Low dose out-of-field radiotherapy,part 2: Calculating the mean photon energy values for the out-of-field photon energy spectrum from scattered radiation using Monte Carlo methods

Purpose

During radiotherapy, leakage from the machine head and collimator expose patients to out-of-field irradiation doses, which may cause secondary cancers. To quantify the risks of secondary cancers due to out-of-field doses, it is first necessary to measure these doses. Since most dosimeters are energy-dependent, it is essential to first determine the type of photon energy spectrum in the out-of-field area. The aim of this study was to determine the mean photon energy values for the out-of-field photon energy spectrum for a 6 MV photon beam using the GEANT 4–Monte Carlo method.

Material and methods

A specially-designed large water phantom was simulated with a static field at gantry 0°. The source-to-surface distance was 92 cm for an open field size of 10 × 10 cm2. The photon energy spectra were calculated at five unique positions (at depths of 0.5, 1.6, 4, 6, 8, and 10 cm) along the central beam axis and at six different off-axis distances.

Results

Monte Carlo simulations showed that mean radiation energy levels drop rapidly beyond the edge of the 6 MV photon beam field: at a distance of 10 cm, the mean energy level is close to 0.3 MeV versus 1.5 MeV at the central beam axis. In some cases, the energy level actually increased even as the distance from the field edge increased: at a depth of 1.6 cm and 15 cm off-axis, the mean energy level was 0.205 MeV versus 0.252 MeV at 20 cm off-axis.

Conclusion

The out-of-field energy spectra and dose distribution data obtained in this study with Monte Carlo methods can be used to calibrate dosimeters to measure out-of-field radiation from 6 MV photons.