Purpose: Monte Carlo (MC) study on the use of PET imaging as an image-guidance tool in Very High-Energy Electron (VHEE) therapy.
Methods: Using the MC code TOPAS, the simulations were aiming to assess the number of positron emitters created along the beam path when irradiating DICOM images for abdomen. While no specific spot was targeted, the 250 MeV VHEE beam was going through a part of a lung and the heart. The main positron emitters created in biological tissue were scored, namely oxygen-15, carbon-11, nitrogen-13, phosphorous-30 and potassium-38 and the corresponding activity was then calculated.
Results: The total number of positron emitters produced by the VHEE beam was averaging 110 emitters.Gy⁻¹.mm⁻³ and can reach a maximum of 330 emitters.Gy⁻¹.mm⁻³. The interface between tissue and air within the lungs can be observed as there is no creation of positron emitters in air. Moreover, the relationship between positron emitter production and material density allows PET imaging to discern structures with strong density differences such as air/tissue or tissue/bone. Finally, an in-beam PET system is required to prevent significant signal losses during acquisition due to the short half-life of oxygen-15 (2.03 min) which is the most abundant emitters. The overall production of positron emitters is lower with VHEE therapy than with proton therapy because it occurs indirectly by photonuclear reactions, whose cross-sections are small. However, the resulting total activity remains sufficient for detection with an average of 0.6 Bq.Gy⁻¹.mm⁻³ and a maximum of 1.8 Bq.Gy⁻¹.mm⁻³.
Conclusion: The potential use of PET imaging as an image-guidance tool in VHEE therapy has been demonstrated as long as an in-beam PET system is used.