Abstract ID: 18870 Title: Off-Axis Dose and Dose Equivalent Due to Secondary Neutrons From Uniform Scanning Proton Beams
Purpose:
The production of secondary neutrons is an undesirable byproduct of proton therapy and it is
important to quantify the contribution from secondary neutrons to patient dose received outside
the treatment volume. The purpose of the study is to investigate the off-axis dose and dose
equivalent from secondary neutrons experimentally using CR-39 Plastic Nuclear Track Detectors
(PNTD) at Procure Proton Therapy Center, Oklahoma City.
Methods:
CR-39 PNTD is generally insensitive to therapeutic protons, but has a sensitivity to the
secondary neutrons similar to that of tissue. In this experiment, we placed several CR-39 PNTDs
laterally inside a 60X20X35 cm3 phantom and in free air respectively at various depths and
angles with respect to the primary beam axis. Three different proton energies (78 MeV, 162
MeV and 226 MeV), 4 cm modulation width, 5cm diameter brass aperture, 38 cm snout to
isocenter distance and 10 cm isocenter point in the phantom were used for the entire experiment
and simulation. Monte Carlo simulations were also performed based on the experimental setup
using a simplified snout configuration and FLUKA.
Results:
The dose and dose equivalent is observed to be higher in air than in the phantom. Preliminary
results show that the measured ratio of secondary neutron dose equivalent to primary proton dose
(H/D) in CR-39 PNTD ranges from 1.04 ± 0.13 mSv/Gy inside the phantom to 43.5 ± 3 mSv/Gy
in free air for 78 MeV proton. The experiment and simulation results follow a similar decreasing
trend with distance but the magnitude varies with a factor of about 2 to 10 depending on
measurement location.
Conclusions:
We are currently investigating the contribution from external neutrons produced in the nozzle
and from the internal neutrons produced inside the phantom to total patient dose outside the
treatment volume as a function of location.