ESTRO 37 Abstract book

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ESTRO 37

3 Central Finland Central Hospital, Department of Medical Physics, Jyväskylä, Finland Purpose or Objective Daily shape changes and swelling of breast may occur during radiotherapy (RT) treatment course. The aim of this work was to quantify the anatomical changes of breast surface to determine the extent of in-air outer PTV margin needed in breast cancer (BCa) RT. Material and Methods The anatomical changes of breast surface during the course of RT were investigated on 40 BCa patients. Patients underwent breast conserving surgery with lumpectomy only ( n =4), with sentinel node biopsy ( n =21) or with radical axillary lymph node dissection ( n =15). Patients were imaged with CT in supine position and arms up. Right sided patients ( n =20) were imaged in free breathing and left sided ( n =20) in deep inspiration breath hold (DIBH) with a guidance of an optical system (C-RAD). Mean age was 63 years (range 49-78 yr) and the PTV volume was 1330 ± 373 cm 3 (mean±SD). All patients were treated with a VMAT technique, 20 with hypo- (40.05Gy/15fr) and 20 with conventional fractionation (50Gy/25fr), with daily CBCT image guidance (Elekta Infinity/XVI). Maximum mutual information (MMI) registration (Mosaiq v2.62, Elekta AB) was performed in 6D with a box shaped region of interest between 800 CBCT and the respective planning CT images. Registrations were first computed for the whole PTV (Fig 1A) and second only for the surface area of the breast (Fig 1B). The difference in orthogonal shifts and rotations was calculated between the two registrations. The maximum value of right/left or anterior/posterior shift was calculated from each fraction to attain the maximum breast surface expansion (MBE).

Fig2. Measured maximum breast surface expansion (MBE) as a function of fraction number. The left sided patients are shown in red and right sided in blue with the average respective values in thick solid red and dotted blue line. Conclusion In addition to conventional PTV margin for set-up errors and breathing motion, an additional margin outside the breast surface is required to cover the entire target volume in whole breast irradiation. The extent of surgery and the side of treatment could affect the amount of breast swelling. EP-2049 Comprehensive robustness test based on a fast Monte Carlo dose engine for PBS proton therapy K. Souris 1 , A. Barragan 1 , J.A. Lee 1 , E. Sterpin 1 1 UCL - IREC Molecular Imaging Radiology and Oncology MIRO, MIRO, Brussels, Belgium Purpose or Objective For ensuring safe treatments with proton therapy delivered by pencil beam scanning (PBS), it is essential to evaluate the robustness of the treatment plans against numerous uncertainties, among which the conversion of Hounsfield Units into stopping powers, setup errors, and breathing motion. For this purpose, we have developed a tool enabling comprehensive robustness evaluation, including against variable breathing motion patterns. Material and Methods In order to assess the robustness of the PBS plan, multiple scenarios of treatment realizations are simulated by randomly sampling uncertainties from probability distributions reported in the literature. Uncertainty scenarios are then modeled by manipulating the planning 4DCT series to simulate realistic daily treatment images: 1. Setup errors are simulated by translating CT images 2. Stopping power errors are simulated by scaling mass densities in the images 3. Variation of the motion amplitude is simulated by generating new 4DCT phases. To do so, velocity fields are first calculated by registering 4DCT images to a reference phase. The concept of velocity field used by Janssens et al (2010) facilitates field manipulation and ensures the physical consistency of deformation. Deformation amplitude is modified by scaling the velocity fields. A new 4DCT series is then created by deforming back the reference image to each phase, employing the scaled fields. Dose distributions are calculated on each simulated 4DCT using the fast Monte Carlo code MCsquare. Its 4D dose calculation algorithm enables the simulation of the interplay between tumor and treatment beam motions. Variable motion periods are simulated in the robustness analysis. Results To validate the amplitude variation model, a 40% increase of the initial breathing amplitude was simulated for a lung tumor case. The motion is then analyzed in the generated 4DCT and compared to the initial 4DCT. An effective variation of 39% of the tumor displacement was measured.

Fig1. Planning CT (magenta) and CBCT (green) image registrations were performed (A) first based on the mutual information of the whole target volume and (B) second only on the surface areas. Results The MBE was on average 2.4 ± 2.1 mm during treatment course. With 25 patients (62.5%) the MBE was ≥ 5 mm at least once during the treatment course. The largest MBE was 12 mm and in 107 fractions (13.4%) the MBE was ≥ 5 mm (Fig 2). An outer margin of 8 mm would have been required to cover the whole breast surface in 95% of the fractions. PTV volume or patient age did not correlate with the MBE. Instead, a weak correlation was observed with the extent of surgery ( r =0.36). The MBEs were also greater with left sided patients (L: 2.6±2.3 mm, R: 2.1±1.9 mm, p <0.01) and the difference was also significant in coronal (L: 1.4±3.0 O , R: -0.8±2.3 O , p <0.01) and transversal (L: - 1.8±2.2 O , R: -0.4±2.5 O , p <0.01) rotations.