ESTRO 36 Abstract Book

S899 ESTRO 36 _______________________________________________________________________________________________

Conclusion IVD with EPID, is a powerful tool that can be inserted in an overload radiotherapy department. It can be helpful daily to monitor the accuracy of the treatment and enable a quickly correction of misalignment or discrepancies occurred during the treatment course. EP-1655 Improved patient setup for breast cancer patients using the predicted (absolute) couch position. M. Essers 1 , S. Hol 2 , I. Maurits 2 , W. Kruijf 1 1 Dr. Bernard Verbeeten Instituut, Department of Medical Physics, Tilburg, The Netherlands 2 Dr. Bernard Verbeeten Instituut, Radiotherapy, Tilburg, The Netherlands Purpose or Objective Usually, patient setup is performed by obtaining a reference position at the first treatment fraction (“relative couch position”) and then applying on-line or off-line setup protocols . In our institute, a method is used in which the couch position is predicted before the treatment (“absolute couch position”) 1 . The purpose of this work was to investigate whether the patient setup for breast cancer patient is improved using the ‘absolute At the time of this study, accurate patient setup was ensured by applying an on-line setup protocol using the patient anatomy (mainly vertebrae, lung tops and sternum) visible on orthogonal (AP and lateral) MV images, and checking the residual deviation of the lung wall on an MV image in the direction of the mediolateral tangential field (ML image). 1. For 83 patients positioned using relative couch positioning as well as 83 patients positioned using absolute couch positioning, the difference in image registration (MV images compared to DRRs) using sternum only or vertebrae only was determined. 2. For the same patients, the residual deviation of the lung wall on the ML image was determined. Results 1. Using relative couch positioning, the difference between sternum and vertebrae match was smaller than 2 mm for 80% and larger than 5 mm for 12% of the fractions, and for absolute couch positioning, 90% of the fractions showed a match difference smaller than 2 mm and 7% larger than 5 mm. These figures indicate that the patient posture at the linac is slightly in better agreement with the posture at the CT for absolute couch positioning. couch position”method. Material and Methods

EP-1656 The inter-fraction variation of the supraclavicular- and the axilla-area in breast cancer patients S. Gerrets 1 , L. Kroon- van der 1 , M. Buijs 1 , P. Remeijer 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Currently a volumetric modulated arc technique (VMAT) for whole breast irradiation, including the supraclavicular- and axilla-area is being implemented at our institute. In contrast to our currently used tangential fields, VMAT requires a CTV and PTV with corresponding margins. From our clinical experience we know that the setup of the shoulder can be very challenging..The purpose of this study was to quantify the inter-fractional variation of the supraclavicular- and the axilla-area in order to quantify So far 6 right sided and 6 left sided breast cancer patients, were randomly selected in this ongoing study. Patients were positioned on a Macromedics MBLXI breastboard with upper- and lower- arm trays. During the acquisition of the planning CT skin marks were drawn extended to the humerus to improve reproducibility of the arm positioning. Setup verification and correction was performed based on bony anatomy registration (ribs and sternum) using Cone beam CT and an offline shrinking action level (SAL) protocol. Retrospectively, the residual inter-fraction errors of the supraclavicular area and the axilla were measured by performing bony anatomy registrations using a rectangular region of interest representative for these areas/regions (see Figure 1 'Region of interest”), and determining their difference from the registration on ribs and sternum. From these residual errors, the random and systematic errors were computed and corrected for the use of a SAL protocol (N=3 and α=9mm). Using previously determined setup data from Topolnjak et al [1], Subsequently, the CTV to PTV margins were determined according to the standard margin recipe: 2.5∑+0.7σ. Results In total 88 Cone beam CT were analyzed; 5-10 scans per patient. Computed residual errors for the supraclavicular region and axilla region are shown in Table 1.The random and systematic residual errors for the axilla regions are larger than the supraclavicular region, as expected. Notable is the small residual error for supraclavicular in LR-direction. The total margins are 0.59cm LR, 0.76cm CC and 0.81cm AP for the supraclavicular region and 0.84cm LR, 0.89cm CC and 0.98cm AP for axilla region. Conclusion For the introduction of a VMAT planning technique for breast and axilla irradiation specific PTV margins adapted for supraclavicular and axilla inter-fraction motion need to be introduced. 1 Topolnjak, et al, IJROBP, Volume 78, Issue 4, 15 November 2010, Pages 1235–1243 EP-1657 Clinical use of transit dosimetry to analyze inter-fraction motion errors F. Ebrahimi Tazehmahalleh 1 , C. Moustakis 1 , U. Haverkamp 1 , H.T. Eich 1 1 Universitaetsklinikum Muenster, Klinik fuer Strahlentherapie, Münster, Germany Purpose or Objective The aim of this work was to inquire the correlation between the target and organ at risks motions and volume discrepancy with the dosimetric variations at hospital levels. The high resolution, large active area, and effectiveness of the Electronic portal imaging devices offers it to be used for in vivo dosimetry during radiation CTV to PTV margins. Material and Methods

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On the ML images, a residual deviation in lung wall position of 5 mm or more was present for 5.1% of the fractions for relative couch positioning and for 2.7% for absolute couch positioning. This also implies a slightly improved patient setup using absolute couch positioning.

Conclusion The patient posture as well as patient setup for breast cancer patients is slightly improved using the predicted, or absolute, couch position. 1 W.J. de Kruijf, R.J. Martens, Reducing patient posture variability using the predicted couch position. Med. Dosim, 40:218-21; 2015.