The effect of energy spectrum change on DNA damage in and out of field in a clinical megavoltage photon beam

Purpose: To quantify the DNA damage induced in a clinical megavoltage photon beam at various depths in and out of the field. Methods and Materials: MCNPX was used to simulate 10x10 cm and 20x20 cm 10 MV photon beams from a clinical linear accelerator. Photon and electron spectra were collected in a water phantom at depths of 2.5, 12.5 and 22.5 cm on the central axis and at off-axis points out to 10 cm. These spectra were used as an input to a validated microdosimetric Monte Carlo code, MCDS, to calculate the RBE of induced DSB in DNA at points in and out of the primary radiation field at three depths. Results: There was an observable difference in the energy spectra for photons and electrons for points in the primary radiation field and those points out-of-field. In the out-of-field region, the mean energy for the photon and electron spectra decreased by a factor of about six and three from the in-field mean energy, respectively. Despite the differences in spectra and mean energy, the change in RBE was less than 1% from the in-field region to the out-of-field region at any depth. There was no significant change in RBE regardless of the location in the phantom. Conclusions: Although there are differences in both the photon and electron spectra, these changes do not correlate with a change in RBE in a clinical MV photon beam as the electron spectra are dominated by electrons with energies greater than 20 keV.