AbstractID: 8501 Title: Photoneutron doses in radiotherapy treatments simulated with
MCNPX code version 2.2.6
According to NCRP report #79 (1984) the dose at patient surface caused by induced
radioactivity just after treatment to 300 cGy with 25 MeV photon beam can be as high as
13 mR/h (decreasing to 2.5 mR/h after 10 min). One may expect higher dose rates caused
by induced radioactivity in room and equipment, including linear accelerators outfitted
with MLC leafs, following IMRT treatments with high energy (18MV -25 MV) beams
(Rawlinson et al., Med.Phys.29, 598-608, 2002). These estimates suggest that induced
radioactivity and neutron contamination can potentially exceed limits of 5 rem/year for
radiation oncology therapists. Therefore detailed re-investigation of dose rates from
neutrons and induced radioactivity for various treatment techniques, and for a variety of
modern equipment utilized in radiation oncology departments, is warranted. Monte Carlo
simulations capable of taking into account all interactions contributing to dose rate from
neutron scattering and induced radioactivity provide the most versatile and accurate
technique for these investigations. MCNPX code version 2.2.6, using full IAEA library of
photo-neutron cross sections, have been utilized in the joint IU-ORNL project to evaluate
doses in the head and the trunk of the ORNL Mathematical Phantom from neutrons,
protons, deutrons, tritons and 3He ions computed for traditional (non-IMRT) 25 MV,
photon beam therapy. Results obtained in these simulations confirm levels of dose
exposure reported by NCRP #79. Generic simulations aiming at calculation of total dose
exposure to therapists from neutrons and induced radioactivity for IMRT treatments
executed with Varian accelerators are under investigation.