By DR. WILLIAM F. SENSAKOVIC
Wilhelm Roentgen serendipitously discovered x-rays in 1895. Scientists and physicians, eager to improve patient care, immediately applied the new technology to imaging the human body. By 1897 reports began to surface of hair loss and skin reddening and in that same year it was confirmed that x-rays induce biological changes when they were used to treat nevi (hairy moles) on the back of a 5-year-old girl.
Extensive research over the last century using animal models and epidemiological data from events such as the atomic bombings of Hiroshima and Nagasaki and Chernobyl has improved our understanding of radiobiological effects. This research has definitively demonstrated that radiation may cause cancer, epilation, sterility, cataracts, erythema, desquamation, tissue necrosis, and death. Further, radiobiological effects are of particular concern for children and pregnant women. Research has demonstrated that children are more sensitive to radiation induced cataracts, hypothyroidism, thyroid nodules, and many forms of cancer. An irradiated embryo/fetus is at risk for miscarriage, childhood cancer, growth retardation, organ malformation, and intellectual disability.
These severe radiobiological effects coupled with the ubiquity of medical imaging are often a source of anxiety for both patients and physicians. Add in damning exposes in the news, exaggerated journal articles, and a general lack of education about radiation and it creates a hysteria that may cause both physicians and patients to avoid essential imaging.
To understand the effects of radiation and gauge its danger one needs to understand how we measure radiation. Though the field is vast, for our purposes it will suffice to say that tissue radiation absorbed dose is measured in Grays (Gy). All radiobiological effects, except cancer, require a minimum dose (threshold) before they occur.
Computed tomography (CT), nuclear medicine, and fluoroscopy typically give the highest radiation absorbed doses (up to 0.1Gy to tissue and up to 0.03mGy fetal). The lowest dose that produces non-cancerous biological effects is 0.25Gy in adults and 0.1Gy in utero. Thus, a typical diagnostic scan will not cause biological tissue effects in adults or a fetus. It should, however, be noted that interventional procedures and radiotherapies can that approach a level of concern.
Radiation dose measurements are modified to account for the varying potential of cancer induction in different tissues. This modified dose is called the effective dose and is measured in Sieverts (Sv). CT typically delivers the highest effective dose (~0.002-0.01Sv) with radiographs and fluoroscopy below that. Though there is some controversy, currently accepted models assume any amount of radiation may induce cancer. That being said a 0.01Sv CT scan increases a typical person's cancer risk from ~40% to ~40.1%. Similarly, a pelvic CT of a pregnant woman increases the fetal risk of childhood cancer from ~0.3% to ~0.5%. Thus, the risk from imaging is very low. However, given the ubiquity of scanning and the possibility of multiple scans on the same patient, it is recommended that radiation is limited to what is diagnostically necessary.
Given the small risk of cancer it would be ideal if we could minimize dose; however, this is not possible. Although the exact relationship is complex, image quality generally decreases as dose decreases. Thus, minimizing dose would result in non-diagnostic image quality. Instead, management of patient radiation should follow the principles of justification and optimization.
Justification states that an exam should only be performed if it does more harm than good. A good mnemonic is .DAM (dot DAM): Don't Order Tests that Don't Affect Management. The physician looking for guidance on appropriate imaging should refer to The American College of Radiology (ACR) Appropriateness Criteria®. These are "evidence-based guidelines to assist referring physicians and other providers in making the most appropriate imaging or treatment decision for a specific clinical condition. By ordering the lowest-dose exam that still conveys relevant clinical information the referring physician can play a large role in reducing patient radiation dose.
Optimization entails ensuring that modern technology is utilized and that imaging protocols are set such that excess radiation is not delivered to the patient. New technology such as iterative reconstruction and automatic exposure control when properly used create images of sufficient quality at reduced dose. How that technology is implemented is determined by the scanning parameters, which have a tremendous impact on image quality and patient dose. The protocol that describes these parameters should be periodically reviewed by a team consisting of, at a minimum, a radiologist, qualified medical physicist, and technologist. The radiologist reviews image quality, the physicist reviews the technology and dose, and the technologist reviews workflow integration and implementation feasibility. Many resources exist to guide optimization. These include journal articles, ACR practice parameters, and publications from Image Wisely, Image Gently, and the American Association of Physicists in Medicine. This information can help a practice provide the best care for their patients by ensuring that patient radiation dose is As Low As Reasonably Achievable (ALARA).
The radiation delivered during imaging is essential for diagnosis, but brings with is a small risk to the patient. It is important that physicians keep this risk in mind, but also in perspective when ordering imaging studies and performing patient scans. Diagnostic imaging exams, when performed correctly, should not induce non-cancerous effects. The probability of inducing cancerous effects is also low to negligible. Qualified Medical Physicists are experts in the application of radiation in healthcare and should be consulted when optimizing protocols, imaging vulnerable populations, and when questions related to dose and image quality arise.
Dr. William F. Sensakovic received his undergraduate degrees (physics and mathematics) and PhD (Medical Physics) from the University of Chicago. His research focused on image processing, computer-aided detection, and imaging biomarkers. He is certified by the ABR for Diagnostic Medical Physics and by the American Board of Magnetic Resonance Safety (ABMRS) as a MR Safety Expert. He is Chair of the AAPM Imaging Physics Curricula Subcommittee and task group on establishing an image quality registry, editor for the physics section of both RadExam and Radiology Assessment and Review (RADAR). He is president-elect for the State of Florida AAPM, the ACR councilor-at-large for Medical Physics, a board member for the ABMRS, and on the board of associate editors for Medical Physics. He is currently a Medical Physicist at Florida Hospital with appointments at the University of Central Florida (Assoc. Prof. of Medical Education), Florida State (Clinical Asst. Prof. of Med. Phys.), and Adventist Universtiy (Adj. Prof. of Research). His current clinical research focuses on the optimization of dose and image quality.