Monte carlo determination of correction factors for dosimetric measurements in gamma knife perfexion small fields

Determination of volume averaging correction factors using an elliptical absorbed dose model for Gamma Knife Perfexion

PURPOSE

The purpose of this study is to calculate volume averaging correction factors for detectors used in the dosimetry of Gamma Knife's narrow photon beams, and to determine the impact of volume averaging on the field output correction factor.

METHODS

Simulations of different Gamma Knife fields were done using elliptical dose model formalism with newly introduced fit functions. To determine volume averaging correction factors a calculation of the absorbed dose over the volume of the detector was performed. The elliptical dose model was tested with respect to absorbed dose distribution for different volumes and compared with the calculations of Leksell GammaPlan v.11.3.1.

RESULTS

The largest differences in absorbed dose calculated by the elliptical model and Leksell GammaPlan are 2.25%, 1.5%, and 0.6% for 16, 8, and 4 mm field sizes, respectively. Volume averaging correction factors were determined for six ionization chambers, five semiconductor detectors, a diamond, and two plastic scintillator detectors. In general, for all examined detectors the impact of volume averaging is more pronounced for smaller field sizes. All studied ionization chambers had a larger volume than other detectors, therefore the volume averaging correction factors for ionization chambers are larger for all investigated field sizes. Besides the fact that plastic scintillator detectors can be considered tissue-equivalent, volume averaging correction factor should be applied.

CONCLUSION

Volume averaging correction factors for different detectors are determined and suitable detectors for dosimetry of Gamma Knife's narrow photon beams are recommended. It is shown that volume averaging has a dominant contribution to a field output correction factor.

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams

Purpose

The goal of this work is to provide a large and consistent set of data for detector‐specific output correction factors, kQclin,Qreffclin,fref, for small static fields for seven solid‐state detectors and to determine field output factors, ΩQclin,Qreffclin,fref, using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS‐483 for small‐field dosimetry were followed throughout the study.

Methods

ΩQclin,Qreffclin,fref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm² to 10 × 10 cm². Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two‐dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. kQclin,Qreffclin,fref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector.

Results

Field output factors and detector‐specific kQclin,Qreffclin,fref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small‐field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for kQclin,Qreffclin,fref are found to differ significantly (P < 0.05) from the corresponding data published in TRS‐483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between kQclin,Qreffclin,fref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm² and 0.8 × 0.8 cm². Significant differences in kQclin,Qreffclin,fref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in kQclin,Qreffclin,fref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant.

Conclusions

Selection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of ΩQclin,Qreffclin,fref for small static fields in megavoltage photon beams. Large set of kQclin,Qreffclin,fref data for seven solid‐state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS‐483 dataset.