Arguing against the Proposition is Cynthia H. McCollough, Ph.D. Dr. McCollough obtained her M.S. and Ph.D. degrees in Medical Physics from the University of Wisconsin, Madison. Upon graduation she began working in the Radiology Department at Mayo Clinic, Rochester, Minnesota, where she is currently Professor of Medical Physics and Biomedical Engineering. As Director of Mayo Clinic's CT Clinical Innovation Center, Dr. McCollough leads a multidisciplinary team of physicians, scientists, research fellows, and graduate students on projects seeking to detect and quantify disease using CT imaging. She has particular expertise in the use of CT for quantitative assessment of material composition, disease progression or regressions, and organ function, as well as methods to quantify and reduce radiation dose. Dr. McCollough is internationally recognized for her contributions to the fields of CT imaging physics and technology, and radiation dosimetry and protection. She has served in numerous capacities in the AAPM including on the Board of Directors and the Editorial Board, and has been elected Fellow of the AAPM, the American College of Radiology, and the American Institute for Medical and Biological Engineering.

Arguing for the Proposition is Jeffry A. Siegel, Ph.D. Dr. Siegel obtained his M.S. degrees in Chemistry and Radiological Physics from the University of Cincinnati and his Ph.D. degree in Medical Physics from the University of California Los Angeles. After working for over 15 years as a medical physicist and Associate Professor, Diagnostic Imaging, at Temple University School of Medicine, as Director, Section of Nuclear Medicine Physics and Physics Research and Development at Cooper Hospital/University Medical Center, and as Clinical Professor of Radiology at the University of Medicine and Dentistry New Jersey – Robert Wood Johnson Medical School, Dr. Siegel assumed his current position as President and CEO, Nuclear Physics Enterprises, Marlton, NJ, USA. This is an international consulting firm specializing, among other things, in evaluation of new radioactive drug therapies and clinical trial design, translational research, biokinetic modeling, quantitative nuclear medicine/radiological imaging, internal and external dosimetry, radionuclide therapy patient release, radiation protection, and FDA and NRC regulatory issues, topics on which Dr. Siegel has published extensively. He has authored more than 350 publications, including two books providing guidance for compliance with NRC regulation of nuclear medicine.

The ALARA (As Low As Reasonably Achievable) principle is based upon the assumption that low doses of radiation might be harmful and, therefore, should be minimized for medical imaging procedures. Some consider, however, that such low doses are not only harmless but might also even be beneficial, and that advocating for use of the ALARA principle in the context of medical imaging fails to recognize that the risk is hypothetical and so serves to reinforce patients' fears of radiation. This is the claim debated in this month's Point/Counterpoint.

Medical imaging achieves a diagnostic purpose and should be governed by the highest science‐based principles and policies (use of proper procedures, appropriately calibrated equipment, etc.). The LNTH is an invalidated anti‐scientific hypothesis, spawning the ALARA policy: neither errs on the side of caution. Rather, LNTH and ALARA are responsible for misguided concerns and uninformed policies promoting radiophobia that leads to actual risks far greater than the hypothetical carcinogenic risk purportedly avoided, all while ignoring imaging's benefits. Therefore, these policies have no place in managing imaging's usage. Radiophobia can no longer be ignored: medical imaging's low‐dose radiation exposure has no documented pathway to harm, while LNTH/ALARA most assuredly do.

Moreover, imaging's dual benefits remain hidden: first, the valuable diagnostic information it provides, which either strengthens confidence in suspected diagnoses or leads to more accurate diagnoses and better treatments; 7 and second, the far more likely low‐dose health benefits of reduced lifetime cancer risk and all‐cause mortality. 8

All medical procedures require the justification of medical indication, but such justification does not involve imaging's radiation levels. The problem is radiophobia , not radiation. Dose “optimization” efforts only multiply illnesses, injuries, and deaths without justification. Therefore, the ICRP‐recommended fundamental principles of radiation protection – justification and optimization – are mutually contradictory and without merit for radiological imaging.

There is nothing prudent about ALARA dosing: radiophobia's far greater actual risks arise from patients' fear‐driven imaging avoidance and physician‐recommended use of alternative procedures, such as long‐duration MRIs in children requiring anesthesia. True iatrogenic risk arises not only from such alternative procedures but also from misdiagnoses that are secondary either to patient refusal of medically indicated imaging or to nondiagnostic scans resulting from insufficient exposure. 6

Many radiologists and medical physicists grant that imaging's radiation‐associated risks are minute, and may be nonexistent, with benefits far outweighing these putative risks, yet nevertheless, advocate the “prudence” of dose “optimization” (i.e., using doses that are ALARA); but this is a radiophobia‐centered approach. For example, the goal of the Image Gently Alliance is to lower the potential risk of CT‐caused cancer in children by providing information on dose management and “optimization” (based on notional LNTH‐predicted risks) creating the false perception that some risk exists.

The LNTH and its offspring ALARA are fatally flawed, focusing only on molecular damage, while ignoring protective, organismal biological responses . DNA double‐strand break repair and other adaptive protections more than eliminate the low‐dose radiogenic damage, repairing or removing even the far greater damage from endogenous processes . 4 , 5

Medical imaging, particularly CT, is said to produce iatrogenic cancer risk from radiation exposure. Yet, credible evidence of imaging‐related low‐dose (< 100 mGy) carcinogenic risk is nonexistent; it is a hypothetical prediction derived from the demonstrably false linear no‐threshold hypothesis (LNTH). On the contrary, low‐dose radiation does not cause , but more likely helps prevent , cancer. Countless experimental and observational studies show this benefit. 1 , 2 Epidemiological studies purporting to establish low‐dose radiogenic risks fail to consider basic scientific research and employ circular reasoning, rendering their conclusions false and indefensible. 3

Against the proposition: Cynthia H. McCollough, Ph.D.

Opening Statement The fundamental principles of radiation protection in medicine require that two criteria are met.9-11The first is justification – any exposures to ionizing radiation must be justified by an anticipated medical benefit. The second is optimization – justified exposures should be applied using the lowest dose necessary to accomplish the required task. This latter principle is referred to as ALARA – As Low As Reasonably Achievable. The premise of Dr. Siegel is that this admonishment to keep doses as low as possible implicitly teaches that radiation is something dangerous, the obvious question being “why aim for low doses unless radiation is a bad thing?” To address this question, I could discuss the topic of whether or not low doses of radiation are in fact dangerous. However, this is irrelevant to the need for the ALARA principle. Large doses of ionizing radiation are a known carcinogen. The evidence for this is unassailable and, because current biological and epidemiological evidence cannot definitively prove that low doses of radiation are safe, the precautionary principle of risk management must be invoked.9-12The precautionary principle is the precept that an action should be undertaken with great care if the consequences are uncertain and potentially dangerous.4 Under the precautionary principle, it is the responsibility of a proponent (e.g., CT provider) to establish that the proposed activity (e.g., receiving a CT) will not result in significant harm (e.g., cancer induction).12 Advising people to take the lowest effective dosage of a medicine (or receive the lowest appropriate dosage of radiation) is always the right thing to do if we know that at high doses, significant harm can occur. Medical imaging providers should not stop aiming to use the lowest radiation dosage that accomplishes the diagnostic task just because Dr. Siegel is concerned about the public perception of radiation. Those who accept existing evidence that the low doses of radiation delivered by medical imaging are associated with risks too small to be definitively demonstrated, including the AAPM,13 IOMP,14 HPS,15 and BEIR VII committee,16 acknowledge that the linear nonthreshold (LNT) hypothesis is a reasonable model for radiation protection. This is absolutely not the same thing as endorsing the hypothesis that risk actually exists from low doses of radiation. Neither does it mean that patient care should ever be compromised in the name of ALARA. ALARA simply means that we should treat radiation as the carcinogen that we know it is (at higher doses) and avoid unnecessary exposures. We should absolutely not abandon such common sense.

Rebuttal: Jeffry A. Siegel, Ph.D. Dr. McCollough asserts that whether or not low‐dose radiation is dangerous is irrelevant to ALARA dosing. But that is precisely the key relevant point. She bases the possibility of low‐dose harm, even if undetectable, on the undisputable fact that high doses are harmful, and advocates erring on the side of caution. However, she ignores voluminous scientific research demonstrating that the body repairs/eliminates low‐dose radiation damage, and at the same time is stimulated to repair the much greater endogenous metabolic damage, resulting in a net benefit, through a variety of protective adaptive mechanisms. At high doses, repair is overwhelmed if not inhibited, indicating a different mechanism of action, thereby invalidating the LNTH. Without this evidence, Dr. McCollough's advocacy of ALARA would indeed, as she says, derive from common sense. Therefore, the question of low‐dose danger could not be more relevant. It is precisely the proven benefit of low‐dose radiation that renders the ALARA principle a source of radiophobia. Furthermore, her invocation of the precautionary principle one‐sidedly ignores the harms of radiophobia, including patient refusals of radiation‐associated medical imaging and numerous deaths caused by unnecessary forced relocations of mass populations in the aftermath of the Fukushima nuclear accident. She refers the reader to the ICRP and other organizations/committees that adhere to the LNTH. They all concede, with her concurrence, that low‐dose medical imaging “risks [are] too small to be definitively demonstrated.” Like Dr. McCollough, they too ignore/dismiss the mountains of evidence that the LNTH‐derived cancer risk is a fiction, and that benefit has been proven as presented in my Opening Statement. ALARA‐dosing fosters radiophobia because denials that low‐dose radiation confers a net benefit, and averrals that it confers risk, are demonstrable falsehoods that neglect the sciences of biology, chemistry, and physics that demonstrate the falsity of the LNTH and the reality of the hazards caused by any policy based on the ALARA principle.