Session Item

Neutrons are a concern for proton beam therapy (PT) due to their high radiobiological effectiveness and the risk for second cancer induction. Neutron dose calculation is not straightforward and therefore studies are carried out to find simple ways for calculating neutron production in PT. Simulations and measurements of proton beam scanning therapy have suggested a linear relation between H*(10) and the field for the neutron field around the patient. The purpose of this work is to test the linearity assumption inside the patient as it could be useful to develop a simple general analytical model for the evaluation of the out-of-field internal neutron doses.

Monte Carlo simulations were carried out to test the effect of field in neutron dose equivalent around the target. A simple geometry, consisting of a proton field entering in water tank, was modelled using the MCNP 6.2 code. A SOBP was created to cover different target volumes at the same depth in water and with the same modulation width, i.e., changing only the field (to maintain the same energy distribution of protons in the source). The field sizes considered were in the interval 5x5 to 15x15 cm². Neutron dose equivalent was evaluated around the target, at distances up to 40cm from the field edge.

The behaviour of the neutron dose equivalent with field depends on the direction relative to the proton beam. Table 1 shows the mean value of the ratio between neutron dose equivalent obtained for a particular field and the value for the 5x5 cm² field, axially and laterally from the proton beam. In the forward direction there is a clear increase in dose as the field increases, while laterally the increase is less pronounced with a no well-defined tendency. However, similar results are obtained when the ratios are calculated normalizing dose equivalent by the projection of the target in the evaluated direction. Another interesting result is that laterally, the dose equivalent can be higher than axially within a range of distances from target.  This interval depends also on the field , being lower as the field increases. For example, lateral dose equivalent is higher from the field edge up to 7.6cm and 1.5cm for the 5x5 and 15x15 cm² fields, respectively.  

Finding an analytical expression for out-of-field neutron doses is feasible with the potential to simplify neutron field characterisation, but the dependence with field is more complex than the linear dependence outside the body. Simulations suggest that the linear behaviour is however maintained with the projection of the target in the observation direction.