eletromagnéticos e leucemia linfocítica aguda em crianças residentes na Região Metropolitana de São Paulo [Masters Thesis
- Pelissari DM
- São Paulo: Faculdade de Saúde Pública…
Leukemia incidence in children has increased worldwide in recent decades, particularly due to the rise in acute lymphoblastic leukemia. Studies have associated exposure to non-ionizing radiation generated by low frequency magnetic fields with childhood leukemia. The current article reviews the case-control studies published on this subject. Of 152 articles tracked in different databases, ten studies from North America, Asia, and Europe met the defined selection criteria, with patients diagnosed from 1960 to 2004. Methodological limitations were observed in these articles, including difficulties with the procedures for assessing exposure. An association may exist between exposure to low frequency magnetic fields and acute lymphoblastic leukemia in children, but this association is weak, preventing the observation of consistency in the findings. Future studies from a wider range of geographic regions should focus on the analysis of acute lymphoblastic leukemia, which is the subtype with the greatest impact on the increasing overall incidence of childhood leukemia. Radiation Exposure; Lymphoid Leukemia; Child Introduction Leukemia is the most common malignant neoplasm in childhood, comprising about one-third of cancers in children under 14 years of age 1. The most common subtype (approximately 80%) is acute lymphoblastic leukemia (ALL) 2. Studies in North America 3, Asia 4, and Europe 5 indicate stability in the overall incidence of malignant neoplasms in children, but an upward trend for leukemia. Subtype analysis of leukemias shows that the increase in these rates is due mainly to the increase in ALL. In Brazil, the leukemia incidence rates (41 and 36 per 1 million, respectively, for boys and girls) show a similar phenomenon 6. Data from the Population Cancer Registry for the city of São Paulo, which has the longest historical series of cancer incidence in Brazil, show an increase in the incidence of lymphoid leukemias in children from 1969 to 2003 7. Although part of this increase may be due to improvement in recording leukemia diagnosis, this does not entirely explain the increasing incidence of ALL. One hypothesis is that the trend results from factors related to modern lifestyle 7, including intensive use of electricity. Epidemiological studies on acute childhood leukemia have examined numerous possible risk factors with the aim of determining the disease etiology. A recent review 8 only identified ionizing radiation as a risk factor for ALL, while other potential factors showed conflicting results. REVISÃO REVIEW Pelissari DM et al. S442 Cad. Saúde Pública, Rio de Janeiro, 25 Sup 3:S441-S452, 2009 In 1979, Wertheimer & Leeper 9 found a positive association between exposure to low frequency magnetic fields and leukemia and brain cancer in children younger than 19 years. Since then, the scientific literature has expanded on the possible health-related effects of exposure to magnetic fields. In addition to brain tumors and leukemia in children, the research has explored the association between exposure to low frequency magnetic fields and birth defects 10 and different types of cancer, like breast cancer in men 11, breast cancer in women 12, lung cancer 13, testis 11, and prostate 14, lymphomas, multiple myeloma, and melanoma 15. Much of the research in the last 20 years on magnetic fields and their effect on cancer has focused on childhood leukemia. In addition to dozens of case-control studies, there was one cohort study in Finland 16, two case-control studies nested in cohorts 17,18, and two meta-analyses 19,20. Based on these studies and particularly on the results of meta-analyses, the International Agency for Research on Cancer (IARC) 21 classified low frequency magnetic fields as class 2B, possibly carcinogenic to humans. Low frequency magnetic fields (fields generated by 50Hz or 60Hz electric power generation, transmission, and distribution lines) display long waves, approximately 3,500km 22. These fields cross the human body without directly depositing any energy. In the environment, nonionizing radiation generated by 50Hz or 60Hz magnetic fields displays values of 0.01 to 0.2μT (microteslas, the measurement of intensity of magnetic fields, or density of the magnetic flows), but exposures greater than 0.3μT can occur in industrial settings 21. The main problem with case-control studies in this area of research is the evaluation and quantification of exposure to low frequency magnetic fields. The difficulty in obtaining precise data, adequately expressing the exposure to given individuals, derives from the retrospective nature of case-control studies, in addition to the complexity of characterizing sources of exposure and estimating the total effect from diverse sources 21. The methodology applied to the investigation of exposure to magnetic fields relates to the original proposal by Wertheimer & Leeper 9, using the so-called “wire code” classification. The authors’ point of departure was the configuration of electric power transmission lines, considering some aspects: (a) probable load in the transmission lines; (b) thickness of the wires; (c) location of the transformers; and (d) proximity of the homes to the power lines. Homes were classified as either HCC (high current configuration) or LCC (low current configuration). Others researchers subsequently introduced some refinements in the wire code system. Thus, Barnes et al. 23 reclassified the categories as: underground (UG), very low current configuration (VLCC), ordinary low current configuration (OLCC), ordinary high current configuration (OHCC), and very high current configuration (VHCC); and Kaune & Savitz 24 as: low (LCC), medium (MCC), and high current configuration (HCC). Savitz et al. 25 quantified the magnetic fields using measurements from a dosimeter in the households’ different rooms. Coleman et al. 26 evaluated exposure to magnetic fields considering the distance to the homes and type and voltage of transmission systems to estimate the magnetic field. Feychting & Ahlbom 17 based their estimates on the distance between residences and power transmission equipment, and defined individuals living close to transmission towers as the population at risk. Such studies have generally evaluated the risk of exposure to magnetic fields for leukemias as a whole. Few studies have specifically investigated ALL 25,27,28,29,30. Only two of these studies showed statistically significant associations with ALL 27,30. The others did not find statistically significant associations with either leukemias as a whole or ALL as a subgroup 25,28,29. The aim of this article was to review the results of case-control studies that evaluated the possible association between exposure to nonionizing radiation generated by low frequency magnetic fields and incidence of ALL, the most common subtype of leukemia in children and the main subtype responsible for the increase in overall leukemia incidence in children in different regions of the world.