Martha Linet, a physician epidemiologist, served as Chief of the Radiation Epidemiology Branch of the National Cancer Institute from 2002–2014. She joined NCI in 1987, and was previously an Associate Professor in the Department of Epidemiology at the Johns Hopkins School of Public Health. Dr. Linet led the collaborative NCI-Children’s Oncology Group case-control study of extremely low-frequency residential magnetic fields and other environmental exposures and risk of childhood acute lymphoblastic leukemia, and was co-principal investigator of the multi-center NCI case-control study of cellular telephones and other risk factors for adult brain tumors. She is the long-standing Principal Investigator of the study of cancer and other radiation-related disease risks in U.S. radiologic technologists and the Co-Principal Investigator of the study of cancer risks in Chinese benzene workers. Dr. Linet currently is a member of the National Academy of Sciences Nuclear and Radiation Studies Board, the National Council on Radiation Protection and Measurements, and the Editorial Board of the American Journal of Epidemiology. During 2004-2005 Dr. Linet was President of the American College of Epidemiology. She also previously served as the NCI liaison to the Committee on Environmental Health of the American Academy of Pediatrics, the Advisory Group on Cancer and the Environment to the American Cancer Society, and the Standing Committee on Epidemiology of the International Commission on Non-Ionizing Radiation Protection. Dr. Linet authored The Leukemias: Epidemiologic Aspects, an internationally recognized text in the field. Among her honors are NIH Merit and Director’s Awards and election to the Johns Hopkins Society of Scholars.
- Assess and quantify cancer risks associated with ionizing and non-ionizing radiation
- Identify etiologies of hematopoietic and lymphoproliferative malignancies and related disorders in adults and children
- Evaluate and quantify risks of hematopoietic, lymphoproliferative and related disorders associated with occupational exposure to benzene
- Study postulated risk factors for malignant lymphoma by subtype in pooled case-control studies conducted by the Interlymph consortium
- Examine risk factors for brain cancer in adults and children
Radiation and Cancer; Etiology of Hematopoietic, Lymphoproliferative, and Primary Brain Cancers
My research focuses on assessing and quantifying cancer risks linked with occupational, residential, and medical exposures to ionizing and non-ionizing radiation. In a series of investigations, my colleagues and I have examined a wide range of postulated risk factors for hematopoietic, lymphoproliferative, and primary brain neoplasms in adults and children.
Ionizing Radiation Exposure and Cancer Risks in Radiologic Technologists
In a collaborative investigation with the University of Minnesota, my colleagues and I have investigated mortality and cancer incidence among a nationwide cohort of 146,022 U.S. radiologic technologists certified 2 or more years during 1926-1982. Using work history questionnaire data provided by 90,000 of the technologists (77% women) during 1983-89 and followed up through 1998, we found no increase in total cancer mortality risks, but a 3-fold excess of breast cancer mortality among those first employed before 1940, modest increases in mortality from the combined grouping of acute myeloid, acute lymphoid, and chronic myeloid leukemia among those first employed prior to 1950, and a 40 percent higher circulatory disease mortality risk among those who first worked before 1940 compared with first working in 1960 or later. Mortality and incidence risks according to work history were similar for breast cancer, and for the combined category of leukemias other than chronic lymphocytic leukemia. Basal cell carcinoma incidence risks were more than 2-fold increased and cutaneous melanoma was also significantly elevated, the latter based on small numbers, for those who first worked before 1940 or before 1950, respectively.
Recently, we have reconstructed doses for individual workers and are currently analyzing incidence risks of cancers and other serious diseases according to individual technologist's occupational and personal radiation doses to specific organs. My colleagues and I are also studying the role of specific molecular and selected genetic factors thought to be directly or indirectly involved in radiation carcinogenesis.
Non-ionizing Radiation, Extremely Low Frequency and Radiofrequency
To address public health concerns about risk of childhood leukemia from exposures to extremely low frequency magnetic fields (50 or 60 Hertz), we conducted a large case control study with the Children's Oncology Group (a NCI funded clinical trials cooperative group) and found that neither high measured residential magnetic field levels (except possibly magnetic field levels greater than 0.4 microtesla) nor high wire code levels (a proxy measure for close distance of residence to power lines) were associated with significantly increased risks of childhood acute lymphoblastic leukemia. Exploratory analyses evaluating various alternative magnetic field exposure metrics, did not alter our conclusions. Childhood acute lymphoblastic leukemia risks were significantly elevated in offspring whose mothers reported use of an electric blanket during pregnancy, and children with postnatal use of electric blankets, hair dryers, video machines in arcades, and video games connected to televisions. However, patterns for duration or frequency of use of these appliances were inconsistent.
My colleagues and I conducted a large hospital-based case-control study and found that risks of gliomas, meningiomas, and acoustic neuromas were not associated with cumulative use, duration or minutes per day of use of hand-held cellular telephones. In a multi-center population-based study of non-Hodgkin lymphoma, we found that risks of lymphoma were not significantly associated with minutes per week, duration, cumulative lifetime or year of first use of cellular telephones.
Using data from 12 NCI SEER population-based cancer registries, we found that age-specific incidence rates of glioma remained generally constant in 2008, a period coinciding with substantial increase in mobile phone use from 0% to almost 100% of the U.S. population. Comparing observed rates of glioma with projected rates (by combining relatives risks from the two key case-control studies forming the basis of the classification of cell phone use as a possible carcinogen by the International Agency for the search on Cancer, and adjusting for age, registry, sex, data on cell phone use, and various latency periods), we found that projected rates would need to have been 40% higher than those observed in 2008 based on results from the Swedish study by Hardell and colleagues but predicted rates were consistent with observed rates based on results from the 2010 Interphone study.
Nonionizing Radiation, Ultraviolet or Sunlight Exposures
My colleagues and I conducted a methodological study to improve the effectiveness of questionnaires in assessing individual UV or sunlight exposures. Comparing assessment of time spent outdoors with an activity-based approach we found higher reproducibility for the activity-based approach during adult ages and over subjects' lifetimes then the time-based approach particularly for women. Currently, we are assessing the extent to which questionnaire-derived data on sunlight exposure and other postulated determinants predict vitamin D levels. We are also studying historical data from NASA (total ozone mapping spectrometer) satellites on UVR ambient exposure in conjunction with questionnaire data on personal exposure to sunlight to assess comprehensively the relationship of UVR with risks for basal cell and squamous cell skin carcinogenesis with USRT.
Benzene Related Hematopoietic Disorders
A cohort study of mortality and hematopoietic malignancy incidence risks was carried out among 75,000 benzene exposed workers employed between 1949 and 1987 in 12 cities in China. We found a three fold excess risk for non Hodgkin lymphoma (NHL), with risks rising with increasing duration of benzene exposure and associated with benzene exposure more than 10 years prior to diagnosis of NHL. In contrast, a three-fold excess of acute nonlymphocytic leukemia (ANLL) was linked with recent exposures; risk did not vary according to exposure duration. Significantly elevated risks were also seen for myelodysplastic syndromes and aplastic anemia, and there were non-significant excesses of acute lymphoid leukemia and chronic myeloid leukemia, but the numbers of cases were small. Underway is an investigation to extend follow-up of mortality risks foe an additional 12 years (1972-1999).
We are currently conducting a case-cohort study to quantify benzene dose-response risk of hematopoietic malignancies, benzene hematotoxicity, and lung cancer during 1972-1999, and also conducting a molecular and genetic case-control investigation of benzene hematotoxiticity.
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