ESTRO 37 Abstract book

S1191

ESTRO 37

sigma=0.04 – 0.13 mm) in the crossplane and inplane orientations respectively (fig. 1). Even though most sources may be considered symmetrical, there were cases were a clear asymmetry was evident (TB6). The source shape reproduced a Gaussian profile with a RMSE of 2.9- 7.7% in the 100 – 10 % fluence region, while systematic deviations were evident in the lower tail region. Conclusion The reconstructed X-ray sources for six Varian TBs are in most cases symmetrical with FWHM values between 1.0 and 1.5 mm. The common approximation of a Gaussian profile, used by most beam models, might not be adequate for the tail region. Further investigation into the appropriate functional form is required. [1] Papaconstadopoulos et al PMB 61 (2016) 1078-1094

Conclusion Distortion Correction software was proved in our study as a helpful tool to detect and adequately correct brain MR distortions. EP-2153 The primary X-ray source: reconstruction and characterization of six Varian TrueBeam sources P. Papaconstadopoulos 1 , K. O'Grady 2 , S. Aldelaijan 1 , S. Devic 1 , I.R. Levesque 3 , J. Seuntjens 3 1 The Jewish General Hospital- McGill University, Radiation Oncology, Montreal, Canada 2 Montefiore Medical Center, Radiation Oncology, New York, USA 3 The Cedars Cancer Institute- McGill University, Medical Physics, Montreal, Canada Purpose or Objective the primary X-ray source size and shape, as generated in the entrance of the linac target, is one of the most important parameters for accurate beam characterization and modeling. In this work we apply a recently suggested and clinically applicable method [1] to fully characterize the primary X-ray source of six Varian TrueBeam (TB). Material and Methods A series of crossplane and inplane photon fluence profile measurements were performed on six clinical linear accelerators (Varian TB including two Stx). The photon fluence profiles were measured using radiochromic film (average of five repetitions) in air with a 2 mm lead (Pb) foil as a build-up layer. A deconvolution kernel was applied to account for the non-zero electron range and photon scattering in the foil. The measurements were performed using the smallest jaw-defined field size (5 mm) in order to maximize the differentiation among the various source sizes and eliminate the impact of scatter sources and other beam parameters. An iterative MLEM reconstruction algorithm was then applied to estimate the source distribution, using a projection/back- projection approach by ray-tracing, to converge on the final source estimate. The exact jaw position was determined by repeating the reconstruction for all possible jaw positions in the range of 4 – 6 mm (step = 0.1 mm) and minimizing the error between reconstructed and measured profile. The technique’s predictive accuracy in reproducing the correct source size and shape was benchmarked using a commissioned Monte Carlo (MC) model (EGSnrc/BEAMnrc) of a Varian TrueBeam with predefined input Gaussian source sizes (FWHM = 0.5, 0.75, 1.0, 1.25, 1.5, 1.75, 2 mm) and fixed jaw position of 5 mm projected at 100 cm. Results At the MC benchmarking stage the MLEM reconstruction algorithm reproduced the Gaussian profile with a root- mean-squared-error (RMSE) of 1.8-4.9 %. The reconstructed source size (FWHM) and jaw position were determined with an accuracy of 0.1 mm in all cases. The algorithm’s ability to reach a unique solution was best for source sizes of FWHM less than 1.5 mm. Experimentally, reconstructed sources presented FWHM values of 1.02 – 1.5 mm (1 sigma=0.04 - 0.18 mm) and 1.08-1.42 mm (1

EP-2154 Lean-six-sigma methodology for improving quality in RT: the breast daily repositioning case P. Mancosu 1 , G. Nicolini 2 , G. Goretti 3 , F. De Rose 1 , D. Franceschini 1 , C. Ferrari 3 , S. Tomatis 1 , M. Scorsetti 4 1 Istituto Clinico Humanitas, Radiotherapy and Oncology, Rozzano Milan, Italy 2 Radiqa Developments, Medical Physics Team, Bellinzona, Swaziland 3 Istituto Clinico Humanitas, Lean Group, Rozzano Milan, Italy 4 Istituto Clinico Humanitas & Humanitas University, Radiotherapy and Oncology & Biomedical Sciences dept., Rozzano Milan, Italy Purpose or Objective According to the ESTRO booklet 4 guidelines, the concept of quality assurance in RT encompasses a comprehensive approach to all activities in the department. When hypofractionation regiment is adopted, the need of a continuous quality procedures updating is self-evident. At this aim, the Lean Six Sigma Methodology (LSSM) has been applied in our institute. LSSM was introduced in industry for providing near-perfect services to large processes, by reducing improbable occurrence. It consists of the synergetic adoption of two methods: (i) create a continuous process flow eliminating waste (Lean); (ii) reduce process variation (Six sigma) achieving the best quality. LSSM has been prospectively applied to breast patient repositioning. At our knowledge, this is the first time the LSSM has been rigorously applied to a RT process. Material and Methods Breast patients treated by hypofractionated RT in SIB delivered through VMAT technology with daily 2D-2D matching were considered. The five DMAIC (define, measure, analyze, improve and control) LSSM steps were applied by an interdisciplinary project team. The process was retrospective measured over 30 months (Jul2014– Dec2016) by querying the RT Record&Verify database.