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Park Lane Clinic Parktown 2193, Johannesburg, South Africa
Are radiologists and other radiation workers at a higher risk for developing myelodysplastic syndrome?
United Nations Scientific Committee on the Effects of Atomic Radiation New Mexico Federal Regional Medical Center Albuquerque, NM 87108
The terms "myelodysplasia" and "myelodysplastic syndrome" refer to clonal hematologic disorders characterized by ineffective hematopoiesis. In approximately 30-40% of cases there is subsequent development of acute myelogenous leukemia (AML). Some authors think that myelodysplastic syndrome is one step in the multistep process leading to AML, whereas others think myelodysplastic syndrome is a distinct clinical entity and should not always be considered early AML. The reader is referred to a review article by Heany and Gold [1] and a recent article by Finch [2] that address radiation causation specifically.
A number of Internet and literature sources suggest possible etiologic factors for myelodysplastic syndrome including benzene, hair dye, pesticides, organic solvents, petroleum products, chemotherapeutic agents, and ionizing radiation. Unfortunately, most of these secondary literature sources rarely provide definitive references that have statistical significance. In addition, the criteria for the diagnosis of myelodysplastic syndrome keep evolving and the definitions used are somewhat imprecise. As a result, in the older literature about the effects of radiation, it is not clear whether the terms "aplastic anemia," "pernicious anemia," and "preleukemic changes" actually refer to myelodysplastic syndrome or to some other condition.
In the more recent literature, follow-up of persons exposed to thorium dioxide (Thorotrast) did not show an increase in myelodysplastic syndrome, but an increase in clonal chromosomal aberrations was seen. In the atomic bomb survivors, there was a clear increase in non-chronic lymphocytic leukemia, but only two of 50 persons who developed AML (and for whom adequate preleukemia data were present) appear to have had a preleukemic phase with myelodysplastic syndrome. In a review of the pathologic slides of 190 atomic bomb survivors who were thought to have developed AML, 11 cases were reclassified as myelodysplastic syndrome [2].
Several recent articles discuss the long-term health effects resulting from occupational exposure of radiologists and radiologic technologists. A 100-year study of British radiologists does not specifically address myelodysplastic syndrome, but it does address total mortality, cancer, and leukemia. The study found that the standardized mortality ratio (SMR) of radiologists employed after 1920 was similar to that of other medical practitioners (SMR, 1.04; 95% confidence interval, 0.89-1.21) and that cancer mortality rates were not elevated for radiologists employed after 1954. Leukemia was elevated in radiologists employed before 1954, but was not statistically significantly increased after 1954 when radiation protection was presumably better [3]. A study of more than 146,000 radiologic technologists employed after 1940 reported on deaths due to both leukemia as well as "diseases of blood and blood-forming organs" (ICD-8 [International Classification of Diseases, 8th revision] codes 280-289 [4]), which presumably includes myelodysplastic syndrome. In neither case was there a statistically significant elevation of mortality [5].
In the past four decades, probably as a result of radiation protection, there is no evidence that the incidence of leukemia, particularly AML, is statistically significantly increased in radiologists. There is also no evidence that myelodysplastic syndrome is increased in radiologists. The risk of myelodysplastic syndrome as a result of radiation exposure, if present, appears to be quite low in spite of the perception that one may get from secondary literature. The rarity of the syndrome and the difficulty in making the diagnosis leave the question of whether radiation can induce myelodysplastic syndrome at all as a matter of conjecture.
Myelodysplastic syndrome should be included in epidemiologic radiation studies of large populations. Studies of the Chernobyl-exposed populations are currently assessing the potential relationship, but even these studies may not be able to answer the question because of a lack of statistical power.
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