ESTRO 36 Abstract Book

S414 ESTRO 36 _______________________________________________________________________________________________

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.

Purpose or Objective PBS, recently developed, scans a tumor with very precise beam of protons that’s accurate within millimeters, sparing the healthy surrounding tissues. But it is able to harmful rather than conventional radiotherapy if the beam is not accurately irradiated as planned. It is very important to measure beam width and spot center of the proton pencil beam for the accurate delivery of dose to the target volume with a good conformity. We have developed the beam monitoring system using Array of Fiber-Optic Cerenkov Radiation Sensor (AFCRS), and conducted feasibility study for proton pencil beam. Material and Methods We have developed a fine segmented detector array to monitor PBS. A prototype beam monitor system using AFCRS has been developed for real-time display of the pencil beam status during the PBS mode operation. The x- y monitoring system with 128 channel readout is mounted to the snout for the in-situ real time monitoring. Beam widths and spot centers of various energies are measured. Two dimensional Gaussian fit is used to analyze the beam width and the spot center. The ability of this system to evaluate Lynx system (Scintillator-based sensor with CCD camera) and EBT3 for PBS was compared. Results The measured Gaussian widths using AFCRS changes from 13 to 5 mm for the beam energies from 100 to 226 MeV. The beam widths of PBS using the AFCRS are well matched with the data acquired by a Lynx system and EBT 3 film. In addition, spot centers for 226 Mev PBS beams are also well matched with RTP system. Conclusion The dosimetric performance of the newly developed system based on AFCRS was comparable to that of the Lynx system and EBT3 film. Not only measuring the spot profile but also monitoring dose map by accumulating each spot measurement will be available. PO-0781 A characterisation of EBT3 Gafchromic film for relative and absolute dosimetry I. Billas 1 , H. Bouchard 2 , A. Subiel 1 , I. Silvestre 1 , S. Duane 1 1 National Physical Laboratory, Radiation Dosimetry, Middlesex, United Kingdom 2 Université de Montréal, Département de physique, Montréal, Canada Purpose or Objective The aim of this work is to investigate the variation in dose response of Gafchromic EBT-3 film within each film and across films from different boxes and lots. In this way the uncertainty of relative and absolute dosimetry using EBT- 3 film is quantified and its potential for use in small field and MRI-guided radiotherapy is better understood. Material and Methods Sheets of Gafchromic EBT-3 film were uniformly irradiated in a cobalt-60 beam in increments of 1 Gy up to a cumulative dose of 10 Gy. Films were scanned repeatedly before the first irradiation and after each step. Software for image processing and analysis was implemented in 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

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 objective of its design is to offer a high spatial resolution 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. with 100% fill factor. Material and Methods