Consistency of dose rates after applying machine-specific reference correction factors for the gamma knife 16 mm collimator field

Abstract

Background: The machine-specific reference (msr) correction factors ( $k_{Q_{\mathrm{msr}},Q_0}^{f_{\mathrm{msr}},f_{\mathrm{ref}}}$ ) were introduced in International Atomic Energy Agency (IAEA) Technical Report Series 483 (TRS-483) for reference dosimetry of small fields. Several correction factor sets exist for a Leksell Gamma Knife (GK) Perfexion or Icon. Nevertheless, experiments have not rigorously validated the correction factors from different studies.

Purpose: This study aimed to assess the role and accuracy of $k_{Q_{\mathrm{msr}},Q_0}^{f_{\mathrm{msr}},f_{\mathrm{ref}}}$ values in determining the absorbed dose rates to water in the reference dosimetry of Gamma Knife.

Methods: The dose rates in the 16 mm collimator field of a GK were determined following the international code of practices with three ionization chambers: PTW T31010, PTW T31016 (PTW Freiberg GmbH, New York, NY), and Exradin A16 (Standard Imaging, Inc., Middleton, WI). A chamber was placed at the center of a solid water phantom (Elekta AB, Stockholm, Sweden) using a detector-specific insert. The reference point of the ionization chamber was confirmed using cone-beam CT images. Consistency checks were repeated five times at a GK site and performed once at seven GK sites. Correction factors from six simulations reported in previous studies were employed. Variations in the dose rates and relative dose rates before and after applying the $k_{Q_{msr},Q_0}^{f_{msr},f_{ref}}$ were statistically compared.

Results: The standard deviation of the dose rates measured by the three chambers decreased significantly after any correction method was applied (p = 0.000). When the correction factors of all studies were averaged, the standard deviation was reduced significantly more than when any single correction method was applied (p ≤ 0.030), except for the IAEA TRS-483 correction factors (p = 0.148). Before any correction was applied, there were statistically significant differences among the relative dose rates measured by the three chambers (p = 0.000). None of the single correction methods could remove the differences among the ionization chambers (p ≤ 0.038). After TRS-483 correction, the dose rate of Exradin A16 differed from those of the other two chambers (p ≤ 0.025). After the averaged factors were applied, there were no statistically significant differences between any pairs of chambers according to Scheffe's post hoc analyses (p ≥ 0.051); however, PTW T31010 differed from PTW 31016 according to Tukey's HSD analyses (p = 0.040).

Conclusion: The $k_{Q_{\mathrm{msr}},Q_0}^{f_{\mathrm{msr}},f_{\mathrm{ref}}}$ significantly reduced variations in the dose rates measured by the three ionization chambers. The mean correction factors of the six simulations produced the most consistent results, but this finding was not explicitly proven in the statistical analyses.

Keywords: correction factor; gamma knife; machine-specific reference field; small field dosimetry.