Unique Considerations Associated with Neutron Ambient Dose Equivalent Measurement for a Gantry-Mounted Synchrocyclotron
Abstract
Purpose
To measure the neutron-ambient-dose-equivalent (H*(10)) and assess the fetal dose due to secondary neutrons in 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).
Methods
IMPT plans employing the DA, SA, and no aperture, each with 3 fields (vertex, lateral, and PA) were optimized to cover a left-sided spherical brain target with 6-cm-diameter with a total dose of 60 Gy (RBE-weighted) in 30 fractions. The 2-Gy-per-fraction 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 and 70 cm distance from the isocenter. To investigate the influence of the couch angle for the PA field, the H*(10) for the PA field was measured at several couch angles including 0°, 180°, and 270°.
Results
WENDI-II, impacted by dead-time associated issues, 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 H*(10) for 60 Gy (RBE-weighted) proton dose was estimated to be 9.75 mSv (8.82 mSv) and 4.5 mSv (4.1 mSv) at 40 and 70 cm distance from the isocenter when the SA (DA) was used. Without an aperture the corresponding values were 10.74 mSv and 4.8 mSv. The 270°-couch-angle resulted in the highest H*(10) from the PA field by ~15% whose impact was an increase in the neutron dose by ~5% for the whole treatment.
Conclusion
WENDI-II is not a suitable 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.