ESTRO 36 Abstract Book

S410 ESTRO 36 2017 _______________________________________________________________________________________________

MATLAB, allowing determination of the correction for scanner inhomogeneity and calibration of film optical density (OD) response in terms of absorbed dose to water. Regions of interest (ROIs) of various sizes were used to sample image data, quantifying the uncertainty associated with variations within each film, from film to film within the same lot, and from lot to lot. 35 sheets of film were used, taken from 7 boxes across 3 lots. Three channels of optical density (OD) data were analysed statistically, both directly as OD and also in the ratios red/blue and green/blue. Net values were obtained by subtracting pre- irradiation values, and a normalisation correction factor, based on large dose saturation values, was applied. Results The figure shows the net ratio of OD, green/blue, before and after applying the normalisation correction, as a function of dose, for ROIs which are 10 x 10 mm 2 . The table lists the relative standard deviation of absorbed dose measurements made using EBT-3 in the present work.

objective of its design is to offer a high spatial resolution with 100% fill factor. Material and Methods The prototype has 2041 liquid ionization chambers of 2.5x2.5 mm 2 effective area and 0.5 mm thickness. The detection elements are arranged in a central square grid of 43x43, covering an area of 107.5x107.5 mm 2 . The central inline and crossline are extended to 227 mm and the diagonals to 321 mm. The active medium is liquid isooctane. We have studied short- and medium-term stability, dose rate dependence, depth and field size dependence, anisotropy and leaf positioning detectability. We have measured output factors, tongue-and-groove, garden fence, small field profiles and irregular fields. Finally we have used it for the verification of patient treatments. Results The detector presents dependency on energy that is reflected in the response variation with depth and field size (2.2% under-response for 6 MV, 20x20 cm 2 at 20 cm depth). The anisotropy study shows important deviations: 28% for lateral incidences and 7% for posterior incidence. The detector sensitivity for leaf positioning measurement is 1.8 % per tenth of millimeter in the penumbra. The output factor corresponding to 6 MV and 1x1 cm 2 shows +2% deviation compared with the measurements obtained using a SFD diode and a CC13 gas ionization chamber. The results are normalized to a 5x5 cm 2 . For a 10x10 cm 2 this deviation is -1%. If the energy increases the deviations decrease (+1% for 1x1 cm 2 and -0.5% for 10x10 cm 2 in 10 MV and 15 MV). In the measurement of small field profiles the gamma comparison between measurements with the liquid ionization array and radiographic film shows 100% passing rates with tolerances 1% - 1mm. Several patient treatments have been verified. In table 1 the comparison between the treatment planning system and the array measurement for a particular case is shown. We show differences in gamma passing rates when anisotropy corrections are applied or not. Figure 1 shows one of such comparisons. Conclusion A new detector array is presented for the verification of patient treatments of high complexity. The detector presents a small dependence on energy, which causes a small over-response for the output factors of small fields and an under-response for output factors of large fields. The anisotropy of the device is significant (28% and 7% for lateral and posterior incidences), but can be compensated during treatment verification by using angle-dependent correction factors. The usefulness for the patient treatment verification has been demonstrated by measuring different patient treatments. The results obtained confirm the validity of this array for dose distribution measurements of complex treatments with small fields and high gradients. PO-0783 Planverification in Robotic Stereotactic Radiotherapy with the Delta4-Dosimetry-System W. Baus 1 , G. Altenstein 1 1 Universität zu Köln, Department of Medical Physics, Köln, Germany Purpose or Objective Stereotactic robotic radiotherapy with the CyberKnife (Accuray, Sunnyvale) might not be fluency modulated radiotherapy (IMRT) in the strict sense. However, the technique is comparable in complexity because of a large number of small (5 to 60 mm), highly non-coplanar fields. Therefore, the manufacturer recommends individual plan verification (DQA, Delivery Quality Assurance), though only on a point dose measurement basis. The report of the

Conclusion By combining the subtraction of pre-irradiation values, with a normalisation correction based on large dose saturation values, it should be possible to reduce the contribution to measurement uncertainty arising from intrinsic variations in the characteristics of EBT-3 film to 0.7 % ( k =1) for doses in the range of up to 5 Gy. PO-0782 New liquid ionization chamber detector of high resolution for treatment verification in Radiotherapy L. Brualla-Gonzalez 1 , A. Vázquez-Luque 2 , M. Zapata 3,4 , D.M. González-Castaño 3,5 , V. Luna-Vega 4 , J. Guiu-Souto 4 , D. Granero 1 , A. Vicedo 1 , M.T. García-Hernández 1 , J. Roselló 1,6 , M. Pombar 3,4 , F. Gómez 3,7 , J. Pardo-Montero 3,4 1 Hospital General Universitario de Valencia, Servicio de Radiofísica ERESA, Valencia, Spain 2 Detection And Radiation Technologies SL, Development, Santiago de Compostela, Spain 3 Instituto de Investigación Sanitaria IDIS, Grupo de Imaxe Molecular, Santiago de Compostela, Spain 4 Complexo Hospitalario Universitario de Santiago de Compostela, Servizo de Radiofísica e Protección Radiolóxica, Santiago de Compostela, Spain 5 Universidade de Santiago de Compostela, Laboratorio de Física de Partículas RIAIDT, Santiago de Compostela, Spain 6 Universidad de Valencia, Departamento de Fisiología de la Facultad de Medicina, Valencia, Spain 7 Universidade de Santiago de Compostela, Departamento de Física de Partículas, Santiago de Compostela, Spain Purpose or Objective In this work we present a new liquid ionization chamber array prototype for patient treatment verification. The