Magnetic resonance biomarkers in radiation oncology: The report of AAPM Task Group 294

A magnetic resonance (MR) biologic marker (biomarker) is a measurable quantitative characteristic that is an indicator of normal biological and pathogenetic processes or a response to therapeutic intervention derived from the MR imaging process.1 There is significant potential for MR biomarkers to facilitate personalized approaches to cancer care through more precise quantification of normal vs pathologic tissue function, response to therapy as well as toxicity to both radiation and chemotherapy. The ongoing integration of MR into routine clinical radiation therapy (RT) planning and the development of MR‐guided radiation therapy systems are providing new opportunities for MR biomarkers to personalize and improve clinical outcomes. Their appropriate use, however, must be based on knowledge of the physical origin of the biomarker signal, the relationship to the underlying biological processes, and their strengths and limitations. Additionally, while significant literature exists providing quantitative values associated with a range of MR biomarkers, these values would not be considered compliant with the stringent requirements necessary to classify the parameter as a quantitative MR biomarker. This is particularly true within the context of using these values to drive radiation oncology (RO) treatment decisions, the most notable of which relate to adaptive treatment planning and delivery. The overall objective of this report is to provide physicists and clinicians with a basic understanding of MR biomarkers within the context of RO. Clarification is provided in terms of the definition of an imaging biomarker as well as the criteria by which a biomarker is considered quantitative. Their physical bases, strengths and limitations, values for normative and disease processes, as well as response to therapy when known are presented. Finally, standardized approaches to quality control and quality assurance programs are described. This educational report serves several purposes: (i) To describe the framework used to define an imaging biomarker and establish clinical validation, utility, and quantification, (ii) to identify MR biomarkers known to provide information relevant to RO, (iii) to describe standardized processes necessary for the quantification and validation of MR biomarkers in RO, and (iv) to provide quantitative values of these biomarkers under both normative and pathologic conditions. While not all potential MR biomarkers for RO are described here, those considered most common and promising are reported. For those measurements that remain undiscovered or unclassified, it is the intent of this report to describe the framework required to translate such measurements into imaging biomarkers for RO application.