EP-1774: The Czech multicenter dosimetric and treatment planning intercomparison of IMRT with a pelvic phantom

S278 measurement, although greater discrepancies were noted in female volunteers. Repeat CT rates dropped from 58/323 in the period leading up to the audit, to 4/254 during the audit period, with rescans now entirely due to unrepresentative scanned rectal volumes.

ESTRO 33, 2014 generally lower than doses calculated for IMRT plan mainly due to the TPS uncertainties in inhomogeneity correction algorithms. The work has been supported by project No. TB01SUJB071. EP-1775 Assessment of organs at risk contour variability within a UK multicentre lung radiotherapy trial (Isotoxic IMRT) E. Miles1, Y. Tsang1, A. Carver2, C. Harris3, C. Rowbottom3, C. FaivreFinn4 1 Mount Vernon Hospital, Radiotherapy Trials QA group, Northwood Middlesex, United Kingdom 2 Clatterbridge Centre for Oncology, Radiotherapy Trials QA group, Merseyside, United Kingdom 3 Christie Hospital NHS Trust, Radiotherapy Physics, Manchester, United Kingdom 4 Christie Hospital NHS Trust, Clinical Oncology Department, Manchester, United Kingdom

Conclusions: The Verathon BVI9400 device can measure bladder volume with accuracy appropriate for use in radiotherapy applications where the principal concern is adherence to a particular drinking protocol. Its clinical introduction has improved workflow for prostate radiotherapy, reducing the need for repeat CT scans. EP-1774 The Czech multicenter dosimetric and treatment planning intercomparison of IMRT with a pelvic phantom I. Koniarova1, I. Horakova1, V. Dufek1 1 National Radiation Protection Institute, Department of Radiotherapy and X-ray Laboratory, Prague, Czech Republic Purpose/Objective: The aim of the project was to develop a program to assess the performance of prostate IMRT treatment in the Czech Republic. All Czech departments (17) where prostate patients are treated participated. End-to-end test with pelvic phantom including inhomogeneities has been performed. In total 21 IMRT treatment plans (including 4 IMAT plans) were evaluated. Of the 19 accelerators, 11 were Varian, 7 Elekta, and one Siemens. Materials and Methods: Pelvic phantom with contrast structures and defined PTV was developed to represent a standard prostate patient indicated for IMRT treatment. Agreement with treatment planning system for dosimetric values (beam calibration, dose in PTV and OAR according to the planned/prescribed dose measured with ion chambers, gamma evaluation of planar dose distribution with gafchromic films) and non-dosimetric values (CT numbers to RED conversion, volumes calculation) was evaluated. DVHs were compared for all plans. Results: The mean ratio of measured to expected doses for beam calibration was 1,007±0,009. For the prostate treatments, the mean ratio of measured (in terms of absolute dose) to planned doses in the PTV was 0,990±0,021. If we exclude the influence of daily output fluctuation we obtain the mean ratio for measured to expected doses 0,983±0,017.The difference from unity for ratio of measured (in terms of absolute dose) to expected doses could be caused by inaccuracies in TPS calculation for inhomogeneities and non-water material and not by the complexity of IMRT plan. To confirm this assumption we assessed the relative difference between simple plan (reference field 10x10 cm2) and complex plan (IMRT plan) for the same prescribed dose at the point of interest in non-reference material as the mean ratio of measured to expected doses, which was 1,001 ± 0,017. Measured doses in OAR were acceptable in all cases. The tolerance limit for relative gamma evaluation was 90% (4%/3mm). 3 sites did not meet this requirement but in 2 cases it had been caused by the uncertainties in phantom position on the treatment couch. When gamma evaluation was performed for dose maps that ideally matched, only 1 site did not meet the criterion. There were no defined constraints and dose prescription from our side as we wanted to evaluate the real clinical practice as much as possible. Therefore we obtained DVHs from sites illustrating common practice of prostate IMRT treatment in the Czech Republic. Dose prescriptions were similar among departments but OAR constraints varied. Conclusions: The results of the study show that all centers were able to deliver their IMRT plans successfully. There was a good agreement in absolute dose calibration as well as between TPS calculation and delivered dose for IMRT of prostate treatment. Measured doses were

Purpose/Objective: Isotoxic Intensity Modulated Radiotherapy (IMRT) is a multicentre, non-randomised feasibility study with the aim to demonstrate the feasibility of delivering isotoxic RT using IMRT in stage III non-small cell lung cancer patients. With the isotoxic design, it is crucial to ensure that all organs at risk (OARs) are delineated according to the trial protocol. Against this background, this study was carried out to determine the potential degree of variation in lung cancer OARs delineation and the relative impact on isotoxic dose escalation using pretrial outlining benchmark quality assurance (QA) cases. Materials and Methods: All five prospective principal investigators in the trial were provided with an atlas of OARs and were required to submit outlining cases which were assessed for protocol compliance. Each case was analysed using trial management group (TMG) consensus OAR contours and Dice coefficient (DC). Sensitivity index (IndexSens) and Specificity index (IndexSpec) was calculated for each individual structure: spinal canal, brachial plexus, oesophagus, heart and mediastinal envelope. A standard IMRT plan was produced using the consensus OARs and dose volume histograms (DVHs) of OARs from each benchmark case were calculated with the standard IMRT plan. Results: A total of 25 individual OAR contours were analysed and the results of calculated conformity indices are summarised in table 1. DC median IndexSens (IQR) median (IQR) Spinal canal 0.91 (0.01) 0.92 (0.12) Brachial plexus 0.42 (0.10) 0.49 (0.14) Oesophagus 0.82 (0.03) 0.74 (0.08) Heart 0.94 (0.01) 0.91 (0.04) Mediastinal 0.92 (0.01) 0.91 (0.01) envelope OAR

IndexSpec median (IQR) 0.94 (0.04) 0.44 (0.03) 0.96 (0.04) 0.97 (0.01)

Overlapping with PTV No No No Yes

0.96 (0.04)

Yes

For DC, the median for spinal canal is 0.91 (Interquartile Range (IQR) 0.01), brachial plexus 0.42 (IQR 0.10), oesophagus 0.82 (IQR 0.03), heart 0.94 (IQR 0.01) and mediastinal envelope 0.92 (IQR 0.01). Comparing against the DVHs of the TMG consensus OARs, the median difference in dose received by 1cc of brachial plexus is 16.47% (IQR 50.40%), dose received by 1cc of oesophagus 12.68% (IQR 1.13%), dose received by 1cc of heart is 0.03% (IQR 0.06%), mean heart dose 1.99% (IQR 1.01%) and dose received by 1cc of mediastinal envelope is 0.24% (IQR 0.02%). No difference in final isotoxic prescription dose was found in any of the benchmark cases. Conclusions: Based on the results, it’s suggested that all Isotoxic IMRT centres have demonstrated a high level of OAR contouring compliance to the trial protocol through this QA exercise, which is proved by the consistent output of the final isotoxic prescription dose across all benchmark cases. Some OAR delineations such as brachial plexus in which surrogates are used to guide the contouring have proven to be subjective and special caution is required especially when it is in close proximity to treatment area. EP-1776 Commissioning results of an automated treatment planning and delivery verification system C. Nelson1, B. Mason1, C. Robinson1, S. Kirsner1 1 UT MD Anderson Cancer Center, Radiation Physics, Houston TX, USA Purpose/Objective: A secondary dose calculation verification system(VS) (Mobius3D, Mobius Systems) was acquired and commissioned as an independent check of the treatment planning system (TPS). The purpose of this system is to have a single system perform all secondary checks on the TPS including both simple geometry cases and IMRT and VMAT. A