U.S. Radiologic Technologists Cohort (USRT)
In order to study cancer and other serious disease risks associated with protracted low-to-moderate dose radiation exposures, scientists identified a nationwide population of 146,022 radiologic technologists, certified for two or more years during 1926-1982, from the American Registry of Radiologic Technologists (ARRT). The technologists have been followed from 1983 to the present to obtain data on employment history and cancer risk factor information for investigations of mortality, cancer incidence, cataract, and cardiovascular disease risks as well as health risks among those conducting certain types of radiologic procedures.
Efforts are underway to estimate annual and cumulative doses of individual technologists to enable assessment of occupational radiation-related dose-response patterns. The potential modification of breast and thyroid cancer risks by genetic susceptibility factors has also been investigated. Genetic studies of breast, thyroid and skin cancer are ongoing.
Summary of Cancer Risks
Leukemia, multiple myeloma, and lymphoma
Relative risks of the combined category of acute myelocytic, acute lymphocytic, and chronic myeloid leukemias were increased among technologists working five or more years before 1950 or holding patients 50 or more times for x-ray examinations, but risks were not significantly related to working in more recent time periods, the year or age technologists first worked, the total years worked, specific procedures or equipment used or personal radiotherapy. Working as a radiologic technologist was not significantly associated with risk of multiple myeloma, non-Hodgkin lymphoma, Hodgkin lymphoma, or chronic lymphocytic leukemia.
Based on 1,050 incident breast cancers, risks were 2.9-fold increased among technologists who began working before 1935. Risks rose with number of years worked before 1940, and were significantly elevated among those who began working before age 17, but were not related to the total years worked in the 1940s or later. The risk was 1.5-fold and significantly greater for those technologists with the highest proxy index of cumulative occupational radiation exposure compared to those with a minimal exposure index.
A statistically significant 1.5-fold elevated risk of thyroid cancer was observed among technologists who held patients for x-ray procedures at least 50 times. Total years worked as a radiologic technologist, years performing diagnostic, therapeutic, and nuclear medicine procedures, first employment at ages younger than 20 years, and calendar period of first employment were not associated with thyroid cancer risk. Risk of thyroid cancers diagnosed before completion of the baseline questionnaire (1983-89) were higher among those who began working in earlier decades and significantly elevated among those working more than five years prior to 1950.
Non-melanoma skin cancer
Among 1,355 incident cases of basal cell carcinoma (BCC) and 270 with squamous cell carcinoma (SCC) in 65,304 white radiologic technologists, relative risk of BCC was 2.2-fold increased among those who first worked before 1940, but declined with more recent first year of employment. Ultraviolet radiation exposure did not modify the effect of year first worked on BCC risk, although risks were significantly stronger among technologists with lighter compared with darker eye and hair color. This study provides some evidence that chronic occupational radiation exposure at low-to-moderate levels can increase the risk of BCC, and that this risk may be modified by pigment characteristics.
Melanoma risk was increased among those who first worked before 1950, particularly among those who worked five or more years, but this finding was based on small numbers of technologists. Risk was also modestly elevated among technologists who did not customarily use a lead apron or shield when they first began working.
Lifestyle Risk Factors
The U.S. Radiologic Technologists Study (USRT) Cohort has been particularly useful in examining issues related to the growing prevalence of obesity, including the nature of the relationship between body mass index and all-cause mortality. In addition, investigators are studying the risk of cancer associated with physical activity, alcohol consumption, and smoking.
Technologists Performing or Assisting with Interventional Radiographic Procedures
Among technologists who completed a questionnaire during 1994-1998 and were followed up through 2003, there were 3,581 deaths. Compared to radiologic technologists who never or rarely performed or assisted with fluoroscopically-guided interventional procedures, mortality was not increased overall, nor from total circulatory disease, total cancer or breast cancer among those working with such procedures daily. Based on small numbers of deaths, there were non-significant excesses (40-70 percent) in mortality from cerebrovascular disease among technologists ever working with these procedures, but no significant increases in technologists performing the highest frequency of such procedures. The findings should be interpreted cautiously in light of the small numbers of deaths, inability to assess risks for less common cancers, and relatively short follow-up. In addition, several genetic studies are under way in this cohort.