ESTRO 36 Abstract Book

S785 ESTRO 36 _______________________________________________________________________________________________

EP-1469 Flattening filter free beam profile analysis using two different normalization methods G. Nicolini 1 , A. Fogliata 2 , E. Vanetti 1 , G. Reggiori 2 , A. Stravato 2 , P. Mancosu 2 , M. Scorsetti 2 , L. Cozzi 2 1 Radiqa Developments, Medical Physics Team, Bellinzona, Switzerland 2 Humanitas Research Hospital, Radiotherapy and Radiosurgery Dept, Milan, Italy Purpose or Objective Flattening filter free (FFF) beams present a profile peaked on the beam central axis (cax), unsuitable for flatness and symmetry description that usually characterize standard beam profiles. Definitions of unflatness and slope have been recently proposed, requiring a preliminar suitable FFF profile normalization. Two main normalization processes as far published are: the inflection point IP (Pönish 2006), and the renormalization factor RF (Fogliata 2012). In both formalisms the FFF dose fall-off at the field edge is superimposed with the corresponding FF profile. The present study aims to compare FFF specific profile parameters using the two normalization procedures. Material and Methods Dosimetric data from a Varian TrueBeam with 6 and 10 MV, FF and FFF modes, have been collected at SSD 100cm and 5 depths. The cax normalization value N was evaluated for the IP method as N=D cax ·(D u /D f ), where D cax and D f are the doses on cax and at the IP of the penumbra region for the corresponding FF beam, D u is the dose at the IP of the FFF beam. The N value for the RF method was evaluated by using the fit dependent on the field size FS and depth: N=(a+b·FS+c·depth)/(1+d·FS+e·depth), where the fitting parameters are taken from published data. The main profile parameters of FFF photon beams were computed: field size, penumbra, unflatness, slope, and peak-position parameters. To systematically investigate the impact of the N value, they were recomputed with a RF value modified of + 1,2,3,5,7,10% (perturbed RF). Results In terms of cax normalization value, in average, the two methods show an agreement within the 2%, with a tendency of a greater N with IP respect RF method for 10MV. In any case, some outliers are present, with a discrepancy that reaches the 10%; this is expected, since the IP method suffers of the uncertainty of IP position determination in the practice. Beam parameters values derived with the approaches (IP/RF) were computed showing, e.g., for both energies 1.00+0.00 for unflatness and, respectively for 6 and 10MV, 0.99+0.05 and 1.02+0.04 for slope. Analysis with perturbed RF values, shows that with a variation up to 10% of N, the peak position remains within 0.05mm, the unflattens within 0.5% and 1% for 6MV and 10MV beams, while the slope has a variation almost of the same amount of N itself. Field size difference is within 1mm if N variation is within 5%. Conclusion The two normalization methods are both suitable for subsequent FFF profile description. Unflatness parameter resulted similar when computed using the two different normalization formalisms with no significant differences. Slope values are more sensitive to normalization value, and therefore some outliers were observed due to uncertainty of IP position in the practice. The RF procedure, with the published fitting parameters is easier to use and more robust respect to measurements sampling and detector size. EP-1470 Determination of paramagnetic gel sensitivity in low energy X-ray beam Y. Ben Ahmed 1 , J. Coulaud 2 , S. Ken 1 , L. Parent 1 1 Institut Universitaire du Cancer Toulouse, Haute

Garonne, Toulouse, France 2 SIMAD, Haute Garonne, Toulouse, France

Purpose or Objective The INTRABEAM® system is a miniature accelerator producing low energy photons (50 keV maximum). The published dosimetric characterization of the INTRABEAM system for flat and surface applicators was based on detectors (radiochromic films or ionization chambers) not allowing measuring the absorbed dose in the first millimeters of the irradiated medium, where the dose is actually prescribed. This study aims at determining the sensitivity of a paramagnetic gel in order to measure the dose deposited with INTRABEAM surface applicators in the first millimeters of irradiated medium. Material and Methods The determination of paramagnetic gel sensitivity was performed with irradiations at different dose levels with the INTRABEAM® Carl Zeiss Surgical system (Oberkochen, Germany). The ferrous gel used in this study is a new « sensitis» material which is described by C. Stien et al and V. Dedieu et al. Gel irradiation in tin and capsule containers was carried out for twelve dose levels between 2 Gy and 50 Gy at the gel surface with a 4 cm surface applicators. The applicator was in contact of the gel during irradiation. For the calibration curve, one batch gel was measured without being irradiated. T 2 weighted multi echo MRI acquisitions were performed on a 1,5 T Magnetom Aera MR scanner of Siemens with surface flex head coil technology. Results The T 2 signal versus echo times can be fitted with a mono- exponential function with 95% of confidence. The first echo time was not considered for the fit. The calibration curve determined from experiments with tins is a linear function (R 2 =0.967) with a sensitivity of 1.04*10 -4 s -1 .Gy -1 . Gels Sensitivity with capsules are of 3.67*10 -4 s -1 .Gy -1 (R 2 =0.979) and 2.54*10 -4 s -1 .Gy -1 (R 2 =0.944). The calibration curve was applied to the irradiation of a surface applicator to obtain the 3D dose distribution in the gel. Conclusion The dose distribution obtained after irradiation at low energies with an INTRABEAM® miniature accelerator can be measured for the first millimeters thanks to ferrous gels. The determination of gel sensitivity was possible with MRI measurements. Results are relevant but must be confirmed with more irradiations with different dose levels at the surface and different surface and flat applicator diameters. EP-1471 Comparison of the integral dose of IMRT, RapidArc and helical tomotherapy prostate treatments J. Martinez Ortega 1 , P. Castro Tejero 2 , M. Pinto Monedero 1 , M. Roch Gonzalez 2 , L. Perez Gonzalez 2 1 Hospital Universitario Puerta de Hierro, Servicio de Radiofísica y PR, Majadahonda - Madrid, Spain 2 Hospital Universitario de la Princesa, Servicio de Radioterapia, Madrid, Spain Purpose or Objective Comparison of integral dose (ID) and normal tissue integral dose (NTID) for Helical Tomotherapy (HT), RapidArc and static fields IMRT. Material and Methods A cohort of ten prostate patients were selected whose prescription was 78 Gy mean dose to the Planning Target Volume (PTV). Seven different plans for every patient were computed. One sliding-window IMRT with XiO planning system and Varian Clinac 21EX, equipped with MLC Millennium 80. Four Intensity-Modulated Radiation Therapy (IMRT) plans were calculated with Varian Eclipse planning system, two step-and-shoot and sliding-window