Study of <i>Bremsstrahlung</i> X‐Rays Emitted During Monoenergetic and Spread‐Out Bragg Peak Proton Irradiations in Preclinical Conditions

ABSTRACT Hadrontherapy treatments generally take advantage of the finite path of the ions in matter to limit healthy tissue irradiation. The absence of a transmitted beam, however, complicates the online assessment of the targeting. This verification becomes even more crucial with low‐energy ion beams used for small animal preclinical irradiations, easily disrupted by changes in the tissues or in the position of the animal, and when comparing different irradiation techniques, such as irradiation in the Bragg plateau of a monoenergetic beam and irradiation in the spread‐out Bragg peak (SOBP). The bremsstrahlung X‐rays emitted during irradiation by a 68 MeV proton beam were studied along a mouse‐sized cylindrical phantom on a realistic beamline, including a ridge filter allowing passive shaping of the SOBP. A compact semiconductor spectrometer was positioned orthogonally to the beam propagation direction. The detector's field of view was controlled using a 2 mm diameter collimator fixed on a tailored tungsten shield. More than 1000 X‐rays/Gy were detected, even in the Bragg peak regions for both monoenergetic and SOBP beams. The absorbed dose at a fixed position in the Bragg curve's plateau can be accurately determined (&lt; 1.5% at 1 Gy) using X‐ray energy spectrum integrals. Furthermore, partial integral ratios, X‐ray average energy and X‐ray energy at the maximum amplitude of the spectrum showed a similar variation with proton mean energy in the field of view for both modalities. Bremsstrahlung X‐rays are a promising probe to monitor in real time the irradiation of small animals at preclinical absorbed doses.