Although ionizing radiation is a well known cause of leukemia, most papers alluding to the topic exclude chronic lymphocytic leukemia (CLL) from consideration since it is believed to be well-established that CLL is not caused by radiation . Just how secure is this assumption?
The primary data come first from studies of atom bomb survivors at Hiroshima and Nagasaki [2-4]. Only ten cases of CLL were identified in survivors between 1945 and 1980, and of those ten, seven turned out to be acute T-cell leukemia/lymphoma (ATLL) when examined more closely. ATLL is endemic in the area around Nagasaki being associated with infection with the human T-cell lymphotropic virus type I (HTLV-1). These findings point out two of the hazards in interpreting the epidemiological data: CLL is an extremely rare diagnosis among those who were bombed (and indeed amongst the Japanese in general), and the definition of CLL has changed down the years. Many who would have been diagnosed as CLL in the past are now recognized as having different sorts of lymphoid malignancies, and since, even as recently as 1975, the diagnosis of CLL required a lymphocyte count of over 15 x 109/L , many low count cases would have gone unrecognized.
The second primary sources of data come from studies of patients treated with ionizing radiation for benign conditions. The initial study  of 14,000 British patients with ankylosing spondylitis treated with radiotherapy between 1935 and 1954 with an average bone marrow dose of 4,400 mSv showed an excess of acute leukemia and chronic myeloid leukemia in the first five years post-irradiation, but no excess of CLL. It is believed that CLL have a very long latent period, making a follow-up of less than two decades unacceptable, but later reports of this study in 1994 and 1995 [7, 8] found only seven deaths attributable to CLL, and while this was greater than expected, it was not significantly so. Similarly, a study of 12,955 women irradiated for benign gynecological disorders found no significant excess of CLL-related deaths .
These studies illustrate other difficulties in the epidemiological data. CLL is seldom recorded as a cause of death in patients who have been diagnosed with it. For many patients, especially those with mutated IgVH genes, it is a very trivial condition that never causes ill health. The traditional methods by which epidemiologists acquire cases – from death certificates and hospital admissions – are ineffective in CLL; many patients never require hospital admission and they die from causes unrelated to their CLL. In one series 75% of cases were diagnosed because they had a blood test for a different condition; presumably there are undiagnosed cases in the community who have not had blood tests.
A recent analysis of the Surveillance Epidemiology and End Results (SEER) database puts the annual incidence in the USA at 3.5 per 100,000 (males 5.0: females 2.5) . However, the Leukaemia Research Fund Data Collection Study which gathered data from individual hematologists responsible for laboratories covering about one third of the population of England and Wales (rather than from death certificates and hospital admissions) found a higher incidence in the U.K. of 6.15 per 100,000, and even this concealed a variation between 1.3 and 13.7 per 100,000 in different health districts, largely dependent on how interested the local hematologist was in the disease. .
Although the US Energy Employees Occupational Illness Compensation Program refuses to acknowledge any claims that CLL is radiation induced , this establishment view has recently been challenged. Although they present no new data, Richardson et al  argue that the data are insufficiently compelling to make this assumption and that the molecular lesions in CLL are sufficiently similar to those in other leukemias, that the presumption should be that radiation can cause CLL unless there are convincing data to the contrary. They argue that the epidemiological studies are simply too weak to carry that burden. On the other hand it should be recognized that one of the co-authors of this paper is the Executive Secretary of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization and another is the recipient of the 2003 Nuclear-Free Future lifetime achievement award.
Relatively new data are available form a study of Czech uranium minors. This study intended not to miss cases by looking at incidence rather than mortality, and ensuring that subjects had annual blood tests. An earlier abstract  from this group had claimed on the basis of 41 cases that the incidence among miners employed for at least one year underground was significantly greater than the general Czech polulation. But, then, the general Czech population does not have a blood test every year. This publication reports a higher incidence of CLL among those exposed to greatest quantity of radon. Unfortunately, the same study did not show a significant increase of myeloid leukemias asociated with a higher exposure to radon, which would have been more likely according to previous experience, nor a significant increase in cases of non-Hodkin’s lymphoma which would have fitted better with the hypothesis of Richardson et al .
In an upcoming issue of Leukemia Research Abramenko et al  report the first study of CLL in individuals exposed to radionuclides following the accident at the Chernobyl nuclear power plant. Previous published abstracts [17,18] have suggested CLL in Chernobyl clean-up workers occrred in younger patients and presented with more advanced symptoms, pursued a more aggressive course and was more resistant to standard agents than in patients not exposed to ionizing radiation. In this study 47 patients with CLL following exposure were compared with 141 patients without a history of exposure. The patients were examined more comprehensively than before and included the mutational status of IgVH genes among their investigations. Among the irradiated group 77.6% had unmutated IgVH genes, but surprisingly this was not significantly different from the non-irradiated group where the figure was 68.3%. Both were much higher than the figure of about 40% reported in most Western countries , and is probably explained by less assiduous screening in the control population and therefore the ommission of the more benign (and mutated) cases. In a sub-group among the clean-up workers who received the greatest dose of irradiation, all bar used germline IgVH genes and especially used the V1-69 and V3-21 genes that are associated with stereotypic B-cell receptors and poorer prognosis.
There was a significantly higher risk of second cancers and Richter’s syndrome among the irradiated group (as might be expected) and especially among those who received the highest dose of irradiation.
What do these new data amount to? They certainly do not establish that CLL may be caused or even made worse by ionizing radiation. On the other hand there is enough suspicion for the case to be sub-judice. Irradiation has been given a clean bill of health with respect to CLL with less than adequate evidence.
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