Produced radioactive isotopes, ambient dose equivalent in TrueBeam room with flattening filter (FF) and Flattening Filter Free (FFF) modes: Monte Carlo simulation.

BACKGROUND

High-energy linear accelerators used in radiotherapy produce unexpected neutrons that can activate materials in the treatment room.

MATERIALS AND

METHODS

The research group used PHITS code to simulate TrueBeam Linac head with 10 MV photons. Then, PHITS codes connected Dchain code to calculate the variation of the induced radioactivity, the ambient dose equivalent during TrueBeam radiation 4 Gy with dose rate 400 MU (Monitor Unit)/minute in FF mode, 24 Gy with dose rate 2400 MU/minute in FFF mode, and cooling time (14 min).

RESULTS

One minute of beam-on TrueBeam with 10 MV photon energy produced six mainly produced nuclides (183 mW, 24mNa, 185 mW, 28Al, 24mNa, and 187W). FFF mode had more activity of the produced isotopes than those in FF mode (1.60 × 1014 Bq compared with 1.57 × 1014 Bq-corresponding to ∼1.71% higher). The differences in the activity of isotopes and ambient dose between FF and FFF mode were mostly less than a few percent; however, considering that the MU per beam current is six times higher in the FFF mode than in the FF mode, the induced activities per MU can be reduced by roughly six times when using the FFF mode instead of the FF mode.

CONCLUSION

The isotope activities of the produced isotopes in the FF and FFF modes are essentially equivalent when normalized per beam current.