Practical considerations in measuring out-of-field neutron dose for pulsed and gantry-mounted proton therapy machines

Objective.Synchrocyclotrons deliver radiation in distinct macro pulses with high instantaneous dose rate that distinguish them from isochronous cyclotrons and synchrotrons. The relatively high instantaneous dose rates associated with synchrocyclotrons require special consideration in measurement of the neutron dose. The objective of this work is to measure the out-of-field neutron ambient dose equivalent (H*(10)) as a conservative operational surrogate to assess the fetal dose due to secondary neutrons for a gantry-mounted synchrocyclotron proton therapy system when an intensity modulated proton therapy (IMPT) brain treatment is delivered with the dynamic aperture (DA), static aperture (SA), and no aperture (NA).Approach.A pencil beam scanning (PBS) gantry-mounted synchrocyclotron was used for beam delivery. IMPT plans employing the DA, SA, and NA, each with 3 fields (vertex, lateral, and posterior-anterior (PA)) were optimized using the treatment planning system to treat a left-sided spherical brain target with 6 cm diameter to a total dose of 66 Gy (weighted by radiobiological effectiveness [RBE] 1.1) in 33 fractions. The plans were delivered to an anthropomorphic phantom and the neutron H*(10) was measured using two commercial rem-meters, a WENDI-II and a LUPIN-BF3-NP counter at 43, 53, 61, and 71 cm distance from the isocenter intended to mimic distances to a fetus at various gestational periods. To investigate the influence of the couch angle for the PA field, the neutron dose from the PA field was measured at several couch angles including 0°, 180°, and 270°.Main results.WENDI-II suffers from dead-time associated issues and underestimated the dose by a factor of ∼1.7-2.7 compared to LUPIN-BF3-NP. H*(10) showed dependence on the measurement location and the use of field shaping system. The total H*(10) for 66 Gy-equivalent proton dose was estimated to be 10.7 mSv (9.7 mSv) and 4.9 mSv (4.5 mSv) at 43 and 71 cm distance from the isocenter, respectively, when the SA (DA) was used. Without an aperture the corresponding values were 11.8 mSv and 5.3 mSv. The 270°-couch-angle resulted in the highest neutron dose from the PA field by ∼15% relative to the 0°-couch-angle, resulting in an increase in the neutron dose by ∼5% for the whole treatment.Significance.WENDI-II is not a reliable device for in-room neutron dose measurement for synchrocyclotrons with high instantaneous dose rates. Employing appropriate use of the aperture system as well as avoiding the 270° couch angle for the PA field can minimize the neutron dose in the gantry-mounted system studied in this work.