Determination of kQmsr,Q0fmsr,fref factors for ion chambers used in the calibration of Leksell Gamma Knife Perfexion model using EGSnrc and PENELOPE Monte Carlo codes.

To calculate the kQmsr,Q0fmsr,fref factors for nine common ionization chamber types following the small fields dosimetry formalism for the calibration of the Leksell Gamma Knife® (LGK) PerfexionTM using Monte Carlo simulation. This study also provides the first independent comparison of EGSnrc and PENELOPE for the calculation of kQmsr,Q0fmsr,fref correction factors and proposes a practical method to predict these factors based on chamber type, chamber orientation and phantom electron density. 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 kQmsr,Q0fmsr,fref factor is determined for all phantom materials and two possible orientations of chamber. The calculated kQmsr,Q0fmsr,fref factors are compared to a previous Monte Carlo study. A relationship between the kQmsr,Q0fmsr,fref factor and the electron density of the phantom material is derived to predict the kQmsr,Q0fmsr,fref factor for any phantom material type. Applying the calculated kQmsr,Q0fmsr,fref factors to the measured dose rate of a recent round robin study improves consistency of reference dosimetry of the Leksell Gamma Knife® (LGK) PerfexionTM . Agreement within uncertainty is observed between kQmsr,Q0fmsr,fref values determined in this study and the previous PEGASOS/PENELOPE study in a liquid water phantom. The difference between kQmsr,Q0fmsr,fref values 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 kQmsr,Q0fmsr,fref values for Exradin-A1SL, A14 and A14SL chambers studies in this work was found to be 0.4%. The kQmsr,Q0fmsr,fref values 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 kQmsr,Q0fmsr,fref values using two methods is found to be 0.15% and 0.16%. Previously measured dose rates corrected with the kQmsr,Q0fmsr,fref values determined in this work leads to absorbed dose values consistent to within 0.8%. The calculated kQmsr,Q0fmsr,fref values 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.

Mirzakhanian L ，Benmakhlouf H ，Tessier F ，Seuntjens J ... -  《-》

Experimental investigation of TRS-483 reference dosimetry correction factors for Leksell Gamma Knife® Icon™ beams.

Radiosurgery using the Leksell Gamma Knife® (LGK) Icon™ is an established technique used for treating intracranial lesions. The largest beam field size the LGK Icon can produce is a 16 mm diameter sphere. Despite this, reference dosimetry on the LGK Icon is typically performed using ionization chambers calibrated in 10 × 10 cm2 fields. Furthermore, plastic phantoms are widely used instead of liquid water phantoms. In an effort to resolve these issues, the International Atomic Energy Agency (IAEA) in collaboration with American Association of Physicists in medicine (AAPM) recently published Technical Report Series No. 483 (TRS-483) as a Code of Practice for small-field dosimetry. TRS-483 includes small-field correction factors, k Q msr , Q 0 f msr , f ref , intended to account for the differences between setups when using small-field modalities such as the LGK Icon, and conventional setups. Since the publication of TRS-483, at least three new sets of values of k Q msr , Q 0 f msr , f ref for the LGK Icon have been published. The purpose of this study was to experimentally investigate the published values of k Q msr , Q 0 f msr , f ref for commonly used phantom and ionization chamber (IC) models for the LGK Icon. Dose-rates from two LGK units were determined using acrylonitrile butadiene styrene (ABS) and Certified Medical Grade Solid Water® (SW) phantoms, and PTW 31010 and PTW 31016 ICs. Correction factors were applied, and the resulting dose-rates compared. Relative validity of the correction factors was investigated by taking the ratios of dose-rate correction factor products. Additionally, dose-rates from the individual sectors were determined in order to calculate the beam attenuation caused by the ABS phantom adapter. It was seen that the dose-rate is underestimated by at least 1% when using the ABS phantom, which was attributed to fluence perturbation caused by the IC and phantom adapter. Published correction factors k Q msr , Q 0 f msr , f ref account for these effects to varying degree and should be used. The SW phantom is unlikely to underestimate the dose-rate by more than 1%, and applying k Q msr , Q 0 f msr , f ref could not be shown to be necessary. Out of the two phantom models, the ABS phantom is not recommended for use in LGK reference dosimetry. The use of newly published values of k Q msr , Q 0 f msr , f ref should be considered.

Häger W ，Kaveckyte V ，Benmakhlouf H 《-》

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

The machine-specific reference (msr) correction factors ( k Q msr , Q 0 f msr , f ref $k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$ ) 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. This study aimed to assess the role and accuracy of k Q msr , Q 0 f msr , f ref $k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$ values in determining the absorbed dose rates to water in the reference dosimetry of Gamma Knife. 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 m s r , Q 0 f m s r , f r e f $k_{{Q_{msr}},\;{Q_0}}^{{f_{msr}},{f_{ref}}}$ were statistically compared. 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). The k Q msr , Q 0 f msr , f ref $k_{{Q_{{\rm{msr}}}},\;{Q_0}}^{{f_{{\rm{msr}}}},{f_{{\rm{ref}}}}}$ 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.

Choi Y ，Chun KJ ，Kim ES ，Bahng J ，Yang HJ ，Kim TH ，Cho GS ，Choi SH ，Seo YC ，Chung HT ... -  《-》

Calibration of the Gamma Knife Perfexion using TG-21 and the solid water Leksell dosimetry phantom.

McDonald D ，Yount C ，Koch N ，Ashenafi M ，Peng J ，Vanek K ... -  《MEDICAL PHYSICS》

On the implementation of a recently proposed dosimetric formalism to a robotic radiosurgery system.

Pantelis E ，Moutsatsos A ，Zourari K ，Kilby W ，Antypas C ，Papagiannis P ，Karaiskos P ，Georgiou E ，Sakelliou L ... -  《MEDICAL PHYSICS》