A new dosimetric protocol for high energy photon radiotherapy beams in Norway using radiochromic film

Abstract

The continual development of high energy photon beam radiotherapy in Norway urges for a revision in dosimetric quality controls. It is favorable to use sharply defined fields, but existing routines for quality assurance are lacking, especially for modalities such as asymmetric fields and IMRT. The thesis suggests a new procedure for implementing radiochromic film as a tool for quality assurance in radiotherapy, in cooperation with the Norwegian Radiation Protection Authority (NRPA). GafChromic®EBT type film has been used for the experiments. The procedure is based on the general method first suggested by Devic et al, with several modications. Films have been calibrated using the Co-60 gammatron at the NRPA Secondary Standard Dosimetry Laboratory, and measurements have been performed on 14 treatment units at 7 different hospitals in Norway. In addition to film irradiation, reference dosimetry according to TRS-398 protocol using an ionization chamber in a water phantom was performed at 13 of 14 linacs. Measurements show that measured doses for a 10x10 cm2 field at reference conditions with an ionization chamber in a water phantom were within 2 % for all linear accelerators, for both 6 MV and 15 MV photon energies. Radiochromic film measurements were inaccurate for absolute dosimetry due to difficulties in the absolute calibration of the film. However, relative measurements using radiochromic film show that values for field size, penumbra, flatness and symmetry are close to expected clinical values. Different asymmetric field setups are also investigated using radiochromic film. In the case where two asymmetric half-collimated fields are spliced together, high over- and underdosages of more than 20 % are detected in the border between the fields, both with and without collimator rotation. These discrepancies are not corrected for in dose planning software. Studies of fields with overtravel collimator parameters in both x- and y-directions show that field sizes vary within 7 % of the desired values, and measured doses do not fully agree with the calculated doses in treatment planning software. Radiochromic film shows promise as a tool in radiotherapy quality assurance, but the existing method is not accurate enough to satisfy needs for clinical use. A modified method is suggested that will potentially improve the detected uncertainties.