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Systematic Out-of-Field Secondary Neutron Spectrometry and Dosimetry in Pencil Beam Scanning Proton Therapy

Worldwide the number of proton therapy centers in operation increases rapidly. Currently more than 70 centers are in operation and over 40 are under construction. Therefore precise data about secondary neutron doses for patients is essential to estimate the risk on secondary cancer. Neutron doses to patients typically are calculated by means of Monte-Carlo (MC) particle transport simulation. To validate these MC simulations, due to a lack of precise high-energy cross-sections, out-of-field secondary neutron spectroscopy measurements are necessary. Furthermore scattered neutrons in the treatment room contributes to the secondary neutron dose of the patient, which has to be quantified. In order to quantify important parameters for the secondary neutron exposure of patients, members of the working group for medical and environmental dosimetry carried out an extensive measurement campaign at Rinecker Proton Therapy Center in Munich. In a pencil beam scanning proton therapy gantry mono-energetic proton beams from 75 MeV up to 200 MeV with a field size of 11 x 11 cm² were used to determine the energy dependence on secondary neutron doses inside the treatment room. Furthermore, the effect of range-shifters on secondary neutron doses were investigated. It was found, that the neutron ambient does equivalent values (H*(10)) ranges from 0.3 µSv/Gy (75 MeV; 90°; 2.0 m distance) to 24 µSv/Gy (200 MeV; 0°; 2.0 m distance). Beside the irradiation with 75 MeV an irradiation with 118 MeV together with a 5 cm range-shifter (PMMA), which corresponds to a similar treatment plan, shows an increase of a factor of 7.2 in neutron dose by using a 5 cm range-shifter.

Out-of-Field secondary neutron doses in pencil beam scanning proton therapy (PBS) strongly depend on the primary beam energy and the angular position. As a result the ambient dose equivalent values inside a treatment room for typical irradiation energies from 75 MeV to 200 MeV ranges over two orders of magnitude. Use of range-shifter could strongly increase neutron doses.