Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the CyberKnife system

Abstract

Monte Carlo (MC) simulation of dose to water and dose to detector has been used to calculate the correction factors needed for dose calibration and output factor measurements on the CyberKnife system. Reference field ionization chambers simulated were the PTW 30006, Exradin A12, and NE 2571 Farmer chambers, and small volume chambers PTW 31014 and 31010. Correction factors for Farmer chambers were found to be 0.7%-0.9% larger than those determined from TRS-398 due mainly to the dose gradient across the chamber cavity. For one microchamber where comparison was possible, the factor was 0.5% lower than TRS-398 which is consistent with previous MC simulations of flattening filter free Linacs. Output factor detectors simulated were diode models PTW 60008, 60012, 60017, 60018, Sun Nuclear edge detector, air-filled microchambers Exradin A16 and PTW 31014, and liquid-filled microchamber PTW 31018 microLion. Factors were generated for both fixed and iris collimators. The resulting correction factors differ from unity by up to +11% for air-filled microchambers and -6% for diodes at the smallest field size (5 mm), and tend towards unity with increasing field size (correction factor magnitude <1% for all detectors at field sizes >15 mm). Output factor measurements performed using these detectors with fixed and iris collimators on two different CyberKnife systems showed initial differences between detectors of >15% at 5 mm field size. After correction the measurements on each unit agreed within ∼1.5% at the smallest field size. This paper provides a complete set of correction factors needed to apply a new small field dosimetry formalism to both collimator types on the CyberKnife system using a range of commonly used detectors.

CyberKnife® fixed cone and Iris™ defined small radiation fields: Assessment with a high‐resolution solid‐state detector array

“…SVs have a pitch of 2.5 mm in the square central area of 5 cm side, and a 5 mm pitch outside. In a recent characterization of the device,41 differences between OFs measurements by the 1000SRS, a synthetic diamond (TM60019, PTW) and a small‐field diode (ETM60017, PTW), were approximately 3.0% for a 5 mm collimator and 1.5% for a 7.5 mm collimator, in agreement with previous investigations 26. The size of the SVs (2.3 × 2.3 × 0.5 mm 3 ) would be responsible for the 3% under‐response for the 5 mm collimator owing to volume‐averaging effect 41.…”

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…The overresponse in small radiation fields of silicon diodes, which incorporate silicon of density 2.3 g/cm 3 and possibly other high‐density material in their sensitive volumes and surroundings, has been well documented 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 38 , 39 , 40 , 41 . Diamond detectors, which have a sensitive volume of density 3.5 g/cm 3 , have been reported to overrespond in small fields 16 , 39 , 41 , 42 .…”

“…Several studies have reported on the response of ionization chambers, diodes, and the microdiamond 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 detector in small radiation fields for flattened beams, using either experiment or Monte Carlo simulations. Where necessary, small field correction factors $false(k_{Q_{clin}{Q}_{msr}}^{f_{clin},f_{msr}}false)$, as defined by Alfonso et al, (18) have been provided.…”

Output factor comparison of Monte Carlo and measurement for Varian TrueBeam 6 MV and 10 MV flattening filter‐free stereotactic radiosurgery system

“…One useful tool in dealing with small field effect is Monte Carlo (MC) simulation. Many recent studies have applied Monte Carlo simulations to examine the output factors of small field sizes used for radiosurgery such as CyberKnife, 6 , 7 Gamma Knife, 8 , 9 and Brainlab (10) . In addition, numerous studies have used Monte Carlo simulation to examine the accuracy of diode detectors measurements for small field dosimetry 11 , 12 , 13 , 14 …”