ESTRO 2020 Abstract book

S1056 ESTRO 2020

Results In the study, mean rectum gEUDs were 55,4 ± 7,5 Gy (1σ), mean bladder gEUDs were 41,6 ± 6,4 Gy (1σ), prescribed doses to PTV were between 72 Gy and 85 Gy. The improvement in dosimetry was observed in terms of film based gamma analysis. There was also improvement in plan quality expressed mainly as statistically significant lower rectum gEUDs; better standardisation in dose prescription (according to ICRU reports); better PTV homogeneity. Surprisingly, gEUDs to bladder were higher for the second group which might be explained by a different approach to the weighting during the optimisation. Conclusion All described observations demonstrate the importance of the role of audits in the quality improvement on the national level. The study was supported by the project No. FV40090, Ministry of industry and trade of the Czech Republic. PO‐1800 Successful Implementation of Treatment Planning Trial QA for the PIVOTALboost Trial H. Mayles 1 , O. Naismith 2 , N. Snelson 1 , I. Syndikus 3 1 Clatterbridge Cancer Centre, Physics, Bebington- Wirral, United Kingdom ; 2 Royal Marsden Hospital, Physics, London, United Kingdom ; 3 Clatterbridge Cancer Centre, Radiotherapy, Bebington- Wirral, United Kingdom Purpose or Objective The PIVOTALboost trial (ISRCTN80146950) tests the role of pelvic node radiotherapy and the effectiveness of a focal intra-prostatic IMRT boost, for patients with intermediate and high risk, localised prostate cancer. The use of a boost is novel in the UK and many centres do not treat pelvic nodes, so a comprehensive QA program is in place and this report analyses the effectiveness of the Dosimetrists at each potential trial centre were asked to prepare a treatment plan on a pre-outlined benchmark case, using draft guidelines, for review prior to a Trial Launch Workshop. The workshop provided a forum for individual queries, advice, planning tips and FAQs. Tailored feedback was given. RT planning guidelines and workshop presentations were made accessible on a website, to help centres that were unable to attend the workshop make submissions. Additionally, the first complex patient plan from every centre was reviewed. Benchmark submissions were passed or failed using a traffic light system, with feedback for centres that failed. Areas of concern were conformance of high dose, high dose spillage around seminal vesicles, excessive bowel dose, and high rectal dose next to the boost volume. If there was concern following the resubmission, the first patient plan from that centre was reviewed prospectively, rather than during treatment. All plan submissions came with a Plan Assessment Form (PAF) with summarised DVH values for PTV coverage and OAR avoidance. The dose constraint values from the PAF’s, were used to quantify the improvement in plan quality by comparing the average of the mean doses to the PTV’s, and the dose constraint values for the OAR’s, for submissions and resubmissions. Results 17 centres attended the workshop. 11 centres submitted plans prior to it. 7 passed, 4 failed then passed after a timely resubmission. Comments from centres were used to improve the RT guidelines prior to publication. 22 additional centres submitted benchmarks, of these 10 needed to resubmit and 6 were referred for prospective review. Of the 14 failures, 2 plans were incorrect (wrong no. of fractions, wrong dose),12 were suboptimal. treatment planning QA. Material and Methods

using Siemens Syngo TPS. Plan quality was assessed through dosimetric parameters. Minor deviations were defined as: i) no more than 5% of any PTV will receive < 90% of its prescribed dose; ii) no more than 5% of any PTV will receive > 110% of its prescribed dose. Major deviations for plan rejection were defined as: iii) no more than 10% of any PTV will receive <90% of its prescribed dose; iv) no more than 10% of any PTV will receive > 110% of its prescribed dose. Results All CI boost plans were judged clinically acceptable and delivered to the pts. 93.3% of the plans fulfilled both the acceptance criteria for major deviations, whereas 73.3% encountered minor deviations for i) criteria and 53.5% for ii). Deviations in tumor coverage i) and iii) were due to the OARs sparing, in particular optic chiasm, optical nerves, brain stem and temporal lobes. Deviations concerning high doses ii) and iv) were due to tissue inhomogeneity in the target and intrinsic PT dose distribution properties. Conclusion No major deviations for target coverage in boost plans except one were met within the patients treated with mixed beam approach. All the plans were judged clinically acceptable for patients treatment. Conformity and Homogeneity index evaluation is ongoing for correlation with treatment outcome. PO‐1799 The quality improvement in prostate radiotherapy based on results from end‐to‐end national audits I. Koniarova 1 , T. Korinek 1 , V. Dufek 1 , I. Horakova 1 1 National Radiation Protection Institute, Department of Radiotherapy and X-Ray Laboratory, Praha, Czech Republic Purpose or Objective In the Czech Republic, independent audits are performed by the National Radiation Protection since 1996. End-to- end audits are included in the auditing system since 2013 for prostate treatment with the anthropomorphic phantom. The audits include dosimetry checks (measurement with ionisation chambers in PTV, OAR, and with gafchromic film) together with plan quality analysis. For all audits, the same anthropomorphic pelvic phantom was used with indicated PTV, rectum, and bladder; and audits were performed by the same auditing group with the same dosimetry equipment to avoid bias. Contours in the phantom were not included and were intended to be drawn by the audited centre based on drilled holes. The local staff should plan the treatment according to the local rules (including the fractionation scheme). All DVHs were recalculated to the normofractionation with LQ model so inter-comparison was possible. In total, 67 on-site audits performed in 2013 – 2019 were analysed in this study for the prostate site. Because there were repeated audits for some departments connected with the installations of new treatment units and with the introduction of new treatment techniques (mostly VMATs) to the clinical practice, it was possible to track the progress at some departments based on previous audits. The results were divided into two groups: the first with results obtained within the national study performed at all departments in 2013 (19 departments, 23 audits), the second with results obtained in 2014 – 2019 (19 departments, 44 audits). Material and Methods The metrics for plan quality evaluation will be described. It included dosimetry results (accuracy of inhomogeneity correction, accuracy of unit calibration, accuracy of MLC treatment delivery, gamma analysis), plan parameters (gEUD rectum, gEUD bladder, gEUD PTV, prescribed to planned dose ratio, D2%, D98%, D50%, homogeneity index, DVH parameters for PTV and organ at risk, NTCPs for organ at risk), optimization constraints, in-house plan verification result, RED-CT numbers calibration curve evaluation.