On April 20, 2022, the International Organization for Medical Physics (IOMP) and International Commission on Radiological Protection (ICRP) organized a webinar on interesting query: Are radiation risks below 100 mGY through recurrent CT procedures of real concern?
Before we learn the details, let us consider the following facts: Presently, every year worldwide, about a million patients might be exposed to doses of the order of 100 mGy of x rays due to recurrent application of medical imaging procedures. [ Gy is a unit of radiation dose. The dose is one Gy when the radiation energy absorbed in tissue is one Joule per kg. Since Gy is a high dose, mGy or one thousandth of a Gy is customarily used. The skin dose in a chest x-ray exam is about 0.1 mGY; CT- Colonography 6 mGy; Coronary CT Angiography 8.7 mGY ]
The webinar reviewed the recent epidemiological evidence on radiation-related cancer risks from a radiation dose of 100mGy. Eminent scientists included in their talks, the recent results of A- bomb survivors (by W. Rühm , Medical and Environmental Dosimetry Group at the Helmholtz Center Munich, Institute of Radiation Medicine, Germany.,),low dose-rate exposures during adulthood (by D. Laurier, Deputy Head of the Health Division, French Institute for Radiation Protection and Nuclear Safety -IRSN) , and in utero and childhood exposures. (by R. Wakeford Honorary Professor in Epidemiology in the Centre for Occupational and Environmental Health at The University of Manchester, UK)
Overall, the reviewers found substantial evidence that ionizing radiation causes cancer at acute and protracted doses above 100 mGy, and growing evidence for doses below 100 mGy. Doses of the order of 100 mGy from recurrent application of medical imaging procedures involving ionizing radiation are of concern, from the viewpoint of radiological protection.
The take -home lesson is that such examination should be carried out only when there is clinical need for it. The physician must weigh the benefit versus risk carefully before carrying it out.
The webinar reminded me of my early days in Atomic Energy Establishment Trombay (AEET). I knew most of the references discussed in the webinar, particularly the paper in The Lancet by Alice Stewart et al (1956),which suggested the possibility of leukemia in newborns if they were exposed to diagnostic x-rays while they are in their mother’s womb.
I had an argument with probably the senior most Gynaecologist in our panel of doctors In 1974, she recommended a pelvimetry exam a few days before my wife delivered.. Pelvimetry assesses the size of a woman’s pelvis aiming to predict whether she will be able to give birth vaginally or not. The physician does it either by clinical examination, or by conventional X‐rays. In 1956, the year in which The Lancet paper was published, frequency of pelvimetry exam came down by 25%
When I expressed my concern, the doctor chided me saying that my concern was because I am working in the Department of Atomic Energy, I explained to her all about pelvimetry, radiation doses, risk of leukaemia etc. I was not amused or surprised; she secured her FRCS from the UK in 1956, the year in which Dr. Alice Stewart working in Oxford University published her seminal paper in The Lancet. stating for the first time the potential risk of leukaemia due to medical x-ray procedures. Later, she led a whole project on the topic: Oxford study on in utero x-ray exposure.
Sadly, Dr. Alice Stewart became an activist. The fall from grace was shocking. In 2000, Dr Alan Brodsky, an eminent US scientist told me that he regretted recommending her name to carry out the Hanford workers study in USA, It became very controversial.
Interview with Dr Roger Clarke
While on the topic of the webinar I could not resist including a part of the interview I had in November 1998 with Dr.Roger Clarke, the then Chairman, ICRP. The full interview was published in AERB Newsletter in December 1998. Some of the issues discussed then remains unresolved even today.
History of radiation protection
KSP: Historically, the International Commission on Radiological Protection (ICRP) was setup to make recommendations on the safe use of ionizing radiation in medicine. However, it took a few decades for ICRP to bring out comprehensive recommendations on the protection of patients in diagnostic radiology. What were the reasons for this? Was it because ICRP did not want any control on medical radiation practice?
RC: In the early years, protection specialists gave more emphasis on therapeutic treatment . The forerunner of the ICRP originated from the recommendations of the British X-ray and Radium Protection Committee. Then the major concern was protection of the workers. This was because deterministic effects such as extensive skin damage were seen among the x-ray workers who handled x-ray units. After the Second World War, radiations that are more penetrating came to be used in medicine. Artificial radioisotopes appeared on the scene. More and more public were exposed. The emphasis shifted to public exposure. It coincided with more developments in fifties the emphasis was on therapy. Later more and more diagnostic technologies were developed.
RC: Radiation generators emitting radiation that is more penetrating began to appear in medical practice. Leukaemia was identified among medical practitioners. The work by an American Physician Mr. Shields Warren is notable. He reported that Leukaemia among radiologists was higher than that among general medical practitioners. It was obvious that ICRP, in this background, started giving more emphasis to patient protection. I would like to mention one recent development. deterministic effects are coming out into focus now. Some of the interventional procedures, if carried out without care, can give substantial doses to patients.
KSP: What are the new concerns?
RC: The concern of the radiologists shifted to finding out what is a better image. Future is in imaging, in medical imaging. For instance, digital imaging in interventional radiology, electronic manipulation of images, fluoroscopy with computer software. These technologies are likely to appear. More and more computers will start controlling x-ray imaging. Surgeons will turn round and expect computers to control the imaging procedure. With the newer techniques being used, a dose reduction of a factor of about 10 is possible to the patient. The workers are also benefited by the dose reduction.
Arguments on Linear-No Threshold hypothesis
KSP: Do you agree that the scholarly discussion on the Linear-No Threshold (L-NT) hypothesis has contributed to the notion that there is no safe level of radiation. Has it not sensitized the large public to greater and unreasonable levels?
RC: I agree. When experts disagree, the credibility of specialists suffers. If experts do not agree, how can people decide which side of the argument is believable? I cannot deny that the arguments on L-NT have created some difficulties. The situation could be bad because there is an increasing possibility that decisions in science may be made by judges and juries in court rooms and not by professional association or by Royal Societies. The judiciary system may not be able to convince itself about the increased possibilities of radiation effects.
KSP: Don’t you think that it is futile to try to get a deterministic answer to a purely probabilistic question?
RC: Yes. However, I do not understand why some people wanted to establish that there is a threshold dose below which there will not be any radiation effect. One of the major difficulties is in tackling the problem of old contaminated sites. Small radiation doses due to residual radioactivities left behind at certain sites can cause very tiny amount of radiation doses. However, when these doses are integrated over several thousand years. One may end up with getting significant amount of doses. We will be left with the estimates of a few hundreds of probable deaths due to these collective doses accumulated over a long period of time. However, I believe that we must have started dialogue on acceptable risks.
Complexities of radiation protection
RC: I am worried that the philosophy of protection has become somewhat complex.
KSP: Even for the professional …
RC: You said that and I agree. We must develop simpler concepts. ICRP must start consultations with other groups and collect ideas for reviewing and consolidating the system of protection. We have started to do that already. It explains why some of our recent documents are better than earlier ones. Consultation with others will help to improve the documents. The document on radon is an instance in point.
KSP: Everyone was keen on the on-going L-NT controversy. While the ICRP and the NRPB supported the argument that there is no threshold for the effects of ionizing radiation, the US Health Physics Society was unconvinced. The NRPB bulletin went to town with the suggestion that the attitude of the Health Physics Society is in tune with the liberalized attitude of US Administration to nuclear power. Can you comment on this development?
RC: Yes, certainly different professional groups looked at this issue very differently. American Health Physics Society has its own stated view. I have been to Health Physicists Society. Many people were interested in the controversy. The arguments put forth by the Health Physics Society are outdated with respect to the recent findings on the Japanese survivors of the atomic weapons. They did not then have the occasion to see the data. The recent data indicated that there could be significant risk at doses as low as 50 mSv, of course with much uncertainty. I do not still understand why they are looking for a threshold. There are many unknown cellular phenomena to be understood. Genomic instability, for instance
RC Unquestionable proponent of LNT concept
KSP: You have become an unquestionable proponent of the linear no threshold theory. Certainly, you did not ask for such a position. Do you really think that this controversy went uncontrolled?
RC: Yes. In my view, there is no need to search for a threshold. Nobody denies that there is evidence for the repair of cellular damage. However, we cannot ignore that the repair mechanisms are also statistical in nature.
KSP: Scientists have been studying effects of radiation for the past 100 years. The stochastic effects such as cancer would not have even been thought about, but for the long and expensive epidemiological studies. Is it not unfair to spend too much of resources, in fact, vast sums of money to carry out studies about an agent which is now known to be much less hazardous than hundreds of toxic chemicals about which practically nothing is known.
RC: I may say that physicists should take the blame for it. The study of nuclear physics progressed rapidly. Some of the best brains entered the profession. The study of physics was intellectually satisfying and scientifically stimulating. Unfortunately, the same was not true for chemicals. Of late, biologists are also starting to use more and more mathematical formulations. Probably natural sciences are getting ready to make quantitative estimate.
Societal impact of ICRP recommendations
KSP: The recommendations of ICRP are scientifically the best available. However, you will agree with me that these recommendations have enormous social impact. Is it justifiable for over a dozen specialists in purely scientific disciplines to take such decisions, which have enormous social impact? Don’t you think that the representation in ICRP should be broadened to include social scientists and economists?
RC: There are various components to this question. ICRP recommendations reflect the best scientific information. We do not say what is acceptable to society or not. There is one recent development. ICRP is currently engaged in more and more consultations with specialists by providing drafts of their recommendations to other specialists and concerned people for review. It would certainly reveal whether there is any inconsistency in the concept and approach put forward by ICRP. It will help to find out whether there is any fallacy in our approach. I believe the recent ICRP document bear testimony to this.
We need not worry over the fact that some of the issues in radiation protection have not been resolved so far. We know enough about the effects of radiation that we can use it safely in many fields