ESTRO 37 Abstract book

S154

ESTRO 37

delivers according to the daily adapted plan. To this purpose, the use of Electronic Portal Imaging Devices (EPID) for independent in vivo dose verification in the MR-Linac is being developed and a proof of concept at gantry angle 0 is presented. Material and Methods The MR-Linac is a combination of a 1.5T magnetic resonance imaging scanner with a 7 MV linear accelerator and is equipped with an on-board EPID. The existing back projection algorithm used with conventional linacs in our institute has been adapted for the MR-Linac at gantry 0, and has been commissioned with measurements of square fields made under a variety of different phantom setups (varying SSD, phantom thickness and field size) acquired with EPID and ionization chamber. Performance of the back-projection algorithm was verified for square fields and for 25 IMRT fields irradiated to a 23 cm slab phantom. The verification was performed by comparing the dose reconstructed with the EPID to the dose measured with the OCTAVIUS 1500 detector array (PTW, Freiburg, Germany), at 10 cm depth at the isocenter plane. The 2- D reconstructed dose distributions were compared to the measured 2-D dose distributions by means of a global 2-D γ-analysis (3%, 3mm, 20% isodose). In both cases, the dose distributions were only compared in the central region of the EPID images, where there is no strong extra attenuation of the MRI structures in the EPID acquired signal ( Figure 1 ), of 11.5 cm at the isocenter plane in the cranial-caudal direction.

potentially fast field by field patient specific QA tool for the MR-Linac. It shows that the challenges arising from the presence of the MR housing in the set-up are not an impediment for the implementation of EPID dosimetry in the MR-Linac. The adaptation of the model for all gantry angles, and the reconstruction of the dose distribution in the attenuated area of the beam are ongoing work.

Proffered Papers: PH 6: Treatment planning 1

OC-0299 Inconsistencies in clinicians‘ final treatment plan evaluations – a need for automation support B. Heijmen 1 , P. Bonomo 2 , G. Goldner 3 , A. Henry 4 , F. Lohr 5 , G. Simontacchi 2 , P. Voet 6 , D. Fransen 1 , J. Penninkhof 1 , M. Milder 1 , A. Akhiat 6 , M. Casati 2 , D. Georg 3 , J. Lilley 4 , L. Marrazzo 2 , S. Pallotta 2 , R. Pellegrini 7 , Y. Seppenwoolde 3 , V. Steil 8 , F. Stieler 8 , S. Wilson 4 , S. Breedveld 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands 2 Azienda Ospedaliero-Universitaria Careggi, Radiation Oncology, Florence, Italy 3 Medical University of Vienna- Christian Doppler Laboratory for Medical Radiation Physics for Radiation Oncology / AKH Wien, Radiation Oncology, Vienna, Austria 4 Leeds Cancer Centre- St James's University Hospital, Radiation Oncology, Leeds, United Kingdom 5 Azienda Ospedaliero-Universitaria Modena- Policlinico, UO Radioterapia- Dip. di Oncologia, Modena, Italy 6 Elekta AB, Elekta, Rotterdam, The Netherlands 7 Elekta AB, Elekta, Milano, Italy 8 University Medical Center Mannheim- Heidelberg University, Radiation Oncology, Mannheim, Germany Purpose or Objective In current interactive trial-and-error treatment planning, final plan quality depends on the skills of the planner and on allotted time for planning. At the end of the planning process, the treating clinician needs to give final plan approval to start with treatment. Ideally, suboptimal treatment plans are recognized by the clinician to prevent treatment with them. In this study, we systematically investigated the consistency of clinicians‘ plan evaluations by comparison of: A. independent plan quality scoring by a clinician of alternative plans of patients, separated in time, with B. side-by-side, direct comparison of alternative plans. Material and Methods Five clinicians from 4 treatment centers in 4 European countries participated in the study. In each center, treatment plans of 20 prostate cancer patients were evaluated (total 80 patients). For each patient, 2 VMAT plans were available, the clinically delivered plan (VMAT clin ) and an alternative plan that was automatically generated with a system for automated planning (VMAT auto ). In each center, plan quality was scored in two sessions: independent, subsequent quality scoring for each of the 20x2 plans in a random order (A.), and, for each of the 20 patients, side-by-side plan comparison and direct scoring of the quality difference between the 2 VMAT plans (B.). Both for A. and B., scoring was performed using visual analogue scales. We compared for each patient the plan quality difference, calculated as the difference in the independent scores for VMAT clin and VMAT auto obtained with approach A., with the plan quality difference obtained by direct plan comparison. For both approaches, plan quality difference-scores ranged from - 100 to +100. Results In the independent plan scoring, the average plan quality difference was -2.5 ± 12, i.e. a small advantage for

Results For the commissioning square fields, X-Y profiles of reconstructed EPID dose distributions were compared to the Octavius 1500 array measured profiles. For the high dose region (above 80% of maximum dose), more than 93% of the points showed local deviations smaller than 4%. The γ-parameters of the commissioning square fields averaged over 20 fields are shown in Table 1 , together with the averaged gamma parameters of 25 IMRT fields. Reconstructions of the dose distributions were calculated in 129.3 ms per segment on average.

Conclusion The EPID dosimetry back projection algorithm was successfully adapted for the MR-Linac at gantry 0. Experiments showed good agreement between measured and EPID reconstructed dose distributions. This result is an essential step towards an accurate, independent, and