ESTRO 37 Abstract book

S1140

ESTRO 37

based thresholds on the balance of plan rejection rate and quality. Results Scenario B deviates the least from the reference plan, resulting in a lower apparent dose to OARs on average (fig. 1). Scenario A shows a large spread in doses, especially the bladder, and the variation is comparable in size to that of scenario C. CTV coverage was always maintained (not shown). The dosimetric difference between SSO+SWO and SWO was small. The mean plan adaptation times were 7 min (SWO+SSO) and 3 min (SWO). In fig. 2 we show as a function of dosimetric thresholds, the plan rejection rate for adapted plans (left), and which fraction of plans under scenario C have the same reject/accept status as in scenario B (right). Conclusion For the first time, dosimetric variations induced by daily online plan adaptation on the MR-Linac were investigated in the context of uncorrected variations induced by anatomical changes. Dosimetric tolerances for online plan adaptation should be tailored to the dosimetric variations induced by uncorrected variability. Anatomical dosimetric variance is likely to decrease once delineations based on daily MRI images are used for plan adaptation.

Conclusion The effect of the ERE on plan quality can be minimised by considering the magnetic field at the beginning of the planning process. Although plans with the magnetic field have higher maximum doses at air-soft tissue interfaces it is unclear if this results in any negative clinical side effects as this is not often considered in radiotherapy plan dose evaluation and assessment of radiotherapy side effects for peripheral sarcoma patients. It could be useful to determine and further explore the clinical relevance of higher doses at air-soft tissue interfaces. EP-2074 Acceptability criteria for pre-beam plan adaptations in MR-guided prostate cancer radiotherapy W. Van den Wollenberg 1 , M.F. Fast 1 , A.J.A.J. Van de Schoot 1 , C. Carbaat 1 , J. Nijkamp 1 , P. Remeijer 1 , T.M. Janssen 1 , J.J. Sonke 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The MR-Linac (Elekta Unity, Elekta AB, Stockholm, Sweden) integrates a 7MV linac with a 1.5T MRI scanner. Target misalignment cannot be corrected by a couch shift and therefore daily online plan adaptations are needed to align the dose with the target. The aim of this study was to explore dosimetric acceptability criteria against dosimetric variances induced by uncorrected inter- fractional anatomical changes. Material and Methods Data of 18 prostate cancer patients with seminal vesicles (SV) invasion were included, each consisting of a planning CT scan and 8-13 follow-up CT scans. On each scan the prostate+SV (CTV), rectum, and bladder were delineated. PTV margins used were 4 mm (prostate) and 7 mm (prostate+SV). For each planning CT a 9-beam IMRT (19x3.4Gy) MR-Linac treatment plan was made using Monaco (research). To simulate anatomical variance, the delineations from the follow-up CTs were copied to the planning CT after prostate registration. To simulate setup errors 40 adapted plans were made from each reference plan, each accounting for a random <1cm isocenter shift. The plans were adapted using segment aperture morphing [Ahunbay et al., Med. Phys., 2008], followed by segment weight optimization (SWO) only, or SWO + segment shape optimization (SWO+SSO). Following our current clinical guidelines on plan quality we evaluated the anal sphincter and bladder D50%, and rectum D35% for three scenarios: A) Reference plan, evaluated on follow-up delineations. B) Adapted plans, evaluated on the reference delineation. C) Adapted plans, evaluated on follow-up delineations. The three groups allow for an estimation of the dosimetric variations due to anatomical changes that are currently clinically accepted (A), plan adaptation induced changes (B), and compound changes (C). Scenarios B and C were used to study correlations and the impact of DVH-

Electronic Poster: Physics track: CT Imaging for treatment preparation

EP-2075 Evaluation of image fusion for computed tomography of head and neck patients with dental implants C.H. LEE 1 , C. Kim 2 , J. Kwak 1 , H. Song 1 , G. Back 1 1 ASAN MEDICAL CENTER, radiation oncology, seoul, Korea Republic of 2 KAIST, KAIST, Daejeon, Korea Republic of Purpose or Objective Computed tomography (CT) is useful for planning radiation treatment of patients with head and neck