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What are the risks of diagnostic medical radiation?

Richard C Smart

It is both ethically and economically desirable to restrict the use of diagnostic medical radiation to only those who will benefit from it. However, patients should not refuse diagnostic tests based on an exaggerated estimation of the risks because most of these tests involve low doses of radiation. It is probable that the risks derived from studies of the atomic bomb survivors, who were exposed to high doses of radiation, overestimate the risks at low doses. No evidence of thyroid cancer, leukaemia or non-Hodgkin's lymphoma has been found in patients exposed to diagnostic levels of ionising radiation. For most diagnostic tests, the risks arising from the radiation exposure are too small to be observed and the benefits will almost always outweigh the risk. (MJA 1997; 166: 589-591)

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Introduction - The effect of dose - The effects of dose rate and fractionation - The effects of age at radiation exposure - Risks of medical diagnostic tests - Risks from natural background radiation - Risks to children from parental radiation exposure - Conclusion - References - Authors' details

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Introduction

 Recently in this Journal, Roebuck suggested that, for many diagnostic radiology and nuclear medicine investigations, the risk from the ionising radiation may outweigh the benefits of these tests.1 He outlined steps which referring doctors and radiologists may consider to reduce patient irradiation, and strongly advocated better education of the medical profession as to the risks of diagnostic radiation. However, Roebuck's estimates of the number of radiation-linked fatal malignancies in Australia each year must be questioned because these data were based on the risk of radiation-associated deaths determined from studies of the survivors of the 1945 atomic bombing of Hiroshima and Nagasaki. Many were exposed to radiation doses hundreds of times greater than those encountered in medical diagnostic procedures.2 Most medical diagnostic studies result in effective doses in the range 1-20 mSv, although chest x-rays give only 0.05 mSv. There is increasing evidence that the risk associated with medical diagnostic radiation exposure is substantially less than that predicted from studies of high-dose radiation.
 

The effect of dose

 The effects of radiation at high doses are dependent on the administered dose.3 Solid tumours show a linear incidence with dose, while leukaemia (chronic and acute myelogenous, and acute lymphocytic) shows a linear-quadratic dose-response function (Figure, below).4 In both cases, the dose-response curve flattens off at high doses (> 3 Sv), probably a direct effect of cell death. This does not necessarily imply that such a relationship continues to zero dose -- the basis of the "linear no-threshold hypothesis", which purports that any dose of radiation carries a risk. An alternative hypothesis is that there is no risk up to a certain dose but that the risk increases above this threshold; another hypothesis (commonly known as "radiation hormesis") proposes that there is a reduced risk for low levels of radiation and a higher risk at higher doses.

The Hiroshima and Nagasaki survivor data showed no increase in the risk of leukaemia at low radiation doses (range, 0-200 mSv).4 For solid tumours, the lowest radiation dose at which there was a statistically significant excess risk was 50 mSv,4 a level well above that of most diagnostic radiological testing.  

The effects of dose rate and fractionation

 The atomic bomb survivors received all their radiation over a short period of time, and thus at high dose rates, while medical diagnostic radiation is given at low dose rates. How does dose rate affect the consequences of radiation exposure? Animal studies have shown a reduction in effect of between two and five for life-shortening and between one and 10 for tumour induction when the same total dose was given at a significantly lower dose rate.3

What happens if the total dose is delivered in a number of smaller doses spread over a prolonged time period (fractionated), such as would be received by a patient having a series of diagnostic x-rays? In the past, patients with tuberculosis were monitored with fluoroscopy every two weeks over a five-year period and hence received a substantial amount of radiation (average lung dose, 840 mGy). In one United States study, there was no evidence of an increased risk of lung cancer in a cohort of such patients (standardised mortality ratio, 0.8).5 However, other studies have shown an increase in breast cancer similar to that found in Hiroshima and Nagasaki survivors.6,7 The risk varied with the total dose received. For those receiving a dose of 1-2 Sv, the relative risk (RR) of breast cancer was 1.38 (95% confidence interval [95% CI], 1.07-1.77), whereas for those receiving a dose of 0.5-0.9 Sv the RR was 1.11 (95% CI, 0.86-1.43).7

The United Nations Scientific Committee on the Effects of Atomic Radiation suggested a dose and dose rate effectiveness factor (DDREF) of 2.0 for leukaemia and 1.4 for other cancers.3 The DDREF is a factor applied to the risk from the radiation observed in the high-dose atomic bomb studies to predict the risk from radiation observed in low-dose and low-dose-rate studies. The Committee stated that the DDREF should be applied if the total dose is less than 200 mGy or the dose rate is below 0.1 mGy/min, as is typically found in diagnostic x-ray studies.3  

The effects of age at radiation exposure

 The most recent report on the mortality of the atomic bomb survivors (which includes an extended period of follow-up until 1990 and which includes an additional 10 500 survivors) provides estimates of excess risk specific to sex and age at exposure.4 Those exposed at age 50 had one-third of the lifetime risk per Sv for solid cancers as those exposed at age 30. Those exposed in childhood had 1-1.8 times the estimates for those aged 30.4 The excess lifetime risk of leukaemia for those exposed at age 50 was about two-thirds of the risk if exposed at an earlier age.4

Similarly, the Canadian studies on breast cancer following extended monitoring of treatment with fluoroscopy for tuberculosis found that the excess relative risk (ERR) decreased with increasing age at exposure.7 Women exposed at age 50 or more showed no increase in breast cancer, while an ERR of 1.25 per Sv was found for those irradiated between birth and nine years.  

Risks of medical diagnostic tests

 Several recent studies have investigated the cancer risk in patients who had received diagnostic x-ray procedures. Inskip et al. identified all patients with papillary and follicular thyroid carcinoma diagnosed between 1980 and 1992 in Uppsala, Sweden.8 An equal number of control patients were matched for age, sex and country of residence. The case patients' hospital medical records were examined and the number and type of x-rays were recorded. Radiation received three to five years before diagnosis was excluded because of the known latent period for thyroid cancers. They found that each of the 484 case patients had received an average of 6.1 x-ray procedures, while the control patients received an average of 6.6 procedures; a similar number in each group received no x-rays (115 and 117, respectively). No association was found between higher doses to the thyroid and cancer, and there was no change in the relative risk with thyroid doses up to the highest dose of 80 mGy.

Iodine 131 (I 131) has long been used for the diagnosis and treatment of thyroid disorders. Hall et al. recently reported on the risk of thyroid cancer in 34 104 patients given diagnostic doses of I 131 of between 0.04-37 MBq.9 An excess of thyroid cancers was observed only among patients referred for a suspected thyroid cancer. No increase was seen among the 23 319 patients referred for other reasons.

In a 1991 United States study, all x-ray procedures were reviewed for 565 patients with all types of leukaemia, 318 patients with non-Hodgkin's lymphoma and 208 patients with multiple myeloma.10 The case patients were selected from a prepaid health plan in Oregon and California from 1956 to 1982 and were matched to 1390 control patients. The probable bone marrow dose was assigned for each x-ray and a cumulative bone marrow dose was estimated for each case patient. A latent period of four years was assumed for both leukaemia and solid tumours. A similar number of case and control patients had not had any x-rays during that time. The average number of x-rays was identical in both groups (11.6). The incidence of leukaemia in the irradiated patients was not significantly increased compared with those who had not been irradiated and there was no evidence of an increased risk with increasing dose (RR, 1.13; 95% CI, 0.7-1.8). Similarly, there were no significant increases in RRs for non-Hodgkin's lymphoma (RR, 1.24; 95% CI, 0.80-2.0) or for multiple myeloma (RR, 1.07; 95% CI, 0.6-2.0). The only group showing any possible effects of the radiation was a small group who had received the highest number of x-rays (average of 35) and who had an RR of 4.5 for multiple myeloma.

Although these studies yielded negative results, they do not imply zero risk from diagnostic medical radiation but suggest that the risk is very small. Further studies involving large numbers of patients are required to detect the low levels of risk at these low doses.  

Risks from natural background radiation

 People are continuously exposed to natural background radiation (e.g., cosmic radiation, terrestrial radiation sources such as soils and building materials, and radon gas). The level of background radiation varies substantially around the world and has provided an alternative means of assessing the risks of low doses of radiation. One study involved 80 000 individuals living in two adjacent regions in China where the levels of background radiation differed by more than a factor of two.11 The leukaemia mortality data indicated that there was no increasing risk with dose; if anything, there was a decreasing risk with dose.

The average background radiation at sea level in Australia has been estimated to be 2.1 mSv per annum.12 Therefore, a patient having a chest x-ray receives the same effective dose as he or she would receive naturally in only six days.  

Risks to children from parental radiation exposure

 The Oxford Survey of Childhood Cancers has estimated that the absolute risk of mortality from cancer following radiation exposure in utero is 1 in 20 000 per mSv.13 Data from the atomic bomb survivors indicate an increased risk of mental retardation to the fetus if the mother is exposed to radiation between eight to 25 weeks' gestation.2 During the most sensitive period (eight to 15 weeks' gestation), there may be a reduction in IQ of 0.03 units per mSv.2 There is no apparent increased risk of congenital malformation below a dose of 100 mSv.14

The possible association of childhood cancer with paternal irradiation has also recently been investigated by the Oxford Survey of Childhood Cancers.15 Using data from 14 869 children dying from cancer in the United Kingdom in the period 1953-1981 matched to an equal number of control patients, paternal irradiation before conception was found not to be a risk factor for childhood leukaemia.

Gardner et al. suggested that there was a risk of childhood cancer following irradiation of the father before conception.15 They investigated a cluster of cases of childhood leukaemia and lymphoma observed in the vicinity of the Seascale nuclear processing plant in the United Kingdom and suggested an association between these cases and external irradiation of the father, particularly in the six months before conception. There have been many attempts to reproduce this finding in France, Germany, Canada and the United States, but none of these studies found a similar association.17 Furthermore, clusters of leukaemia were observed at six potential nuclear sites in the United Kingdom and two nuclear installations that had been built but not operated.18 As no increased levels of radiation existed at these sites, it was apparent that radiation exposure was not the cause of these clusters.  

Conclusion

 It is both ethically and economically desirable to restrict the use of diagnostic radiation to only those who will benefit from it. Wherever possible, diagnostic procedures which do not use ionising radiation should be used if these alternative techniques can give the same information. However, when a radiological study is clinically indicated it is equally important that patients do not refuse such tests based on an exaggerated estimation of the risks. For example, about 2400 women die of breast cancer in Australia each year.19 Periodic mammographic screening of women over the age of 50 has been shown to reduce breast cancer mortality by 30%.20 In the most recent report of the Canadian Breast Cancer Study, Howe and McLaughlin concluded that "even a very small benefit to women from routine mammographic screening would outweigh any possible risks of radiation-induced breast cancer".7 If diagnostic radiation studies are used appropriately, with all routine steps taken to minimise patient radiation exposure,1 the benefits will almost always outweigh the risk.  

References
  1. Roebuck DJ. Ionising radiation in diagnosis: do the risks outweigh the benefits? Med J Aust 1996; 164: 743-747.
  2. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. (ICRP Publication 60.) Oxford: Pergamon Press, 1991.
  3. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Sources and effects of ionizing radiation, Annex A. Epidemiological studies of radiation carcinogenesis. 1994 Report. New York: UNSCEAR, 1994.
  4. Pierce DA, Shimizu Y, Preston DL, et al. Studies of the mortality of atomic bomb survivors. Report 12, Part 1. Cancer: 1950-1990. Radiat Res 1996; 146: 1-27.
  5. Davis FG, Boice JD Jr, Hrubec Z, et al. Cancer mortality in a radiation-exposed cohort of Massachusetts tuberculosis patients. Cancer Res 1989; 49: 6130-6136.
  6. Boice JD Jr, Preston DL, Davis FG, et al. Frequent chest x-ray fluoroscopy and breast cancer incidence among tuberculosis patients in Massachusetts. Radiat Res 1990; 125: 214-222.
  7. Howe GR, McLaughlin J. Breast cancer mortality between 1950 and 1987 after exposure to fractionated moderate-dose-rate ionizing radiation in the Canadian fluoroscopy cohort study and a comparison with breast cancer mortality in the atomic bomb survivors study. Radiat Res 1996; 145: 694-707.
  8. Inskip PD, Ekbom A, Galanti MR, et al. Medical diagnostic x-rays and thyroid cancer. J Natl Cancer Inst 1995; 87: 1613-1621.
  9. Hall P, Mattsson A, Boice JD Jr. Thyroid cancer after diagnostic administration of iodine-131. Radiat Res 1996; 145: 86-92.
  10. Boice JD Jr, Morin MM, Glass AG, et al. Diagnostic x-ray procedures and risk of leukaemia, lymphoma and multiple myeloma. JAMA 1991; 265: 1290-1294.
  11. Wei L, Zha Y, Tao Z, et al. Epidemiological investigation of radiological effects in high background radiation areas of Yangjiang, China . J Radiat Res 1990; 31: 19-136.
  12. Costello JM. Radioactivity in the environment. Radiat Prot Aust 1983; 1: 21-27.
  13. Mole RH. Childhood cancer after prenatal exposure to diagnostic x-ray examinations in Britain. Br J Cancer 1990; 62: 152-168.
  14. Mettler FA, Moseley RD. Medical effects of ionizing radiation. Grune & Stratton, 1985.
  15. Sorahan T, Lancashire RJ, Temperton DH, Heighway WP. Childhood cancer and paternal exposure to ionizing radiation: A second report from the Oxford Survey of Childhood Cancers. Am J Ind Med 1995; 28: 71-78.
  16. Gardner MJ, Snee MT, Hall AJ, et al. Results of a case-control study of leukaemia and lymphoma among young people near Sellafield nuclear plant in West Cumbria. BMJ 1990; 300: 423-429.
  17. McLaughlin JR, Clarke EA, Nishri D, et al. Childhood leukaemia in the vicinity of Canadian nuclear facilities. Cancer Causes Control 1993; 4: 51-58.
  18. Cook-Mozaffari P, Darby SC, Doll R. Cancer near potential sites of nuclear installations. Lancet 1989; 2: 1145-1147.
  19. Australian Institute of Health and Welfare. Cancer in Australia, 1989-90. Canberra: AIHW, 1996.
  20. Fletcher SW, Black W, Harris R, et al. Report of the International Workshop on Screening for Breast Cancer. J Natl Cancer Inst 1993; 85: 1644-1656.

 

Authors' details

Department of Nuclear Medicine, St George Hospital, Kogarah, NSW.
Richard C Smart, MSc, PhD, Principal Medical Physicist.

Reprints: Dr R C Smart, Department of Nuclear Medicine, St George Hospital, Kogarah, NSW 2217.
E-mail: r.smart @ unsw.edu.au

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