The analysis of the measurement uncertainty with application of small detectors made of Gafchromic EBT films for the range of doses typical for in vivo dosimetry in teleradiotherapy

Abstract

IntroductionThe required overall uncertainty in dose delivery in modern radiotherapy is 3.5% of a prescribed dose (one standard deviation). In vivo dosimetry is the simple method in dose delivery control. In this work we present our results concerning the precision of dose measurement with Gafchromic EBT films used within the dose range measured in the in vivo dosimetry of photon external beam radiation therapy. Materials and methodsThe rectangular pieces of Gafchromic EBT films of 1 cm × 2 cm were prepared. They were prepared from three different films from two batches. For each film four sets of samples consisting of 18 pieces were made. Each set of films was exposed perpendicularly to 6 MV photon beams at several dose levels from 2 cGy to 190 cGy. The net optical density was read with the flatbed document scanner Epson Expression 10000 XL and the ImageJ software. The calibration curves for each set of Gafchromic EBT samples were obtained. The mathematical formulae describing the calibration curve was fitted to the measurements results. Four different calibration procedures were analyzed based on one or four sets of samples and with or without use of image filtration (median filter). Two sources of uncertainty were analyzed separately: the calibration curve measurements, and the precision of formulae fitting. ResultsFor the protocol described in this paper, the overall dose measurement uncertainty of less than 2% (one standard deviation) for doses of about 25 cGy and higher was found for calibration procedure based on four sets of film samples with image filtration, and less than 3% for calibration procedure based on one set of film samples with image filtration. For two films from the same batch almost identical calibration curves were obtained. It allows to use one calibration curve for all films from the same batch. There were larger differences for films from two different batches. ConclusionsThe Gafchromic EBT films have sufficient accuracy for dose measurements above 20 cGy and may be an interesting alternative for the semiconductor or thermoluminescent detectors as an in vivo dosimeter. Contrary to these detectors Gafchromic EBT films are transparent for radiation.