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Med Phys. 2019 Mar;46(3):e53-e78. doi: 10.1002/mp.13390. Epub 2019 Feb 14.

Report of the AAPM TG-256 on the relative biological effectiveness of proton beams in radiation therapy.

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Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA, USA.
Biological Systems and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
Department of Radiation Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA, USA.
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, CT, USA.
New York University Langone Medical Center & Laura and Isaac Perlmutter Cancer Center, New York, NY, USA.
Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.
Department of Radiation Medicine and Applied Sciences, University of California San Diego, La Jolla, CA, USA.
Department of Radiation Oncology, School of Medicine, University of Washington, Seattle, WA, USA.


The biological effectiveness of proton beams relative to photon beams in radiation therapy has been taken to be 1.1 throughout the history of proton therapy. While potentially appropriate as an average value, actual relative biological effectiveness (RBE) values may differ. This Task Group report outlines the basic concepts of RBE as well as the biophysical interpretation and mathematical concepts. The current knowledge on RBE variations is reviewed and discussed in the context of the current clinical use of RBE and the clinical relevance of RBE variations (with respect to physical as well as biological parameters). The following task group aims were designed to guide the current clinical practice: Assess whether the current clinical practice of using a constant RBE for protons should be revised or maintained. Identifying sites and treatment strategies where variable RBE might be utilized for a clinical benefit. Assess the potential clinical consequences of delivering biologically weighted proton doses based on variable RBE and/or LET models implemented in treatment planning systems. Recommend experiments needed to improve our current understanding of the relationships among in vitro, in vivo, and clinical RBE, and the research required to develop models. Develop recommendations to minimize the effects of uncertainties associated with proton RBE for well-defined tumor types and critical structures.


RBE; proton therapy; relative biological effectiveness


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