A review on reference dosimetry in radiation therapy with proton and light ion beams: status and impact of new developments

Abstract

This review focuses on reference dosimetry of proton and light ion beams used for cancer treatments. The background is the recommendation from the International Atomic Energy Agency in the Technical Report Series 398 and the recent publication of the International Commission on Radiation Units and Measurements Report 93 (Prescribing, Recording, and Reporting Light Ion Beam Therapy). These recommendations are briefly summarised, focusing on protons and carbon ions, as the most used ion types worldwide. Following these guidelines, the different aspects involving absolute dose measurements with ionisation chambers are discussed. A description of the current challenges and adaptions for the future is given for new developments. One of them, with a direct impact on ionisation chambers, is the influence of magnetic fields since efforts are made to integrate magnetic resonance imaging techniques into ion beam treatments, allowing online imaging with high soft-tissue contrast. Additionally, upcoming treatments with helium beams call for more experimental studies and simulations characterising the response of ionisation chambers, and at the same time following the international recommendations. Lastly, since the use of helium ion beams may present the advantage of reduced number of secondary particles as compared to carbon ion beams, the role of secondary neutrons and their impact on secondary cancer incidence needs to be assessed.