An effective calibration technique for radiochromic films using a single-shot dose distribution in Gamma Knife

Purpose: A method of calibrating radiochromic films for Gamma Knife (R) (GK) dosimetry was developed. The applicability and accuracy of the new method were examined. Methods: The dose distribution for a sixteen millimeter single-shot from a GK was built using a reference film that was calibrated using the conventional multi-film calibration (MFC) method. Another film, the test film, from a different set of films was irradiated under the same conditions as the reference film. The calibration curve for the second set of films was obtained by assigning the dose distribution of the reference film to the optical density of the test film, point by point. To assess the accuracy of this single-film calibration (SFC) method, differences between gamma index pass rates (GIPRs) were calculated. Results: The SFC curves were successfully obtained with estimated errors of 1.46%. GIPRs obtained with the SFC method for films irradiated using a single-shot showed differences less than one percentage point when dose difference criterion (Delta D) was 2% and the distance to agreement criterion (Delta d) was 1 mm. The GIPRs of the SFC method when the films were irradiated following a virtual target treatment plan were consistent with the GIPRs of the MFC method, with differences of less than 0.2 percentage points for Delta D = 1% and Delta d = 1 mm. Conclusion: The accuracy of the SFC method is comparable to that of conventional multi-film calibration method for GK film dosimetry. (C) 2016 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.