Monte Carlo simulations of neutron spectral fluence, radiation weighting factor and ambient dose equivalent for a passively scattered proton therapy unit

Abstract

Stray neutron exposures pose a potential risk for the development of secondary cancer in patients receiving proton therapy. However, the behavior of the ambient dose equivalent is not fully understood, including dependences on neutron spectral fluence, radiation weighting factor and proton treatment beam characteristics. The objective of this work, therefore, was to estimate neutron exposures resulting from the use of a passively scattered proton treatment unit. In particular, we studied the characteristics of the neutron spectral fluence, radiation weighting factor and ambient dose equivalent with Monte Carlo simulations. The neutron spectral fluence contained two pronounced peaks, one a low-energy peak with a mode around 1 MeV and one a high-energy peak that ranged from about 10 MeV up to the proton energy. The mean radiation weighting factors varied only slightly, from 8.8 to 10.3, with proton energy and location for a closed-aperture configuration. For unmodulated proton beams stopped in a closed aperture, the ambient dose equivalent from neutrons per therapeutic absorbed dose (H*(10)/D) calculated free-in-air ranged from about 0.3 mSv/Gy for a small scattered field of 100 MeV proton energy to 19 mSv/Gy for a large scattered field of 250 MeV proton energy, revealing strong dependences on proton energy and field size. Comparisons of in-air calculations with in-phantom calculations indicated that the in-air method yielded a conservative estimation of stray neutron radiation exposure for a prostate cancer patient.

Reducing stray radiation dose to patients receiving passively scattered proton radiotherapy for prostate cancer

“…This is in good agreement with previous H/D determinations made in the presence of a phantom. Specifically, measured H/D values from Yan et al (2002) and simulated H/D values from and Zheng et al (2008) ranged from 1 mSv Gy −1 to 15 mSv Gy −1 , depending on the location and other variables.…”

Monte Carlo simulation of a proton therapy beamline for intracranial treatments

“…Monte Carlo methods also turn out to be powerful in complex situations where measurements are difficult to achieve. They also have many other applications in proton therapy: verification of shielding design efficiency (Newhauser et al, 2002), assessment of secondary doses received by proton therapy patients (Agosteo et al, 1998;Jiang et al, 2005;Zacharatou Jarlskog and Paganetti 2008) and neutron fluence and ambient dose equivalent calculations (Zheng et al, 2008;Perez-Andujar et al, 2009). It should be noted that each Monte Carlo model must first be carefully validated with experimental data in order to verify the accuracy of results.…”

Spot scanning proton therapy minimizes neutron dose in the setting of radiation therapy administered during pregnancy

“…This underestimation is greatly reduced at the out‐of‐field location, since the evaporation neutrons with energy spectrum peaked around 1 MeV are significant 16 , 20 . Neutron H measured by etched track detector, which can measure neutrons up to at least 40 MeV was compared extensively with REM500 measurement in previous study (20) .…”

“…The value decreased significantly with lower beam energy and increasing distance of fetus from the field edge. There are other studies of out‐of‐field secondary neutron both with Monte Carlo (MC) calculations or experimental measurements that could be used to estimate the neutron H to the fetus for passive scattering and uniform scanning beams 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 . However, most of these studies were made for a specific proton beam configuration, and the reported values of neutron H vary markedly because of different machine designs, experimental setups, and measurement instruments.…”