ESTRO 36 Abstract Book

S237 ESTRO 36 2017 _______________________________________________________________________________________________

1 Academic Medical Center, Radiotherapie, A msterdam, The Netherlands Purpose or Objective Image-guided radiation therapy (IGRT) for p atients who will be treated to both the breast/chest wall and the axillary lymph node region (ALNR) is performed with Conebeam-CT (CBCT). The position verification is based on the breast/chest wall registration. The current planning technique is a combination of a quarter field fast- forward intensity modulated radiation therapy (IMRT) for the breast/chest wall with an AP-PA beam technique for the ALNR. This technique is robust for the daily position variation of the ALNR but is not very conformal. We are planning to introduce a volumetric arc therapy (VMAT) technique which is highly conformal but the margins to account for daily position variation are not known. The aim of this study is to determine the daily positional variation of the ALNR relative to the breast. Material and Methods The study population consisted of 20 female patients treated with locoregional radiotherapy for stage II to IV breast cancer. For all 20 patients the target volume was the breast/chest wall and the ALNR level 1 and 2 or level 1 to 4, depending on the TNM classification. A standard clinical target volume (CTV) to planning target volume (PTV) margin of 5 mm is used for the entire axilla area and the delineated breast is the PTV. The patient positioning was supine with both arms up on a CQual breastboard including a wedge position combined with a knee support (CIVCO, USA). The clinical IGRT protocol with CBCT is based on the position variation of the breast/chest wall. A bony registration with a region of interest (ROI) on bony anatomy (ribs and sternum) is used as a surrogate for the breast position (XVI 4.5, Elekta). For this study 138 CBCTs were retrospectively registered on level 1 to 3 and 66 CBCTs on level 4. The CBCT analysis was based on: 1. registration of the breast/chest wall using a ROI (figure 1) on bony anatomy of the ribs and sternum; 2. registration of the ALNR using shaped ROI (SROI, figure 1) around level 1, 2, 3 and 4. The SROI is a ROI that can be designed in the shape of each level separately. No bony elements were included in these SROIs. The geometric variation was expressed as the displacement of the ALNR relative to the breast. The mean, systematic and random setup errors of the displacement of level 1 to 4 were calculated.

to variation in arm / shoulder positioning on the breastboard armrests.

Conclusion With the current patient set up there is a considerable geometric variation in Level 1 in VD direction. Introducing a highly conformal technique requires adaptation of currently used margins for adequate target coverage of both the breast/chest wall and the ALNR. OC-0451 Effect of cardiac motion on displacement of LAD artery in gated left breast treatment using MRI S.Y. Ng 1 , W.K. Fung 1 , K.M. Ku 1 , O.L. Wong 2 , G. Chiu 1 1 Hong Kong Sanatorium & Hospital, Department of Radiotherapy, Hong Kong, Hong Kong SAR China 2 Hong Kong Sanatorium & Hospital, Medical Physics & Research Department, Hong Kong, Hong Kong SAR China Purpose or Objective Respiratory control has been promoted to minimize dose to heart during left sided breast radiotherapy. However, there is limited data to address the effect of intrinsic cardiac motion during actual treatment. This study quantified the effect of both cardiac motion and respiratory motion on variation in distance between left anterior descending artery (LAD) and chest wall, D LAD , for gated left-sided breast radiotherapy using MRI. Material and Methods Eighteen healthy female volunteers aged 32.1±5.0 were scanned in a 1.5T MR simulator (MAGNETOM Aera, Siemens Healthcare) with cine mode for respiratory motion (images resp ) and cardiac triggered cine mode for cardiac motion (images card ), at the middle slice locations of three equal segments of LAD (proximal, middle and distal). The images were sorted into 10 phases for respiratory cycle and cardiac cycle respectively. D LAD was measured in each slice of images resp as shown in Figure 1. The maximum LAD displacement along the direction of D LAD (Maxdisp LAD ) was measured in images card .

Results The mean displacement for each level of the ALNR is small (table 1). Considerable geometric variation was found for Level 1 in ventral-dorsal (VD) direction. This may be due