title: An analytical model of leakage neutron equivalent dose for passively-scattered proton radiotherapy and validation with measurements.
abstract: Exposure to stray neutrons increases the risk of second cancer development after proton therapy. Previously reported analytical models of this exposure were difficult to configure and had not been investigated below 100 MeV proton energy. The purposes of this study were to test an analytical model of neutron equivalent dose per therapeutic absorbed dose  at 75 MeV and to improve the model by reducing the number of configuration parameters and making it continuous in proton energy from 100 to 250 MeV. To develop the analytical model, we used previously published H/D values in water from Monte Carlo simulations of a general-purpose beamline for proton energies from 100 to 250 MeV. We also configured and tested the model on in-air neutron equivalent doses measured for a 75 MeV ocular beamline. Predicted H/D values from the analytical model and Monte Carlo agreed well from 100 to 250 MeV (10% average difference). Predicted H/D values from the analytical model also agreed well with measurements at 75 MeV (15% average difference). The results indicate that analytical models can give fast, reliable calculations of neutron exposure after proton therapy. This ability is absent in treatment planning systems but vital to second cancer risk estimation.
authors.affiliation: Department of Physics and Astronomy, Louisiana State University and Agricultural and Mechanical College, 202 Nicholson Hall, Baton Rouge, LA 70803, USA. cschn19@tigers.lsu.edu.|Mary Bird Perkins Cancer Center, 4950 Essen Lane, Baton Rouge, LA 70809, USA. cschn19@tigers.lsu.edu.
authors.forename: Christopher
authors.initials: C
authors.lastname: Schneider
authors.affiliation: Department of Physics and Astronomy, Louisiana State University and Agricultural and Mechanical College, 202 Nicholson Hall, Baton Rouge, LA 70803, USA. newhauser@lsu.edu.|Mary Bird Perkins Cancer Center, 4950 Essen Lane, Baton Rouge, LA 70809, USA. newhauser@lsu.edu.
authors.forename: Wayne
authors.initials: W
authors.lastname: Newhauser
authors.affiliation: Institut de Radioprotection et de Sûreté Nucléaire, Service de Dosimétrie Externe, BP-17, 92262 Fontenay-aux-Roses, France. jad.farah@irsn.fr.
authors.forename: Jad
authors.initials: J
authors.lastname: Farah
keywords: analytical model; neutron equivalent dose; proton therapy