ESTRO 36 Abstract Book

S404 ESTRO 36 2017 _______________________________________________________________________________________________

Campus Pius Hospital Carl von Ossietzky University, Oldenburg, Germany 2 Prof em. Medical Physics and Biophysics, Georg August University, Göttingen, Germany Purpose or Objective The development of therapeutic devices combining clinical linear accelerators and MRI scanners for MR guided radiotherapy leads to new challenges in the clinical dosimetry since the trajectories of the secondary electrons are influenced by the Lorentz force. In this study, the lateral dose response functions of a clinical air- filled ionization chamber in the presence of a magnetic field were examined depending on beam quality and magnetic field following the approach of a convolution model (Looe et al 2015, Harder et al 2014). Material and Methods In the convolution model, the 1D lateral dose response function K ( x-ξ ) is defined as the convolution kernel transforming the true dose profile D ( ξ ) into the disturbed signal profile M ( x ) measured with a detector. For an air- filled ionization chamber, type T31021 (PTW Freiburg, Germany), the lateral dose response functions were determined by Monte-Carlo simulation using 0.25 mm wide 60 Co and 6 MV slit beams. The chamber was modelled according to manufacturer’s detailed specification and placed at 5 cm water depth in three different orientations, i.e. axial, lateral and longitudinal. For each chamber orientation, a magnetic field oriented perpendicular to the beam axis was applied. Simulations were performed for magnetic fields of 0, 0.5, 1 and 1.5 T using the EGSnrc package and the egs_chamber code. To verify the simulation results, the lateral dose response functions without magnetic field were compared against measurements with a 5 mm wide collimated 6 MV photon slit beam using tertiary lead blocks following the approach of Poppinga et al 2015. Results Fig. 1 shows good agreement between the simulated and measured dose response functions K ( x-ξ ) of the investigated ionization chamber in the three investigated orientations. The structures of the measured functions are not as evident as those of the simulated functions possibly due different scanning step widths used in the experiment and the calculation. Fig. 2 shows the lateral dose response function K ( x-ξ ) with and without magnetic field obtained exemplary for the detector in lateral orientation. The distortion of the dose response function K ( x-ξ ) corresponds to the change in the trajectory lengths of the secondary electrons in the air of the ionization chamber due to the Lorentz force, as compared to the trajectories in a small sample of water.

Fig. 2. Area-normalized dose response functions K ( x - ξ ) for the T31021 in lateral orientation for magnetic fields of 0, 0.5,1 and 1.5 T Conclusion The distortions of the lateral dose response function K ( x- ξ ) will alter the measured signal profile M ( x ) of a detector in magnetic field, as demonstrated in this study. The variety of the possible combinations of detector orientation and magnetic field direction and the strong dependence of the distortion on the magnetic field strength require careful consideration whenever a non- water equivalent detector is used in magnetic field. PO-0772 Patient-specific realtime error detection for VMAT based on transmission detector measurements M. Pasler 1 , K. Michel 2 , L. Marrazzo 3 , M. Obenland 4 , S. Pallotta 5 , H. Wirtz 4 , J. Lutterbach 6 1 Lake Constance Radiation Oncology Center, Department for Medical Physics, Friedrichshafen, Germany 2 Lake Constance Radiation Oncology Center- Martin- Luther-Universität Halle-Wittenberg, Department for Medical Physics- Naturwissenschaftliche Fakultät II, Friedrichshafen, Germany 3 AOU Careggi, Medical Physics Unit, Florence, Italy 4 Lake Constance Radiation Oncology Center, Department for Medical Physics, Singen, Germany 5 University of Florence- AOU Careggi, Medical Physics Unit- Department of Biomedical- Experimental and Clinical Sciences, Florence, Italy 6 Lake Constance Radiation Oncology Center, Radiooncology, Singen, Germany Purpose or Objective To investigate a new transmission detector for online dose verification. Error detection ability was examined and the correlation between the changes in detector output signal with γ passing rate and DVH variations was evaluated. Material and Methods The integral quality monitor detector (IQM, iRT Systems GmbH, Germany) consists of a single large area ionization chamber which is positioned between the treatment head and the patient. The ionization chamber has a gradient along the direction of MLC motion and is thus spatially sensitive. The detector provides an output for each single control point (segment-by-segment) and a cumulative output which is compared with a calculated value. Signal stability and error detection sensitivity were investigated. Ten types of errors were induced in clinical VMAT plans for three treatment sites: head and neck (HN), prostate (PC) and breast cancer (MC). Treatment plans were generated with Pinnacle (V.14) for an Elekta synergy linac (MLCi2). Geometric errors included shifts of one or both leaf banks for all control points toward (i) and away (ii) from the central axis of the beam and unidirectional shifts of both leaf banks (iii) by 1 and 2mm, respectively. Dosimetric errors were induced by increasing the number of MUs by 2% and 5%. Deviations in dose distributions between the original and error-induced plans were compared in terms of IQM signal deviation, 2D γ passing rate (2%/2mm and3%/3mm) and DVH metrics (D mean , D 2% and D 98% for PTV and OARs). Results For segment-by-segment evaluation, calculated and measured IQM signal differed by 4.7%±5.5%, -2.6%±4.6% and 4.19%±6.56% for MC, PC und HN plans, respectively. Concerning the cumulative evaluation, the deviation was -1.4±0.25%, -6.0±0.3% und -1.47%±0.97%, respectively. Signal stability for ten successive measurements was within 0.5% and 2% for the cumulative and the segment- by-segment analysis. The IQM system is highly sensitive in detecting geometric errors down to 1mm MLC bank displacement and dosimetric errors of 2% if a measured signal is used as reference. Table 1 reports IQM signal deviations for a range of introduced errors.

Fig. 1. Area-normalized simulated and measured dose response functions K ( x - ξ )