Out-of-field neutron radiation from clinical proton, helium, carbon, and oxygen ion beams

In hadron therapy, out-of-field doses, which may in the long-term cause secondary cancers, are mostly due to neutrons. Very recently, ⁴He and ¹⁶O beams have been added to protons and ¹²C ions for clinical therapy.

The focus of this article is to compare secondary neutron doses produced by clinical protons, ⁴He, ¹²C, and ¹⁶O ion beams.

Ambient dose equivalent, H*(10), measurements were performed, with five types of rem counters, of the neutron field produced by the four primary ions impinging on a water phantom. This experiment was performed at the Heidelberg Ion Beam Therapy Center (HIT) in the framework of the activities of the European Radiation Dosimetry Group (EURADOS). The experimental data are normalized to both unit primary particle and target dose, and are further compared to Monte Carlo (MC) simulations performed with the FLUKA and MCNP codes.

The intensity of the neutron field increases with ion mass, and the trend is more significant in the forward direction. The minimum H*(10) for all ions, 5µSv/Gy to 10µSv/Gy, was measured in the transverse and backward directions, whereas the maximum measured value was about 1.3 mSv/Gy for primary ¹⁶O ions in the forward direction. Additional MC simulations are presented for a more detailed analysis of the rem counters' response in the presence of heavy charged fragments. In the downstream direction, for ¹²C and ¹⁶O ions, approximately only 30% of the instruments' counts are due to neutrons.

The four extended-range instruments provide reliable and consistent results, whereas the conventional rem counter underestimates H*(10) in a neutron field with a large high-energy component. FLUKA and MCNP provide consistent predictions, within a factor of 1.6 for the downstream position and lower differences in the other cases, and are in agreement with the experimental data. It was found that under certain conditions neutrons do not represent the only secondary radiation field to be monitored; the presence of charged particles affects the performance of moderator-type neutron detectors.