ESTRO 36 Abstract Book

S847 ESTRO 36 _______________________________________________________________________________________________

multiple collimator angles), followed by VMAT, electronic compensator, and field-in-field. The optimal IMRT and VMAT plans were typically considered clinically acceptable, while the electronic compensator and field- in-field plans were not (poor homogeneity). Our original clinical plans were generally more homogeneous than those created for the prototype MLC design. The optimal treatment plans for treatments that are typically treated with two beam angles (breast, whole brain) used IMRT with the conventional beam angles. For large breasts, 2 additional IMRT fields were needed to improve coverage and homogeneity (see figure)

decreased doses in organs at risk (OAR). Simultaneous irradiation of regional lymph nodes (RLNs) is done only in supine position, losing this beneficial effect. We have performed a feasibility study of irradiating large (> 780 ml) breasts with RLNs in prone setup. Material and Methods Target volumes including breast, supra-, infraclavicular, Rotter’s, axillary lymph nodes with or without internal mammary (IM) chain were contoured on six tomography scans of 5 patients immobilized in prone position using two commercial breast boards. Delineation was done in accordance with European Society for Radiotherapy and Oncology (ESTRO) consensus. Radiotherapy plans using static (3D CRT) and dynamic (IMRT) conformal techniques were prepared. Dose-volume limits were based on QUANTEC review. Results In all plans mean doses to the heart, lung (ipsilateral, contralateral and both), left descending artery (LAD) were obtained. Volumes receiving more than 20 and 25 Gy were reported in lungs and heart, respectively. Mean values from all plans are presented in Table 1.

The best VMAT plans created with the prototype MLC were typically less homogeneous but more conformal than IMRT plans, when 4 or more arcs were used (see figure comparing IMRT (left) and VMAT (right)). Based on our current experience, we suggest the use of IMRT for this prototype MLC design - because these plans are significantly faster to optimize, and usually give the best treatment plans (for this MLC) .

Radiotherapy to breast and RLNs with IM was associated with significantly higher doses in all OARs independently of the technique used. 3D CRT plans resulted in lower doses than IMRT to nearly all structures.

Beam modulation was similar for IMRT and VMAT (3.2 vs 3.4 MU/cGy. When comparing the calculated dose and delivered dose the average gamma passing rate (3%/3mm) was 99.5% (range: 91%-100%) and 99.0% (97.7%-100%) for IMRT and VMAT, respectively. Conclusion It was possible to plan and deliver clinically acceptable plans for all treatment sites using the prototype 1.0cm MLC design. Initial experience was that IMRT plans outperform the VMAT plans in terms of homogeneity. EP-1572 Feasibility study of prone position in radiotherapy of breast with regional lymph nodes E. Pawlowska 1 , A. Prawdzik 1 , M. Narkowicz 1 , M. Damięcka 1 , R. Zaucha 1 1 Medical University of Gdansk, Department of Oncology and Radiotherapy, Gdansk, Poland Purpose or Objective Prone position radiotherapy has been successfully used to treat breast cancer in women with large, pendulous breasts. The benefit of this technique comes from

Image 1 presents differences in dose distribution between IMRT (bottom) and 3D CRT (top). PTV includes breast and RLNs with IM