ESTRO 36 Abstract Book

S775 ESTRO 36 2017 _______________________________________________________________________________________________

head and neck (H&N), brain and sarcoma in tomotherapy; breasts at Linac and skin tumors in Plesio-Röntgen therapy. For each patient films were located in 1 to 3 reproducible points (see figure 1 and 2) and measurements were repeated on average in three consecutive fractions. EBT3 films were read with a flatbed scanner Epson 10000XL and images were analyzed using the red channel calibration. In vivo dose evaluations were compared with measurements performed on Cheese phantom both with and without thermoplastic mask at Linac and in Tomotherapy.

assurance in radiotherapy, since the TPS does not give accurate dose values in the first millimiters of skin. 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

Results A total of 117 film measurements were performed on 21 patients. The absolute value of the mean difference between measured and TPS-calculated dose and its standard deviation was 11.3% ± 6.5% for all treatments. A mean absolute difference of 17.7% for Linac plans, 11.6% in Tomotherapy and 4.6% in Plesio-Röntgen therapy were achieved. Both at Linac and in Plesio-Röntgen therapy there was not a clear trend of overestimation of the TPS with respect to measurements. Instead in Tomotherapy there was an underestimation of the TPS (-9.1%) for H&N and brain treatments (in these case measurements were performed with thermoplastic mask) and an overestimation for the sarcoma (9.2%). This trend was confirmed by the measurements made on the Cheese phantom in Tomotherapy, where there was an overestimation of the TPS without mask (28.6% vs -0.7% with mask). Moreover, an improvement of the agreement between EBT3 measurements and Pinnacle and Tomotherapy dose estimation was shown in presence of mask (28.6% to -0.7% in Tomotherapy and -20.7% to -16.3% at Linac). Conclusion Gafchromic films are suitable detectors for skin dose measurements in radiotherapy. In vivo surface dose measurements with EBT3 are a useful tool for quality