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Determination of scatter factor parameters and electron disequilibrium for monoenergetic photon beams
Author(s) -
McDonough James,
Bloch Peter,
Bjärngard Bengt E.
Publication year - 1999
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 - 10.1118/1.598504
Subject(s) - physics , attenuation , photon , imaging phantom , monte carlo method , electron , attenuation coefficient , computational physics , photon energy , range (aeronautics) , optics , atomic physics , nuclear physics , materials science , mathematics , statistics , composite material
The tissue‐phantom‐ratio (TPR) is expressed as the product of the phantom scatter factor (SF), an electron disequilibrium factor, and an attenuation factor, equal to the zero‐area TPR. The scatter factor, as a function of depth d and field size s , has been described by two parameters a and w , SF ( d , s ) = 1 + asd / ( ws + d ) . We have determined the parameters a and w for 20 monoenergetic photon beams between 1 and 20 MeV. Pencil‐beam energy‐deposition kernels were obtained using Monte Carlo simulations. The kernels were used to calculate broad‐beam depth‐dose data, which were converted to TPR and fitted to the equation above using an iterative search over a ‐ w space. The parameter a is nearly equal to the attenuation coefficient for all energies while the parameter w increases with energy. The resulting a and w compare favorably to values determined for clinical photon beams, as a function of the measured attenuation coefficient. With the scatter factor determined, we isolated the electron disequilibrium factor for each monoenergetic beam. It can be characterized as a quadratic function of the depth. The coefficients of the quadratic function can be related to the range of the most energetic secondary electron produced.