Therapeutic rather than diagnostic medical physicists should lead the development and clinical implementation of image‐guided nonionizing therapeutic modalities such as MR‐guided high‐intensity ultrasound

Arguing against the Proposition is R. Jason Stafford, Ph.D. Dr. Stafford obtained his Ph.D. in Medical Physics from the University of Texas Health Science Center at Houston and M. D. Anderson Cancer Center Graduate School of Biomedical Sciences, Houston, TX in 2001, where he was subsequently appointed to the faculty and is currently Associate Professor in the Department of Imaging Physics. He is certified by the ABR in Diagnostic Radiological Physics. His major research interests include MR-guided interventions such as nanoparticle-directed photothermal ablation and MR thermal imaging. He serves on several AAPM committees and Task Groups and the Editorial Boards of both Medical Physics and the JACMP. He is Past-President of the AAPM Southwest Chapter. Why should therapeutic medical physicists, rather than diagnostic physicists, lead the development and clinical implementation of image-guided nonionizing therapeutic modalities such as MR guided high-intensity ultrasound? Our diagnostic colleagues have had great success applying unfocused low intensity ultrasound in the field of oncology to detect and characterize tumors. The detection of breast tumors1 and the quantification of liver tumors using dual frequency ultrasound2 are two such examples. Clearly, as an imaging modality, unfocused low intensity ultrasound falls naturally into the domain of the diagnostic medical physicist. However, high intensity focused ultrasound (HIFU) is not an imaging but a therapeutic modality and hence its development as a therapeutic modality and its clinical implementation falls more naturally into the domain of the therapeutic medical physicist. Safe and efficient patient treatment using any therapeutic modality involves site-specific patient immobilization and virtual simulation of the treatment to choose the best treatment approach that allows for maximal sparing of normal tissue while allowing for adequate target coverage. This is followed by treatment planning, delivery verification, target localization using some form of imaging before and during treatment, and pretreatment delivery and treatment quality assurance. The aim of these processes is to mitigate normal tissue injury as far as possible and to maximize local tumor control through accurate and reproducible patient setup and adequate choice of treatment margins to deal with residual treatment uncertainties such as tumor motion and random setup errors. In fact, therapeutic medical physicists have spent the last decade and a half with great success perfecting these processes in radiation therapy through the development and implementation of image guided radiation therapy using ultrasound, CT, and MR imaging. These areas of patient care have not been traditionally part of the training of diagnostic medical physicists and, hence, only therapeutic medical physicists can assure the safe and effective delivery of HIFU, since the above areas of patient care are part of their clinical expertise and training. Moreover, HIFU is ablative therapy and hence is, by definition, a local therapy modality that can only be directed against the gross disease visible on imaging. Therefore, HIFU has to be combined with other regional therapies such as fractionated radiotherapy to treat microscopic disease extensions to afford patients the best chance for local/regional disease control. Currently, the American Board of Radiology defines therapeutic medical physics as the branch of medical physics that deals with “(a) the physical aspects of therapeutic applications of x-rays, gamma rays, electrons and other charged particles beams, neutrons, and radiation from sealed radionuclide sources and (b) the equipment associated with their production and use…”3 From the discussion above, it follows that this definition of therapeutic medical physics is unnecessarily narrow and should be broadened to include image-guided nonionizing therapeutic modalities such as MR guided high-intensity ultrasound and radiofrequency ablation, since only therapeutic medical physicists have the training and experience to safely and efficiently implement these technologies in the clinic for patient treatment. Image-guided nonionizing therapies encompass a broadening arsenal of approaches (e.g., cryoablation,4 thermal ablation,5 hyperthermia,6 and irreversible electroporation7). Some of these modalities, such as HIFU,8 as well as emerging techniques based on alternating magnetic field activation of nanoparticles,9 incorporate extracorporeal delivery of nonionizing radiation. However, the majority in use today utilizes minimally invasive applicators which deliver energy locally. Together these approaches constitute a rapidly proliferating array of minimally invasive image-guided interventions aimed at achieving a variety of clinical goals reaching well beyond just cancer therapy and into applications in cardiology and neurology. Many of these modalities have matured over the last decade and are becoming commercially available to a wider variety of physicians, including surgeons, urologists, interventional radiologists, and radiation oncologists. To directly address the verbiage of the proposition, the development and, in particular, clinical implementation, of these emerging therapeutic approaches should be led directly by physicians who are fully aware of the potential impact of the proposed nascent technologies in the management of their patients. Many of these procedures, such as cryoablation or radiofrequency ablation, are already delivered safely and effectively to a variety of anatomical sites under CT, ultrasound or MRI guidance.10 In response to the proposition, the safety and efficacy of many of these procedures could benefit from the inclusion of physicists providing support and taking leadership roles in various aspects of these procedures, such as assisting in the design of techniques and protocols for treatment delivery as well as image-based planning, targeting, monitoring, and verification of treatment delivery, especially during the technical development and initial clinical implementation phase of research. However, these procedures encompass a range of techniques using nonionizing energy which propagates and interacts with tissue in a fundamentally different manner than ionizing radiation. Some techniques, for example, HIFU in deep seated tissue,8 rely heavily on the proper implementation and interpretation of nonstandard, advanced imaging techniques for real-time beam targeting and therapy monitoring. Very often it is the scientists who have helped shepherd these technologies through inception, preclinical, and early clinical trials who have developed the necessary expertise to lead clinical implementation efforts. Regarding the question of “therapeutic” or “diagnostic” medical physicist, at the current time, there is no distinction in the CAMPEP accredited didactic course work nor Medical Physics board certification processes to indicate that members of either of these professions possess the necessary expertise to advance, let alone support, these approaches without substantial additional training. It would appear that the emergence of these therapies is an excellent opportunity to recruit scientists from these laboratories into medical physics in an effort to further diversify and enrich our research and clinical expertise portfolio. Given this, and the fact that medical physicist participation and support is likely to be governed primarily by the clinical service performing the procedures, it seems the question that the Medical Physics community should focus on is not “who,” but “how” we will accommodate emerging image-guided nonionizing therapies into our academic and professional programs as the traditional boundaries separating disciplines such as therapy and imaging physics continue to blur in an era of multidisciplinary patient care. I am in full agreement with my valued colleague that it should be a team of physicians and physicists trained in the safe application of these new therapeutic radiation modalities that should lead the clinical implementation of this “broadening arsenal of image-guided nonionizing therapy approaches” if they are to become successful. To that end, we tend to focus very much on the clinical subspecialty that tends to apply these new therapeutic modalities. However, clinical skills can only be helped by appropriate science to back it up. While basic scientists are investigating the biological mechanisms of HIFU, which are different from those of ionizing radiation, a medical physicist has the right experience and knowledge for appropriate clinical implementation of therapeutic ultrasound. The clinical workflows for patient treatment for both ionizing and nonionizing radiation therapy are similar. As pointed out in my opening statement, safe and efficient patient treatment using any image-guided therapeutic delivery of physical energy would involve site-specific patient immobilization and virtual simulation of the treatment to choose the best approach that allows for maximal sparing of normal tissue while allowing for adequate target coverage. This would be followed by treatment planning to quantify the treatment dose to be delivered, treatment verification, image-guided real-time tumor/target localization, and pretreatment delivery and treatment quality assurance. The aim of these processes is to mitigate normal tissue injury as far as possible and to maximize local tumor control through accurate and reproducible patient setup and adequate choice of treatment margins to deal with residual treatment uncertainties such as tumor motion and random setup errors. Therapeutic medical physicists routinely perform these duties in the clinic. They have the appropriate training in respiratory gating, 4D simulation, and treatment planning for radiation. It is likely that some of the same techniques could accelerate the progress and success of clinical application of therapeutic ultrasound. Moreover, these newer nonionizing image-guided treatment modalities, just like surgery, are ablative and by definition local therapies that can only be directed against visible disease. Hence, they will need to be combined with some form of regional therapy such as radiation therapy. Therefore, these new nonionizing image-guided treatment modalities fall naturally into the purview of radiation oncology and therapeutic medical physics. I sympathize with my colleague on the narrowness of ABR professional medical physics definitions. “Diagnostic medical physics” might be more appropriately called “imaging medical physics.” However, these definitions aim to concisely describe an expected minimum training scope for certificate holders. Broadening these definitions without concurrent changes in didactic and clinical training requirements, as well as examination content, is unwarranted, at best. My colleague makes another excellent observation with respect to image-guided radiation therapy and the successful collaboration with imaging expertise. As nonionizing modalities proliferate in other services, collaboration with therapeutic medical physicists is likely to provide value. However, extension to all nonionizing therapeutic modalities, much less all MR-guided high-intensity focused ultrasound (MRgFUS) is not warranted. The recent FDA PMA of MRgFUS for treatment of painful bone metastases in patients who do not respond to, or cannot undergo, radiotherapy, exemplifies an application that may be performed within a radiology service, as other image-guided nonionizing modalities are now. MRgFUS is unique in that imaging potentially provides a closed-loop mechanism for continuous periprocedural planning, localization, and quantitative monitoring of therapy delivery followed by post-treatment verification imaging, often in a single session. Prospective planning is unlikely to provide precise predictions of delivered dose, but rather serves as a tool for assessing feasibility of approach and likelihood of failure. Safety and efficacy, in most scenarios, is achieved via direct imaging feedback of heating or tissue changes at defined intervals during delivery to ensure proper localization of energy, heavily favoring imaging expertise in the design, implementation and interpretation of periprocedural imaging feedback and assessment. Such modalities will benefit from onsite scientists or physicists properly trained to support the equipment and procedures, and current focus should likely be aimed at defining what that training should encompass.