Quantification Of Unwanted Dose In Perfexion

Mark Ruschin1, Nazanin Nayebi2, Håkan Nordström3, Jonas Johansson, PhD4, Markus Eriksson3, Per Kjäll3, Young-Bin Cho2, David Jaffray2

1Toronto, Canada 2Princess Margaret Hospital 3Elekta SA 4Stockholm, Sweden

Keywords: Dose, dosimetry, gamma knife, dose delivery, complications

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     The purpose of this study was to categorize, model, and quantify the contributions of various sources of unwanted radiation dose (i.e. any dose that is not part of the intended treatment) resulting from the Perfexion radiation unit.
     Aside dose resulting directly from entrance and exit beams, three main categories of unwanted dose were identified in this work. Type 1 includes in-patient scattered radiation originating from the primary beams to regions outside of the treated volume. Type 2 includes leakage radiation that passes directly through the collimator body and then reaches regions outside of the treated volume either directly or via scattering. Type 3 dose includes the summed background leakage radiation that, for a given cobalt source, may reach the patient through a different collimator opening other than the intended opening due to multiple scattering events.
     The total unwanted dose was measured in-phantom for various treated volumes and collimator sizes using EBT Gafchromic film. A model of unwanted dose was made in an attempt to separate the three sources of unwanted dose and was tested with Monte Carlo simulations and additional film measurements. All measurements and simulations were performed at a distance from the isocentre along the z-direction (cranio-caudal) where there the amount of dose-gradient is minimized.
     The magnitude of the sum unwanted dose along the Z-direction ranged from 0.35% to 1.7% of the prescription dose for a 4.0 cm3 and a 60 cm3 treated volumes respectively. For single shot placement, the measured dose rate 70 mm away in the Z-direction was of the order of 0.05 cGy/min, 0.15 cGy/min, and 0.25 cGy/min for the 4-mm, 8-mm and 16-mm collimators respectively (the in-phantom calibration dose-rate was 3.143 Gy/min at the time of these measurements). Preliminary Monte Carlo simulations exhibited the same trends in dose-rate fall-off as the single-isocentre measurements.
     This is a retrospective study.
     The overall magnitude of the unwanted dose arising from treatment on Perfexion is small but measurable.
      The magnitude depends on treated volume as well as collimator size. The separation of the sources of unwanted dose may be useful in further optimizing the Perfexion for internal shielding.


Project Roles:

M. Ruschin (), N. Nayebi (), H. Nordström (), J. Johansson (), M. Eriksson (), P. Kjäll (), Y. Cho (), D. Jaffray ()