ICRU Report 93: Prescribing, Recording, and Reporting Light Ion Beam Therapy

Light ion beam therapy is a radiation therapy modality that uses light ions (Z < 10), that are heavier than protons. Presently, all clinical light ion facilities focus on carbon ions. Light ions combine the physical advantage of an absorbed dose vs. distribution depth trajectory similar to protons, with an even more pronounced so-called Bragg peak, providing an additional benefit of an increased biological effectiveness in the Bragg peak region compared to the entrance region. In 2019, there are 10 clinical facilities in operation worldwide. This number is slowly but steadily increasing. Meanwhile, the efficacy of ion beam therapy is well established for a number of tumor types. Unfortunately, comparisons of clinical results are hindered by significant differences in the prescription of RBE-weighted doses and the modeling of RBE. The present Report provides information necessary to standardize techniques and procedures and to harmonize the clinical prescription, recording and reporting of ion beam among the centers and with other modalities.

The Report’s Introduction briefly outlines the rationale and historical development of light ion beam therapy. Then the biological effects of light ion beams of different LET, as compared to x-rays, are described in detail, along with their clinical potential and the models used for RBE calculation in treatment planning. Moreover, the techniques used to produce and deliver these beams in clinical applications and their dosimetry are explained. The ICRU definition of volumes and geometric concepts to be used in light ion beam therapy is described and the concepts of treatment planning, including the clinical use of RBE models is explained. This is followed by an update of the strategies used to mitigate the effects of organ motion in proton and light ion beam therapy. An overview of uncertainties and quality assurance measures is included. Then the final section deals with the actual recommendations for prescribing, recording, and reporting treatments. The Report ends with six clinical examples of light ion beam therapy for different tumor entities and from different clinical facilities to illustrate the application of the recommendations.

The specific recommendations in this Report include the use of the quantity RBE-weighted dose (like in proton therapy) in units of Gray (Gy) for the product of absorbed dose with RBE. To avoid confusion, the specific type of dose (absorbed vs. RBEweighted) should always be included in any
related description. For dosimetry, the update of basic quantities for dosimetry from ICRU 90 are included and recommendations of IAEA TRS (technical report series) 398 are adopted.

The Report should be an important and useful reference for all practitioners in light ion beam therapy and should facilitate comparisons of clinical results from different centers, using different prescription and modeling schemes for RE-weighted dose. For all new users and interested readers, the description of the basic concepts and background of light ion beam therapy should enable them to understand the techniques involved in light ion beam therapy.