Carcinogenic risk in diagnostic nuclear medicine: biological and epidemiological considerations

An update on radiation absorbed dose to patients from diagnostic nuclear medicine procedures in Tehran: A study on four academic centers

Purpose:

Use of radiopharmaceuticals for diagnostic nuclear medicine procedures is one of the main sources of radiation exposure. We performed this study with respect to the rapid growth in nuclear medicine in Iran and lack of updated statistics.

Materials and Methods:

The data were obtained for all active Nuclear Medicine Centers affiliated to Shahid Beheshti University of Medical Sciences during 2009 and 2010.

Results:

The most frequently performed procedures were bone (30.16%), cardiac (28.96%), renal (17.97%), and thyroid (7.93%) scans. There was a significant decrease in the number of thyroid scintigraphies with ¹³¹I and ^99mTc-sulfur colloid liver/spleen scans and tremendous increase in the frequencies of cardiac and bone scintigraphies compared to one decade ago.

Conclusion:

Compared to previous studies, there were striking changes in trends of diagnostic nuclear medicine procedures in Tehran. This field is still evolving in the country, and this trend will further change with the introduction of positron emission tomography scanners in future.

Correlation of 18F-FDG PET/CT assessments with disease activity and markers of inflammation in patients with early rheumatoid arthritis following the initiation of combination therapy with triple oral antirheumatic drugs

PURPOSE

This study evaluated the potential of functional imaging to monitor disease activity and response to treatment with disease-modifying antirheumatic drugs (DMARD) in DMARD-naive patients with early rheumatoid arthritis (RA).

METHODS

The study involved 17 patients with active RA in whom combination therapy was initiated with methotrexate, sulfasalazine, hydroxychloroquine, and low-dose oral prednisolone. Clinical disease activity was assessed at screening, at baseline and after 2, 4, 8 and 12 weeks of therapy. (18)F-FDG PET/CT of all joints was performed at baseline and after 2 and 4 weeks of therapy.

RESULTS

(18)F-FDG maximum standardized uptake values showed a reduction of 22 ± 13 % in 76 % of patients from baseline to week 2 and a reduction of 29 ± 13 % in 81 % of patients from baseline to week 4. The percentage decrease in (18)F-FDG uptake from baseline to week 2 correlated with clinical outcome, as measured by the disease activity score (DAS-28) at week 12. In addition, changes in C-reactive protein levels and erythrocyte sedimentation rate were positively associated with changes shown by PET.

CONCLUSION

(18)F-FDG PET/CT findings after 2 and 4 weeks of triple combination oral DMARD therapy correlated with treatment efficacy and clinical outcome in patients with early RA. (18)F-FDG PET/CT may help predict the therapeutic response to novel drug treatments.

Hormesis, an update of the present position

The ongoing debate over the possible beneficial effects of ionising radiation on health, hormesis, is reviewed from different perspectives. Radiation hormesis has not been strictly defined in the scientific literature. It can be understood as a decrease in the risk of cancer due to low-dose irradiation, but other positive health effects may also be encompassed by the concept. The overwhelming majority of the currently available epidemiological data on populations exposed to ionising radiation support the assumption that there is a linear non-threshold dose-response relationship. However, epidemiological data fail to demonstrate detrimental effects of ionising radiation at absorbed doses smaller than 100-200 mSv. Risk estimates for these levels are therefore based on extrapolations from higher doses. Arguments for hormesis are derived only from a number of epidemiological studies, but also from studies in radiation biology. Radiobiological evidence for hormesis is based on radio-adaptive response; this has been convincingly demonstrated in vitro, but some questions remain as to how it affects humans. Furthermore, there is an ecologically based argument for hormesis in that, given the evolutionary prerequisite of best fitness, it follows that humans are best adapted to background levels of ionising radiation and other carcinogenic agents in our environment. A few animal studies have also addressed the hormesis theory, some of which have supported it while others have not. To complete the picture, the results of new radiobiological research indicate the need for a paradigm shift concerning the mechanisms of cancer induction. Such research is a step towards a better understanding of how ionising radiation affects the living cell and the organism, and thus towards a more reliable judgement on how to interpret the present radiobiological evidence for hormesis.

Stress echocardiography: a historical perspective

Stress echocardiography is a popular cardiac imaging technique that provides similar diagnostic and prognostic accuracy as radionuclide stress perfusion imaging. Similar accuracy, however, should not be synonymous with being "clinically interchangeable." Because of economic cost, environmental impact, and individual biohazard exposure, a nuclear examination should be performed only when it cannot be replaced by other techniques that do not employ ionizing radiation. Nuclear medical imaging procedures, of which cardiological procedures are an important part, account for 4% of the annual effective radiation dose received by the average person in the United States. At the individual level, the radiological exposure of a single nuclear cardiology procedure conveys a low, but measurable, risk of fatal cancer, which varies from 1 in 1000 to 1 in 10,000, depending on the type of examination (higher for thallium scans; lower for technetium 99m methoxyisobutylisonitrile scans). Because the information provided by stress echocardiography and stress radionuclide perfusion imaging are similar, the choice of test should be made in the context of the environmental, biological, and economic effects of that choice.

Do physicians correctly estimate radiation risks from medical imaging?

Proper use of medical imaging tools requires knowledge of their associated radiation risks, as well as their possible benefits. The authors assessed physicians' knowledge of the radiation risks associated with bone scintigraphy (bone scan) during an annual meeting of the Israeli Orthopedic Society. The mortality risk of radiation-induced carcinoma from bone scan was identified correctly by less than 5% of respondents. The most frequent answer (38.4%) was the option that was least correct. Senior orthopedists estimated lower risks than did residents. Overall, respondents grossly underestimated the potential radiation risk from bone scan.

Somatic effects in nuclear medicine and radiology

Probable risks for the occurrence of somatic effects due to diagnostic radiology and nuclear medicine are summarised. The biological background of radiation carcinogenesis and epidemiological results are discussed. At the Leiden University Medical Centre the average effective dose per examination due to diagnostic radiology and nuclear medicine amount to 0.95 and 4.4 mSv, respectively. These values correspond well with the average values of 0.82 and 3.0 mSv reported for The Netherlands as a whole. Since radiological examinations are performed at a much larger frequency than nuclear medicine the relative collective dose for the first type of examinations is higher than the latter. Risk for occurency of malignancies are at least one order of magnitude lower than the hypothetical risk due to the background radiation typical of The Netherlands.

Risk assessment of the nuclear medicine patient

Two types of risk are identified following the administration of a radiopharmaceutical to a patient: the risk to the patient, and the risk to critical groups exposed to the patient. The method for quantifying the risk to the patient is described in terms of estimating the effective dose. The main limitations in these estimates for adult and paediatric patients are uncertainties in the biokinetic data, and the assumption of a uniform distribution of activity in each organ. Effective doses from most nuclear medicine procedures will not exceed twice the annual dose from natural background radiation in the UK. Lack of human placental transfer data is now the main limitation to estimating fetal doses. The characteristics of two methods which can be used to derive the dose to critical groups exposed to nuclear medicine patients are reviewed. It is shown that studies using either method have indicated that the current recommendations in the UK for restricting the exposure of these groups and the recommendation recently proposed for restricting the exposure of pregnant members of staff are not appropriate. Revised recommendations for restricting the behaviour of patients administered iodine-131 should await the results of a current multicentre trial. The method to estimate the dose to a breast-fed infant from a mother administered a radiopharmaceutical is outlined, and the recently revised guidance for interrupting breast feeding is summarised. When a recommendation for controlling risk is to be derived from dosimetry data obtained from a number of individuals, an outstanding issue to be resolved is the value (e.g. 95% upper confidence limit) on which it should be based.