In-Beam and Off-Beam PET Measurements of Target Activation by Megavolt X-Ray Beams

Abstract

In-beam positron emission tomography (in-beam PET) is a valuable in situ method for quality assurance in radiation therapy. It is well investigated for therapy with carbon ions and has been successfully implemented clinically at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. The extension of this efficient technique to other radiation treatment modalities may be worthwhile. For protons, 3 He, 7 Li, and 16 O the feasibility has already been experimentally shown.Furthermore, it seems to be feasible for the case of radiotherapy with high-energy photons, since positron emitters are generated by photons with energies above 20 MeV due to ( ) photo-nuclear reactions (predominantly 11 C and 15 O in tissue). In this regard, promising conclusions have been obtained by Geant4 simulations as well as by off-beam PET experiments using a conventional PET scanner. The next step was the installation of a small double head positron camera consisting of two bismuth germanate (BGO) block detectors at the irradiation site to measure the generated + activity distribution simultaneously to the irradiation. The relation between deposited dose and + activity density was quantified. The obtained results are presented and compared to that of off-beam PET experiments. Higher activities as well as an improved contrast between materials of different stoichiometry are achieved by measuring in-beam, showing the advantage of in-beam PET over off-beam PET. Thus, the application of in-beam PET to radiation therapy with high-energy photons can be useful for quality assurance, comprising monitoring of dose delivery, patient positioning and tumor response.Index Terms-Dose monitoring, high-energy photon therapy, positron emission tomography.

Dynamic PET/CT measurements of induced positron activity in a prostate cancer patient after 50-MV photon radiation therapy

“…As the radionuclides produced in the patient during photon irradiation all originate from the body tissue (and not from the beam itself), the measured PET activity will, with some corrections, be strictly proportional to the body tissue composition. The idea of being able to measure the true body tissue composition and map anatomical structures is interesting [29] and could have future applications. One possible aspect of this method would be to measure and determine the tumor tissue composition in order to identify any hypoxic or necrotic regions that can be used in further therapy planning process.…”