Performance Evaluation of an EBT Film in Small Radiation Fields Dosimetry

Abstract

Purpose: In order to evaluate suitability of available detectors for small-field dosimetry, absorbed doses were measured and compared. Material and Methods: The measurements were performed in a home made-multilayer solid phantom for 6 MV photon beams from a 6 EX Varian accelerator. Output factors with an ionization chamber (PTW-Feiburg, Model 31015), diode detector (Scanditronix Wellhofer, SFD), and Gafchoromic@EBT film were measured for five radiation field sizes ranging from 2×2 cm2 to 0.5×0.5 cm2. To remove the ionization chamber volume averaging effect which leads to dosimetric uncertainty, a spatial convolution kernel was measured and corrections were made. For EBT film dosimetry, film was scanned using a flat-bad scanner with RGB mode using a red channel filter, and the obtained pixel value at the center of the irradiated field were converted into the dose according to the darkness-dose calibration data. The output factors which are normalized to the field size of 2×2 cm2 were computed. Results: Three detectors are agreed with each other for radiation sizes of 2×2 cm2. For smaller than 1.5×1.5 cm2, output factors measured with ionization chamber were 7.3 % and 29.5 % smaller than those with EBT for 1×1 cm2 and 0.5×0.5 cm2, restrictively. Output factors with the diode were over-estimated up to 3.4 % compared to the EBT film. Conclusion: In field size smaller than 1.5×1.5cm2 notable differences of output factor with different detectors were observed due to the increased effect of non-water equivalent materials inside the radiation field. The results suggested that choice of detector type is important for accurate dosimetry for small field sizes. Therefore, using EBT film for small field dosimetry can provide simple and accurate dose measurements due to the near-water equivalency of the film material.