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Maximizing the Availability of Positron Emitting Nuclei for Proton Therapy Verification Using Different Beam Irradiation Sequences

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M Yagi

M Yagi1,3*, D Oxley1, P Dendooven1, S Brandenburg1, M Koizumi2, T Teshima3, (1) Kernfysisch Versneller Instituut, Groningen, Groningen, (2) Osaka University Hospital, Suita, Osaka, (3) Osaka University, Suita, Osaka

SU-E-T-294 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To demonstrate that the amount of nuclei available for post-irradiation proton treatment verification using positron emission tomography (PET) can be enhanced by reversing the beam delivery sequence in proton scanning beam irradiations.
Methods: A time-dependent analytical model is used to calculate the distributions of positron emitting nuclei for three different irradiation sequences: a scattered beam and a scanning beam in both the conventional sequence, distal edge first, and reverse sequence, distal edge last. The simulated geometry emulates reference dosimetry measurements conducted at the Paul Scherrer Institute (PSI). The reference measurements irradiate a 10 x10 cm² field, delivering about 1 Gy to a 10 cm wide spread-out Bragg peak (SOBP). Positron emitter availability with different beam sequence and imaging times and the impact of the different irradiation sequences on the statistical error on a range extrapolation were investigated.
Results: The ratio of the amount of positron emitters from the distal last beam sequence to that from the distal first sequence was 2.22 in the last centimeter of the SOBP. The comparison between distal last and a scattered beam gave a ratio of about 1.7 in the same region. In the distal last irradiation, more isotopes decay within a 120 second window, than in a 240 second window using a distal first irradiation. The statistical fluctuation on a range extrapolation was also smallest in the distal last beam sequence.
Conclusions: We demonstrated the effect of the irradiation beam sequence on the isotope production relevant for the verification of proton spot scanning therapy with PET. The largest amount of isotopes is available by irradiating the distal edge last. This new beam sequence reduces the PET measurement time while still offering higher counts and accuracy compared with both the conventional beam sequence and the scattering method.

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