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During proton therapy, neutrons originate by interaction of the protons in the beam with any material that they could find along their path to the patient’s treatment volume. This includes accelerator elements present in the beam line, potential beam deviations leading to collisions with the beam pipe or other accelerator elements and, the patient’s tissues. The resulting neutron field represents parasitic radiation, extending throughout the irradiation room, with no therapeutic use, increasing the unwanted dose delivered to the patient’s healthy tissues. Correct determination of this dose and of the related risks of the patient developing radiation induced cancers in the future requires knowledge of the neutron spectrum at the measurement points.
Two projects (INTEND and MENPRO) are in progress for fully characterizing the neutron fields present in the Clinica Universidad de Navarra (CUN) proton therapy treatment room. Measurements of neutron ambient dose equivalent H*(10) and personnel dose equivalent Hp(10) are in progress using several types of instruments, including both rem-meter -type and passive detectors at several specified points inside and around the treatment room.
Neutron spectrometry was performed in four points inside the room, at different directions with respect to the incoming beam. Irradiation took place with proton beams of a single energy layer at 70, 150 and 229 MeV, as well as with a realistic clinical treatment, impinging in a solid water phantom. Spectra were determined from measurements of two different Extended Range Bonner Sphere Spectrometers, based on $^3$He detectors at the center of different sets of moderating spheres, made of polyethylene and with different metal layers. Measurements were unfolded using different codes to obtain the spectra for each energy at each measurement points. This work will summarize the spectrometry results obtained.
Acknowledgements: This work has been partially supported by Spanish projects MENPRO (CSN: PR-128-2024) and INTEND (PID2025-168369NB).