Radial dose distribution, dose to water and dose rate constant for monoenergetic photon point sources from 10 keV to 2 MeV: EGS4 Monte Carlo model calculation

A comprehensive set of dose distributions from monoenergetic photon-emittin g isotropic point sources in a medium can be used as a reference database f or the dosimetry of photon emitter sources in that medium. Data of this typ e for water over the photon energy range from 15 keV to 2 MeV have been pub lished based on calculations using a one-dimensional photon transport model . The present work, based on a previously published EGS4 Monte Carlo code, updates the classic data set of Berger and provides more extensive calculat ions than previously available. Air kerma strength per unit photon emission rate from an isotropic point emitter is obtained as a function of energy u sing published data for mass energy absorption coefficients. The TG-43 dose rate constant for water as a function of energy is calculated for monoener getic photon emitters as the ratio of dose rate to water at 1 cm to air ker ma strength for unit photon emission rate. Results for the radial dose dist ribution agree well with the data of Berger between 40 and 400 keV. For ene rgies greater than or equal to 500 keV, a previously undescribed buildup re gion for the radial dose function is identified. Thickness of the buildup r egion ranges from 1 mm at 500 keV to 8 mm at 2 MeV. Between 15 and 30 keV, the radial dose function within a few millimeters of the emitter is calcula ted to be 4%-5% higher than values derived from Berger's data. The maximum dose rate constant for monoenergetic photon emitters occurs at an energy of 60 keV, and has the value 1.355 cGy h(-1) U-1, where U is the unit of air kerma strength, 1 mu Gy m(2) h(-1). This would correspond to the maximum hy pothetical dose rate constant for a brachytherapy photon source emitting ph otons of energy less than or equal to 2 MeV. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01012-3].