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Trinkl, S. ; Mares, V. ; Englbrecht, F.S.* ; Wilkens, J.J.* ; Wielunski, M. ; Parodi, K.* ; Rühm, W. ; Hillbrand, M.*

Systematic out-of-field secondary neutron spectrometry and dosimetry in pencil beam scanning proton therapy.

Med. Phys. 44, 1912-1920 (2017)
Verlagsversion DOI
Open Access Green möglich sobald Postprint bei der ZB eingereicht worden ist.
BACKGROUND AND PURPOSE: Systematic investigation of the energy and angular dependence of secondary neutron fluence energy distributions and ambient dose equivalents values (H*(10)) inside a pencil beam scanning proton therapy treatment room using a gantry. MATERIAL AND METHODS: Neutron fluence energy distributions were measured with an extended-range Bonner sphere spectrometer featuring ³He proportional counters, at four positions at 0°, 45°, 90° and 135° with respect to beam direction and at a distance of 2 m from the isocenter. The energy distribution of secondary neutrons was investigated for initial proton beam energies of 75 MeV, 140 MeV and 200 MeV, respectively, using a 2D scanned irradiation field of 11 x 11 cm² delivered to a 30 x 30 x 30 cm³ PMMA phantom. Additional measurements were performed at a proton energy of 118 MeV including a 5 cm range-shifter (PMMA), yielding a Bragg peak position similar to that of 75 MeV protons. RESULTS: Ambient dose equivalent values from 0.3 μSv/ Gy (75 MeV; 90°) to 24 μSv/Gy (200 MeV; 0°) were measured inside the treatment room at a distance of 2 m from the isocenter. H*(10) values were lower (by factors of up to 7.2 (at 45°)) at 75 MeV compared to those at 118 MeV with the 5 cm range-shifter. At 0° and 45° an evaporation peak was found in the measured neutron fluence energy distributions, at neutron energies around MeV, which contributes about 50% to total H*(10) values, for all investigated proton beam energies. CONCLUSIONS: This study showed a pronounced increase of secondary neutron H*(10) values inside the proton treatment room with increasing proton energy without beam modifiers. For example, in beam direction this increase was about a factor of 50 when protons of 75 MeV and 200 MeV were compared. The existence of a peak of secondary neutrons in the MeV region was demonstrated in beam direction (0°). This peak is due to evaporation neutrons produced in the existing surrounding materials such as those used for the gantry. Therefore, any simulation of the secondary neutrons within a proton treatment room must take these materials into account. In addition, the results obtained here show that the use of a range-shifter increases the production of secondary neutrons inside the treatment room. Using a range-shifter the higher neutron doses observed mainly result from the higher incident proton energy (118 MeV instead of 75 MeV when no range-shifter was used), due to higher neutron production cross-sections.
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Publikationstyp Artikel: Journalartikel
Dokumenttyp Wissenschaftlicher Artikel
Schlagwörter Geant4 ; Extended-range Bonner Sphere Spectrometer ; Neutron Dosimetry ; Neutron Spectrometry ; Pencil Beam Scanning ; Proton Radiotherapy; Bonner Sphere Spectrometer; Dose-equivalent; Radiation-field; Carbon-ion; Radiotherapy; Risk; Sets
ISSN (print) / ISBN 0094-2405
e-ISSN 1522-8541
Zeitschrift Medical Physics
Quellenangaben Band: 44, Heft: 5, Seiten: 1912-1920 Artikelnummer: , Supplement: ,
Verlag American Institute of Physics (AIP)
Verlagsort Hoboken
Begutachtungsstatus Peer reviewed