Secondary neutron spectra and ambient dose equivalent measurements with an extended range Bonner sphere spectrometer in clinical pencil beam scanning using protons, helium, carbon, and oxygen ions

<i>Objectives.</i>Particle therapy is an advanced radiotherapy technique primarily using protons and carbon ions, with helium and oxygen ions also being considered for clinical applications. A critical concern in ion therapy is the production of secondary neutrons due to nuclear reactions, which may contribute to unwanted dose deposition within the patient.<i>Approach.</i>This study investigates neutron production for different ion species and energies, providing essential data for assessing secondary neutron exposure. Protons, helium, carbon, and oxygen ions were used to irradiate a PMMA phantom at two energies, corresponding to penetration depths of approximately 5 cm and 14 cm in water. The secondary neutron fluence and ambient dose equivalent (<i>H</i>*(10)) were measured using an ERBSS at four angular positions (0°, 45°, 90°, and 135°) relative to the beam direction.<i>Main results.</i>Results showed significant differences in neutron production depending on ion species and energy. The neutron ambient dose equivalent per primary ion in the beam direction varied by a factor of about 50 across the different ion species. When normalized to the absorbed dose in the pristine Bragg peak, variations of up to a factor of 10 were observed between proton and oxygen ions. However, at off-axis positions, neutron ambient dose equivalent per absorbed dose was relatively similar across ion species, even lower for ions heavier than protons when normalizing to the biologically effective treatment dose.<i>Significance.</i>This study presents the first measurement-based comparative analysis of fluence energy distributions and neutron equivalent doses for protons, helium, carbon, and oxygen ions in a synchrotron-based clinical facility for monoenergetic beams. These findings are highly relevant for evaluating secondary neutron exposure in particle therapy and optimizing treatment strategies to reduce long term-risks of radiation induced second cancers.