Premium
SU‐E‐T‐554: Comparison of Electron Disequilibrium Factor in External Photon Beams for Different Models of Linear Accelerators
Author(s) -
LIU B,
Zhu T
Publication year - 2014
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4888889
Subject(s) - photon , linear particle accelerator , electron , physics , kerma , imaging phantom , dosimetry , computational physics , inverse square law , range (aeronautics) , field (mathematics) , atomic physics , beam (structure) , optics , nuclear physics , nuclear medicine , materials science , mathematics , quantum mechanics , medicine , gravitation , composite material , pure mathematics
Purpose: The dose in the buildup region of a photon beam is usually determined by the transport of the primary secondary electrons and the contaminating electrons from accelerator head. This can be quantified by the electron disequilibrium factor, E, defined as the ratio between total dose and equilibrium dose (proportional to total kerma), E = 1 in regions beyond buildup region. Ecan be different among accelerators of different models and/or manufactures of the same machine. This study compares E in photon beams from different machine models/ Methods: Photon beam data such as fractional depth dose curve (FDD) and phantom scatter factors as a function of field size and phantom depth were measured for different Linac machines. E was extrapolated from these fractional depth dose data while taking into account inverse‐square law. The ranges of secondary electron were chosen as 3 and 6 cm for 6 and 15 MV photon beams, respectively. The field sizes range from 2×2 to 40×40 cm 2 . Results: The comparison indicates the standard deviations of electron contamination among different machines are about 2.4 – 3.3% at 5 mm depth for 6 MV and 1.2 – 3.9% at 1 cm depth for 15 MV for the same field size. The corresponding maximum deviations are 3.0 – 4.6% and 2 – 4% for 6 and 15 MV, respectively. Both standard and maximum deviations are independent of field sizes in the buildup region for 6 MV photons, and slightly decreasing with increasing field size at depths up to 1 cm for 15 MV photons. Conclusion: The deviations of electron disequilibrium factor for all studied Linacs are less than 3% beyond the depth of 0.5 cm for the photon beams for the full range of field sizes (2‐40 cm) so long as they are from the same manufacturer.