Evaluation of the usefulness of small detectors made of Gafchromic EBT3 film for measurements in areas with high dose gradient and without charged-particle equilibrium (CPE)

Abstract

Introduction: In some clinical cases a full therapeutic dose needs to be delivered in the area close to the skin surface where a high dose gradient and there no charged-particle equilibrium (CPE) exists. The accuracy of dose distribution calculations performed in this region with the treatment planning system is limited. In this work we investigated the usefulness of small pieces of Gafchromic EBT3 film for measurements of the absolute dose value in the area close to the skin surface. Material and methods: The Gafchromic EBT3 film detectors of size 1.0 cm x 1.5 cm were prepared. The film samples were calibrated in 6 MV photon beam (Elekta Versa HD). Calibration was performed in a dose range of 0 – 250 cGy. Films were scanned using the EPSON EXPRESSION 10000 XL flatbed scanner in 48-bit RGB mode, with a resolution of 72 dpi. ImageJ software was used to calculate the dose. Triple-channel film dosimetry was applied. The uncertainty of the dose measurement method was estimated. Film measurements were compared with the dose measurements using ionization chamber. The conformity of measurements was assessed using the metrological compliance test. Results: The relative differences between dose measurements using Gafchromic EBT3 film detectors and ionization chamber for a single square photon beam were -0.8% and 0.3% for a depth of 0.5 cm and 5.0 cm (CPE) respectively. The values of the metrological compatibility test factor ζ were 0.3 and 0.1 respectively. The maximum relative differences for dynamic beams were 0.9% and −1.0% for a depth of 0.5 cm and 5.0 cm respectively. Metrological compatibility test showed also good agreement (ζ=0.3). Conclusions: Small film detectors made of Gafchromic EBT3 film allow for the accurate dose measurements in the high dose gradient region and without CPE. They can be used to validate the calculation of the treatment planning systems also for VMAT techniques.