ESTRO 2020 Abstract book

S843 ESTRO 2020

obtained with breath hold techniques in 3D-CRT technique. VMAT and IMRT should be reserved only for especially challenging cases of WBI. PO-1478 The effect of magnetic port density on dose distribution in 3D, IMRT, VMAT for breast radiotherapy A. Rygielska 1 , A. Walewska 1 , M. Gabor 1 , D. Pruska-Pich 1 , B. Czeremszynska 2 , L. Kepka 2 1 Military Institute od Medicine, Radiotherapy Department- Laboratory of Medical Physics, Warsaw, Poland ; 2 Military Institute od Medicine, Radiotherapy Department, Warsaw, Poland Purpose or Objective Postmastectomy radiotherapy following immediate breast reconstruction is increasingly used in the treatment of patients with breast cancer. It means that patients are irradiated with a tissue expander (TE) in place before permanent implant exchange. The purpose of this study is to evaluate the influence of magnetic port (MP) in expander on dose distribution for 3D, IMRT and VMAT techniques in case of lack of full technical specification of the magnetic port. The effect of relative electron density (rED) values given to different parts of port was examined too. Material and Methods The CIRS phantom (CIRS Inc. USA) and expander (Nagor Ltd. the United Kingdom) were used. Three single MOSFET detectors (Best medical, Canada) were placed on expander surface under the 2 mm orfit (used for attach expander to phantom). Mosfets were positioned on the top (named A), proximal (B) and distal (C) part of “breast” (Fig. 1). 1 cm silicon bolus (Klarity Medical, USA) was placed on the top. PTV was delineated as usual in such a treatment situation. The total dose was prescribed to PTV mean dose as 39.9 Gy in 15 fractions. Three types of plan: 3D-CRT, IMRT and VMAT (all 6X) was prepared using Monaco v.5.1 (Elekta) treatment planning system (TPS). As we hadn’t a full technical specification of magnetic port (material and dimensions) we made contour for port and divided it into two parts with different density. For planning, we used two types of CT: with the overwritten rED (equals the average number of rED in each volume of MP) and with ‘row’ rED (without overwritten rED). Every plan was irradiated three times, and doses from every mosfet were collected.

prescribed dose for WBI was 39.9 Gy in 15 fractions. For each patient in TPS Monaco v.5.1 (Elekta) 3 plans were generated: 3D-CRT, IMRT and VMAT. The following parameters were compared separately for left and right breast: PTV coverage by the 95% isodose (PTV 95% ), mean heart and lung) doses (D mean H, D mean L), percentage volume of ipsilateral and contralateral lung receiving doses: 5 Gy (V 5Gy ipsL, V 5Gy contrL), 20 Gy (V 20Gy ipsL, V 20Gy contrL). Also percentage volume receiving 5Gy for contralateral breast (V 5Gy contrB) and whole body volume receiving 5 Gy (V 5Gy Body) were compared. Maximum dose for left anterior descending coronary artery (D max LAD) and mean dose for left ventricle (D mean LVentr) were compared for left-sided only. The Wilcoxon signed rank test was used to compare 3D-CRT, IMRT, VMAT plans differences, while the data were analyzed by STATISTICA v.13.1software (Statsoft, Poland). The results were considered statistically significant at a p-value less than 0.05. Results The results are presented in Table 1. There was no difference in the PTV coverage for compared techniques. For left breast: IMRT and VMAT decreased significantly D max LAD compared to 3D-CRT; VMAT , but not IMRT was beneficial over 3D-CRT for D mean LVentr; mean heart dose was not different for all of compared techniques.For right breast: there was no significant clinical improvement for OAR seen. Additionally, IMRT and VMAT increased mean heart dose, however in the range of very low doses. For both sides, V 5Gy ipsL, V 5Gy contrB, and V 5Gy Body increased significantly for IMRT and more for VMAT than for 3DCRT.

Results We obtained agreement with TPS within 4.8% using for planning CT with overwritten rED for magnetic port for all of the tested techniques. In case of using ‘row’ CT (including artefacts, without overwritten density) the difference is higher (up to 6.8 %). In both situations, the biggest difference was noticed for IMRT plan (point A, the nearest to MP). Conclusion In case of lack of the full technical specification of expander (especially magnetic port) the CT with the average number of rED in each part of magnetic port may be used in routine.

Conclusion All 3 compared techniques provided adequate PTV coverage. The dose reduction for LAD and left ventricle in left WBI with dynamic techniques was obtained at the price of higher body integral dose related to higher volume of low dose given to OAR. Such a reduction may be