S728 ESTRO 35 2016 _____________________________________________________________________________________________________ Purpose or Objective: In this work we carried out a series of measurements of a small field to investigate the shape of the dose deposition kernel of a radiotherapy beam. Starting with 2D dose distributions measured with radiochromic films a deconvolution process is followed to obtain the dose deposition kernels. Material and Methods: Radiochromic films Gafchromic EBT2 were

more »

... ed to measure 6 MV beams from a Varian Clinac 2100 linear accelerator. The nominal field size of the beams was 0.5 cm x 0.5 cm (at isocenter), and the films were places in a PMMA phantom at 100 cm source to film distance. The dose delivered to the film was 300 cGy. The films were read 6 hours after irradiation with a Microtek ScanMaker 9800XL flatbed scanner. In order to minimize the inhomogeneity variations of the film-digitizer system a procedure, as described in [1] was followed. The procedure uses a number of film cut-outs, taken from one sheet of an RC film, to produce a number of measurements of the same field. After reading the films, the resulting images are registered and averaged. It's worth noting that the film pieces used for the calibration of the film-digitizer response are taken from the same sheet of RC film that the pieces used for dosimetric purposes [1]. In this way, the inter-digitization variability is drastically reduced. Deconvolution of measured dose distributions was carried out by minimizing its Euclidean distance to a calculated dose distribution. The calculated dose distribution was obtained as the convolution of a rectangular aperture with a parameterized kernel,