Premium
Determination of k Q msr , Q 0 f msr , f ref factors for ion chambers used in the calibration of Leksell Gamma Knife Perfexion model using EGSnrc and PENELOPE Monte Carlo codes
Author(s) -
Mirzakhanian Lalageh,
Benmakhlouf Hamza,
Tessier Frederic,
Seuntjens Jan
Publication year - 2018
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12821
Subject(s) - imaging phantom , monte carlo method , ionization chamber , dosimetry , physics , calibration , radiosurgery , computational physics , nuclear medicine , ionization , optics , mathematics , ion , radiation therapy , statistics , medicine , quantum mechanics
Purpose To calculate the k Q msr , Q 0f msr ,f reffactors for nine common ionization chamber types following the small fields dosimetry formalism for the calibration of the Leksell Gamma Knife ® (LGK) Perfexion TM using Monte Carlo simulation. This study also provides the first independent comparison of EGSnrc and PENELOPE for the calculation of k Q msr , Q 0f msr ,f refcorrection factors and proposes a practical method to predict these factors based on chamber type, chamber orientation and phantom electron density. Methods The ionization chambers are modeled using the EGSnrc and PENELOPE Monte Carlo codes based on the blueprints provided by the manufacturers. The chambers are placed in a half‐sphere water phantom and five spherical phantoms made of liquid water, solid water, ABS, polystyrene, and PMMA, respectively. Dose averaged over the air cavity of the chambers and a small water volume are calculated using EGSnrc and PENELOPE Monte Carlo codes for both conventional and machine specific reference ( msr ) setups. Using the calculated dose ratio, the k Q msr , Q 0f msr ,f reffactor is determined for all phantom materials and two possible orientations of chamber. The calculated k Q msr , Q 0f msr ,f reffactors are compared to a previous Monte Carlo study.[1][Johansson J, 2012], [2][Andreo P, 2014] A relationship between the k Q msr , Q 0f msr ,f reffactor and the electron density of the phantom material is derived to predict the k Q msr , Q 0f msr ,f reffactor for any phantom material type. Applying the calculated k Q msr , Q 0f msr ,f reffactors to the measured dose rate of a recent round robin study[3][Drzymala R, 2015] improves consistency of reference dosimetry of the Leksell Gamma Knife ® (LGK) Perfexion TM . Results Agreement within uncertainty is observed between k Q msr , Q 0f msr ,f refvalues determined in this study and the previous PEGASOS/PENELOPE study[1][Johansson J, 2012], [2][Andreo P, 2014] in a liquid water phantom. The difference between k Q msr , Q 0f msr ,f refvalues in parallel and perpendicular detector orientations is most significant for the PTW 31010 (1.8%) chamber. The percentage root‐mean‐square (%RMS) deviation between EGSnrc and PENELOPE calculated k Q msr , Q 0f msr ,f refvalues for Exradin‐A1SL, A14 and A14SL chambers studies in this work was found to be 0.4%. The k Q msr , Q 0f msr ,f refvalues increase linearly with electron density of the phantom material for all chamber types mainly due to the linear dependency of photon energy fluence ratios on electron density. The average percentage difference between the calculated and predicted k Q msr , Q 0f msr ,f refvalues using two methods is found to be 0.15% and 0.16%. Previously measured dose rates corrected with the k Q msr , Q 0f msr ,f refvalues determined in this work leads to absorbed dose values consistent to within 0.8%. Conclusions The calculated k Q msr , Q 0f msr ,f refvalues in this work will enable users to apply the appropriate correction for their own specific phantom material only knowing the electron density of the phantom material.