying and resolving inaccuracies, which can ultimately improve patient care. PH-0363 Barium-contrasted bone-cements: impact on dosimetry S. Crowe 1,2 , M. Lathouras 1 , C. Lancaster 1 , T. Kairn 1,2 1 Royal Brisbane and Women's Hospital, Cancer Care Services, Herston, Australia ; 2 Queensland University of Technology, Science and Engineering Faculty, Brisbane, Australia Purpose or Objective Polymethyl methacrylate (PMMA) based bone cements are a commonly used cranioplasty material and are specifically used when tumour involvement prevents the re-insertion of bone flaps. These cements are contrast enhanced (e.g. with barium), to provide a CT number similar to bone in CT images. Where density overrides are not employed, radiotherapy treatment planning systems may treat the bone cement as bone-equivalent, despite it having substantially different radiological properties for MV treatment beams. For treatments of volumes near the bone flap, this may result in disagreement between calculated and delivered dose. Material and Methods To assess the effective radiological properties of barium- contrasted PMMA bone cements, measurements were made with a physical sample slab of DePuy CMW 1 bone cement. Specifically, relative electron density (RED) was acquired using CT images with nominal energies of 120 kVp and approximately 3.5 MV, and transmission measurements for therapeutic beams with energies 100 kVp / 6.3 mm Al HVL and 6 MV. Monte Carlo dose calculations were performed, using the chemical composition of the cement and measured mass density. A 6 MV beam was simulated, incident on a simple slab geometry of 6 mm water, 6 mm flap (modelled alternatively as either water, bone cement, cortical bone, or tri-layered cortical and cancellous bone), and 12 cm water. Results

PH-0362 Beam modeling variations and potential contributions to treatment delivery errors M. Glenn 1 , D. Followill 1 , R. Howell 1 , J. Pollard-Larkin 1 , C. Peterson 2 , S. Kry 1 1 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA ; 2 The University of Texas MD Anderson Cancer Center, Biostatistics, Houston, USA Purpose or Objective In recent years, IROC Houston has identified a link between treatment planning system (TPS) deficiencies and phantom irradiations with dose disagreements greater than 5%. This work aims to assess common trends in TPS parameter definitions and evaluate the dosimetric impact of adopting extreme parameter values based upon current A 6 MV beam for a Varian Clinac 2100iX was commissioned in Eclipse (AAA 13.5) to represent an average-performance linear accelerator, based upon IROC site visit measurements (percent depth dose curves, output factors and off-axis factors) and a beam modeling parameter survey provided to IROC service users. The survey included parameters modeling the radiation source and multileaf collimator (MLC) characteristics. A sensitivity analysis of both dosimetric and non-dosimetric modeling characteristics was then performed on five clinically- acceptable head and neck phantom treatment plans (3 IMRT, 2 VMAT) by manipulating parameters of interest following the 2.5th, 25th, 50th, 75th, and 97.5th percentiles of survey responses (for non-dosimetric parameters) and IROC site visit measurement agreement (for dosimetric characteristics). Results The non-dosimetric parameter survey yielded 2818 responses from 642 institutions, and site visit measurements used in beam model definition were derived from 349 Varian linear accelerators. Several non- dosimetric modeling parameters exhibited poor community consensus, including the dosimetric leaf gap (DLG) in Eclipse and most parameters reported by Pinnacle users. Dosimetric properties such as the PDD reported measurement errors up to 5% for small fields. When beam modeling parameters were modified, dose calculations for the standard Varian linear accelerator were most sensitive to changes in the DLG, which yielded deviations of up to - 4.5% to +3% of the calculated dose to the primary target; dosimetrically, the PDD yielded deviations of up to -1%. community practice. Material and Methods