Ambient dose equivalent and secondary neutron spectrometry during radiotherapy of a head tumour in a paediatric phantom with protons, helium ions and carbon ions

OBJECTIVE: Secondary neutrons are an unavoidable by-product of ion beam therapy and contribute to out-of-field doses. Quantifying these doses is crucial for ensuring the safety of the patients, particularly for paediatric patients. APPROACH: Out-of-field neutron measurements were performed for protons, helium ions and carbon ions, treating a 10-year-old paediatric phantom with a virtual brain tumour. Neutron spectrometry was performed using a Bonner Sphere Spectrometer (BSS), and ambient dose equivalents, H*(10), were additionally measured with an extended-range rem counter (WENDI II). Both measurements were carried out at two locations in the treatment room, positioned upstream and downstream of the primary beam. Monte Carlo (MC) simulations were used to obtain a priori spectra for the unfolding and to complement the measurements. MAIN RESULTS: The measured neutron energy spectra showed the characteristic features of secondary neutrons in ion beam therapy, consistent with the MC simulations. For all three ions, both the fast neutron and high-energy neutron peaks were observed at the downstream position, along with thermal and epithermal neutron contributions, while no high-energy neutron peak was observed upstream. The highest ambient dose equivalent H*(10) measured with WENDI II was 11.8 μSv per 4 Gy(RBE) for helium ions at the downstream position, and the lowest value of 1.1 μSv per 4 Gy(RBE) was obtained by the MC simulations for carbon ions upstream. SIGNIFICANCE: The energy distribution of the secondary neutrons during ion beam treatment of a paediatric phantom was successfully measured and used to estimate the ambient dose equivalent.