Program Information
New Formalism Correction Factors for Ionization Chambers in the Calibration of Leksell Gamma Knife Perfexion Model Based On Monte Carlo Simulation
L Mirzakhanian1*, H Benmakhlouf2 , J Seuntjens3 , (1) McGill University, Medical Physics Unit, Montreal, Quebec, (2) Department of Medical Physics, Karolinska University Hospital, Stockholm, ,(3) McGill University, Montreal, Quebec
Presentations
TH-AB-201-9 (Thursday, August 4, 2016) 7:30 AM - 9:30 AM Room: 201
Purpose:
To determine the k_(Q_msr,Q)^(f_msr,f_ref ) factor, introduced in the small field formalism for five common type chambers used in the calibration of Leksell GammaKnife Perfexion model over a range of different phantom electron densities.
Methods:
Five chamber types including Exradin-A16, A14SL, A14, A1SL and IBA-CC04 are modeled in EGSnrc and PENELOPE Monte Carlo codes using the blueprints provided by the manufacturers. The chambers are placed in a previously proposed water-filled phantom and four 16-cm diameter spherical phantoms made of liquid water, Solid Water, ABS and polystyrene. Dose to the cavity of the chambers and a small water volume are calculated using EGSnrc/PENELOPE codes. The calculations are performed over a range of phantom electron densities for two chamber orientations. Using the calculated dose-ratio in reference and machine specific reference field, the k_(Q_msr,Q)^(f_msr,f_ref ) factor can be determined.
Results:
When chambers are placed along the symmetry axis of the collimator block (z-axis), the CC04 requires the smallest correction followed by A1SL and A16. However, when detectors are placed perpendicular to z-axis, A14SL needs the smallest and A16 the largest correction. Moreover, an increase in the phantom electron density results in a linear increase in the k_(Q_msr,Q)^(f_msr,f_ref ). Depending on the chambers, the agreement between this study and a previous study performed varies between 0.05-0.70% for liquid water, 0.07-0.85% for Solid Water and 0.00-0.60% for ABS phantoms. After applying the EGSnrc-calculated k_(Q_msr,Q)^(f_msr,f_ref ) factors for A16 to the previously measured dose-rates in liquid water, Solid Water and ABS normalized to the dose-rate measured with TG-21 protocol and ABS phantom, the dose-rate ratios are found to be 1.004±0.002, 0.996±0.002 and 0.998±0.002 (3σ) respectively.
Conclusion:
Knowing the electron density of the phantoms, the calculated k_(Q_msr,Q)^(f_msr,f_ref ) values in this work will enable users to apply the appropriate correction for their own specific phantom material.
Funding Support, Disclosures, and Conflict of Interest: LM acknowledges partial support by the CREATE Medical Physics Research Training Network grant of the Natural Sciences and Engineering Research Council (Grant number: 432290)
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