Radiotherapy dose measurement uncertainty evaluation

Abstract

Introduction The verification of dose in external radiotherapy benefit of the high resolution offered by radiochromic films, a characteristic that is hard to be matched by other measuring devices. A new generation of this type of films, EBT-3 (Gafchromics® – ISP) was added to the already existing. Equal-Estro laboratory uses for the moment in its audits EBT-2 film but plans to start the use of EBT-3 films. Before starting, an assessment of the uncertainty of dose measurements is required for these films. Materials and methods The study focused on a batch of EBT-3 film (three boxes containing 10 films of 14” × 17”). In order to investigate the homogeneity within a film as well as for the whole batch of film allowing deducing associated uncertainty, pieces of EBT-3 film were scanned using a professional scanner – Epson Expression 10000 XL, used in transmission mode. In view to check the effect of irradiation on the repeatability and reproducibility of measurements, the films were irradiated in a gamma beam from a Cobalt-60 using an Alcyon machine. The calibration of the new batch of films was performed using a photon beam with energy of 6 MV of a Varian linac. The irradiation of the films was performed in a water equivalent solid phantom, (Easy- Cube® – Euromechanics). Digital images of the films were processed with Image-J® and Omni- Pro-I'mrT® softwares. Results The study on the homogeneity of the films EBT-3 gave a maximum standard deviation of 2%. For batch homogeneity the maximum standard deviation is less than 2.5%. Repeatability and reproducibility measures using the scanner gave very encouraging results. The other components of the total uncertainty (as for example the uncertainty of the dose delivered during the calibration of films) are identical to those estimated for the EBT-2 film, in a previous study. Conclusion The study allowed concluding that EBT-3 films show a major improvement in their homogeneity. The overall uncertainty for dose measurements using EBT-3 films is lower than that obteined for the EBT-2 films.