ESTRO 36 Abstract Book

S476 ESTRO 36 _______________________________________________________________________________________________

planning-CT and post-treatment CBCTs were recorded. Inter-fraction patient positioning uncertainty was null as online patient position correction was always performed. Margins were determined by combining systematic (Σ) and random (σ) errors. Van Herk (2.5∑ + 1.7σ) and McKenzie (1.3∑ ± 0.5σ) analytic solutions were used for PTV and PRV margin expansions, respectively. Results Tumor bed and OARs mean CoM displacements were less than 3-mm for all directions for both inter- and intra- fraction motion. Largest displacements were seen in the cranio-caudal (CC) direction (Figure 1). Inter-fraction motion was larger than intra-fraction motion (Figure 1). Mean intra-fraction patient positioning uncertainty was considered negligible (translation <1-mm; rotation <1 o ). Σ and σ errors differed less than 2.5-mm for organ motion and 0.5-mm for patient positioning uncertainty. The calculated PTV and PRV margins (Table 1) were up to a maximum of 6/5-mm in the CC direction, respectively.

Liver had median displacement of 12mm on 4DCT and 43mm from EE to DIBH. It was only in the beam path for D targets. Even though volume in the beam path decreased with median 1.1% (EI) and 2.6% (DIBH) compared to EE, EE was still optimal in 2 and DIBH only optimal in 5 patients. Conclusion Lung sparing can be achieved in DIBH for proximal, medial and most distal esophagus targets. For some medial and distal targets heart sparing can be achieved. As the optimal phase is not always DIBH, lung vs. heart sparing must be prioritized. No general conclusions can be drawn for liver. Further investigations are warranted. PO-0873 Inter- and intra-fraction motion of the tumor bed and organs at risk during IGRT for Wilms' tumor F. Guerreiro 1 , E. Seravalli 1 , G. Jansses 1,2 , M. Heuvel- Eibrink 2 , B. Raaymakers 1 1 UMC Utrecht, Department of Radiotherapy and Imaging Division, Utrecht, The Netherlands 2 Princess Máxima Center, Pediatric Oncology/Hematology, Utrecht, The Netherlands Purpose or Objective Radiotherapy planning for Wilms' tumor (WT) is currently done according to the SIOP-2001 protocol. The planning target volume (PTV) is defined as the clinical target volume (CTV) plus a margin of 10-mm while no planning risk volume (PRV) margins are recommended. The aim of this study is to assess inter- and intra-fraction motion of the tumor bed and organs at risk (OARs) as well as patient positioning uncertainty to estimate PTV and PRV margins for flank irradiation in WT. Material and Methods Computed tomography (CT), 4D-CT and daily cone-beam CTs (CBCTs), acquired during planning and treatment of 10 pediatric patients (mean 3.9 ± 2.1 years) were used. OARs (kidney, liver and spleen) were delineated without accounting for any motion in all image sets. OARs motion was quantified in terms of absolute displacements of the center of mass (CoM) in all orthogonal directions. Tumor bed motion estimation was assessed using a quadratic sum of the CoM displacements of 4 clips positioned at the superior, lateral, medial, and inferior border of the tumor during surgical resection. Intra-fraction motion was estimated by calculating the CoM displacements between the maximum inspiration and expiration phases of the 4D- CT. For inter-fraction motion assessment, CoM displacements were calculated using the planning-CT as reference and daily pre-treatment CBCTs. For intra-fraction patient positioning uncertainty, translational and rotational bone off-sets between the

Conclusion Imaging data collected before and during radiotherapy demonstrated limited motion of the tumor bed and OARs and reduced patient positioning uncertainty. By combining 4D-CT and daily CBCTs information, PTV margins can be reduced to 6-mm in the CC direction compared with the existing protocol. The use of PRV margins for OARs protection is also advised. In addition, margins should be applied anisotropically and individualized for each patient. PO-0874 The impact of rectal filing on rectal tumor position J.J.E. Kleijnen 1 , M. Intven 1 , B. Van Asselen 1 , A.M. Couwenberg 1 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiotherapy department, Utrecht, The Netherlands Purpose or Objective In 15% of rectal cancer patients, a pathological complete response (pCR) is observed after neo-adjuvant chemoradiotherapy. To increase this pCR rate, many studies are being performed, in which the GTV dose is escalated. To avoid an increase in toxicity and potential surgical complications, PTV margins must be minimized and geometrical miss has to be avoided. However, rectal filling can change from day-to-day as can be observed in daily practice (see figure 1, A & B), which might alter the