gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

High Energy Neutron Measurements and Monitoring for a 230 MeV Proton Radiotherapy Facility

Meeting Abstract

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  • V. Nazaryan - Hampton University Proton Therapy Institute, Hampton, VA, Hampton, USA
  • C. Keppel - Hampton University Proton Therapy Institute, Hampton, VA, Hampton, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog148

DOI: 10.3205/09ptcog148, URN: urn:nbn:de:0183-09ptcog1482

Published: September 24, 2009

© 2009 Nazaryan et al.
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Outline

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Background: Primary shielding walls in modern proton therapy facilities are typically made of an over 7' thick of ordinary (2.3 g/cm3) concrete. Our analysis demonstrate that behind such a thick wall in forward direction with respect to the proton beam neutrons with energies greater than 8 MeV still contribute considerably to the total dose. High energy neutron contribution in lateral direction is potentially even larger. Traditionally neutron rem-meters are designed to have their response function match well an appropriate (ICRP 1990, or NCRP-38) fluence-to-dose conversion function over an energy range extending from thermal (0.025 ev) to 10 MeV. The dose equivalent response of these detectors monotonically decreases above 7 MeV. We have reexamined the neutron field spectrum and angular distribution characteristic to medium-energy (230 MeV) proton accelerator facilities, and performed high efficiency measurements of the high energy neutron component of the radiation fields present.

Method and materials: We have performed over thirty measurements of the neutron dose equivalent at various locations in and around a state-of-the-art 230 MeV proton therapy facility using the Wide Energy Neutron Detection Instrument (WENDI) that has a useful energy response in the energy range from thermal to 5 GeV. It features a He-3 counter tube for superior sensitivity, and a tungsten powder shell that surrounds the counter tube acting as a neutron generator material via (n,2n) reaction above the 8 MeV threshold, and also acting as an absorber facilitating energy response contouring at intermediate neutron energies and excellent gamma rejection.

Results: We have obtained neutron attenuation lengths in forward and lateral directions from our measurements most appropriate for use at 230 MeV proton therapy facilities (PTFs) and compared our results with some previously published values. We have also obtained a new parameterization for neutron attenuation in the maze suitable for modern PTFs.

Conclusion: In surveying and area monitoring modern PTFs the neutron detector of choice must be capable of detecting with sufficient efficiency the high energy component of the neutron field to avoid large dose underestimation in an environment where greater than 8 MeV neutrons may have a significant contribution. New maze attenuation parameterization will provide for adequate maze design in these facilities, where previously used attenuation models provided at best for a marginal maze design.