Monte Carlo calculated and experimentally verified correction factors for clinical reference dosimetry for the Leksell Gamma Knife®: Application of a new IAEA dosimetry formalism

Keywords: Monte Carlo, gamma knife, dosimetry, physics, Dose Prescription

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     Dosimetrical measurements for the Leksell Gamma Knife® (LGK) are challenging due to the possible lack of electronic equilibrium, spectral changes with field size, and partial occlusion of the primary source. The IAEA working group on reference dosimetry of small and nonstandard fields have published a new dosimetry formalism.
     The objective of this work is to report Monte Carlo (MC) calculated and experimentally verified correction factors for a number of ionization chambers that are needed for the application of the new IAEA formalism to LGK dosimetry.
     According to the new IAEA formalism, the absorbed dose to water is given by this formalism links the standard reference field (fref) used at a SSDL and the machine specific reference field (fmsr), used in the clinic, via the correction factor . Correction factors for eight different small volume ionization chamber types (0.007-0.125 cm3) from various manufacturers used for LGK dosimetry were determined using MC simulations. Verification of MC simulated correction factors were done at three LGK facilities and the Swedish SSDL through an international collaboration. Measurements followed a prescribed procedure to provide consistency and accuracy in the acquired datasets. The absorbed dose to water was determined using IAEA TRS-398 and the new IAEA small field formalism.
     Applying the correction factor using the new formalism leads to, depending on the ionization chamber and phantom configuration, an increase in the dose-rate between 0.8%-1.6% for PFX and 0.3%-0.5% for LGKC compared to IAEA TRS-398. The average standard deviation in the dose-rate measurement decreases from about 1% applying TRS-398 to about 0.6% applying the new formalism.
     The findings of this report are specific to the methods we used.
     With the new formalism, average dose rate is higher and the standard deviation is decreased slightly compared to IAEA TRS-398.
     This suggests that small improvements in the accuracy of Gamma Knife® dosimetry can be accomplished through the implementation of the new formalism using the correction factors reported in this study.


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