ESTRO 36 Abstract Book

S239 ESTRO 36 _______________________________________________________________________________________________

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.

relevant organs at risk (OAR) were pre-contoured. The planning target volume (PTV) was derived at each centre (5-10 mm). All plans were checked, PTV conformality (PTV V 95% /PTV V total ) and PTV compromise (OAR V 95% /OAR&PTV V Total overlap ) indexes were also calculated. The relevant OAR for this case were the femoral head and neck (FHN) and the femur in treatment field (FTF). The IMRT dose fall-off gradient for FHN (FHN V95%/ FHN V80%) was also assessed. Normal tissues and the joint were not analysed, as their tolerances were easily met for this specific case. Results 19 centres completed 20 IMRT plans. The plan quality of 9/20(45%) submissions was suboptimal and had to be repeated. The results (see table) include the resubmitted cases (total 29 plans). The case was particularly challenging near FHN and FTF, due to an overlap of OARs with the PTV. Depending on PTV margins, overlapping FHN volumes varied from 13.8% (for PTV margins of 5mm) to 33.0% (for PTV margins of 10mm). FTF overlapping volume with PTV ranged from 24.7% to 51.1%.Plans were very conformal to PTV; however, the PTV conformality index was not useful for areas where PTV overlapped with OAR. We therefore calculated a compromise index for the PTV areas overlapping with FHN and FTF, which support the visual assessment of plans. The graph below highlights plans in which V80% was suboptimal in relation to the V95% (in total 5 plans had a suboptimal IMRT fall-off dose gradient).

Conclusion Limb STS tumours are a heterogeneous group of tumours with significant variation in PTV shape and size. Evaluating the plans for a newly implemented technique can be challenging, particularly when determining if a plan is optimal. We developed an objective assessment method that is applicable to all limb STS. The first planning results show that 45% of plans had either compromised PTV coverage in favour of meeting OAR dose constraints, or had not created a steep enough dose gradient near the OAR. We attribute this to a change in the planning technique paradigm, as many of the centres were using IMRT for limb STS for the first time. OC-0450 Geometric variation of the axillary lymph node region in locoregional breast/chest wall irradiation. K.N. Goudschaal 1 , N. Bijker 1 , A. Bel 1 , N. Van Wieringen 1 , M. Kamphuis 1

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