Monte Carlo calculated and experimentally determined output correction factors for small field detectors in Leksell Gamma Knife Perfexion beams
(English)Manuscript (preprint) (Other academic)
The measurement of output factors (OF) for the small beams of the Leksell Gamma Knife® (LGK) is a challenge for the physicist due to the over- or underestimation of these factors by a vast majority of commercially available detectors. Output correction factors, introduced in the new international formalism published by Alfonso et al. (2008) standardizes the determination of OFs for small photon beams by correcting the detector reading ratios with output correction factors in order to yield the correct OF. Output correction factors have, in this work, been determined for LGK Perfexion™ 60Co γ-ray beams by Monte Carlo (MC) calculations and measurements. The MC calculations were performed using the MC system PENELOPE scoring the doses to the active volumes of the detectors and to a small volume of water. Two silicon diodes, one liquid ionization chamber (LIC), one alanine and one TLD detector were included in the MC derivation of the output correction factors. The LIC resulted in correction factors within ±0.4% and was therefore selected as the reference detector for the measurements. Twelve detectors were used in the experimental determination of the output correction factors by normalizing their detector readings to those of the LIC. The MC-calculated and experimentally determined output correction factors for the silicon diodes resulted in up to a -4% correction for the smallest collimator size. The air ionization chamber measurements resulted in extremely large output correction factors, due to the well-known effect of partial volume averaging (PVA). The natural diamond detector resulted in 6% correction for the 4 mm collimator, also due to PVA, whereas the smaller synthetic diamond detector resulted in a correction within ±1%. The LIC, requiring the smallest correction, was used to explore machine-to-machine differences in the OFs by performing measurements in four LGK units with different dose rates. This resulted in OFs within ±0.6% and ±0.3% for the 4 mm and 8 mm collimators, respectively, favouring the use of generic OFs. Using these experimentally derived correction factors, OFs can now be measured using a wide range of commercially available detectors.
Research subject Medical Radiation Physics
IdentifiersURN: urn:nbn:se:su:diva-114411OAI: oai:DiVA.org:su-114411DiVA: diva2:792300