Study Design

Overview

The USRT Study is an historical cohort study. The cohort is comprised of 146,022 radiologic technologists who were certified for at least two years by the American Registry of Radiologic Technologists (ARRT) from registry inception in the mid-1920s through 1982, and who resided in any of the U.S. states, commonwealths, territories, or the District of Columbia at enrollment (in 1982). Active cohort follow-up is conducted through annual renewals with the ARRT. Inactive registrants are linked with national address and death record databases, such as National Change of Address, Social Security Administration, and National Death Index, to identify address changes and new deaths.

More than 110,000 technologists completed at least one of four comprehensive questionnaire surveys and approximately 22,000 (15%) of the original target population are deceased.

As expected, an assessment of mortality outcomes in the full cohort of radiologic technologists demonstrated that these workers are significantly healthier than individuals in the general population. Follow-on mortality studies, plus studies of incident cancers and other diseases, have focused on evaluating cancer and other serious disease risks according to cumulative occupational estimated radiation dose.

Blood or buccal cell samples have been collected from about 2,500 case and 8,600 comparison subjects for nested case-control genetic studies of primarily breast and thyroid cancers. Studies have evaluated main effects of genetic variation, as well as the interaction between genetic variation and radiation exposure, on cancer risks.

Cohort Description and Participation

When the U.S. Radiologic Technologists (USRT) Study began in 1982, we enrolled into the study 143,517 individuals who were certified by the American Registry of Radiologic Technologists (ARRT) for at least two years between when the registry started in the mid-1920s through 1982. The vast majority were certified in radiology only (92%); small percentages were certified in nuclear medicine (<3%), radiation therapy (<1%), or more than one specialty (5%). Most were female (73%), born during the 1940s (32%) or 1950s (41%), and certified during the 1960s (28%) or 1970s (48%). Four cohort questionnaire surveys were conducted since 1983. Between the First and Second Surveys, an additional 2,505 eligible technologists were identified and added to the cohort, bringing the total number to 146,022.

The First Survey was sent during 1983-1989 to all registrants who were living and located at the time (n=133,298). There were 90,305 (68%) technologists who completed the full-length questionnaire, providing data on work history (employers, type of facility, years began and ended; procedures performed, start year, number of years; apron and shield use), medical outcomes (cancers, thyroid conditions, myocardial infarction), selected cancer risk factors (education, cigarette smoking, height, weight, personal diagnostic and therapeutic radiation procedures, gynecologic and reproductive factors, family history of breast cancer), and cancer and birth defects in offspring. An additional 14,028 (11%) technologists completed an abbreviated telephone survey that inquired about medical outcomes only.

The Second Survey was undertaken during 1994-1998 to identify incident cancers diagnosed since the First Survey. Among 126,628 living and located subjects who were mailed a questionnaire, 90,972 (72% overall; 83% of first survey responders) participated, providing updated and more detailed information on employment history (years trained and worked, frequency performed various procedures, apron and shield use, and holding patients by calendar period), cancer and other medical outcomes, and cancer risk factors (religion, height, weight, gynecologic and reproductive factors, breast biopsies, personal diagnostic and therapeutic radiation procedures by calendar period, census of children and cancers and birth defects in offspring, census of siblings and first-degree family cancer outcomes, multivitamins and selected medications, abbreviated dietary questionnaire, and physical activity).

The Third Survey was administered during 2003-2005 to 101,694 living and located technologists who completed at least one of the first two surveys. A total of 73,625 (72%) responded to one of three questionnaire formats, providing updated information on medical outcomes and additional detailed work history information. Cohort members who began working before 1950 were interviewed by telephone in 2003. A total of 2,744 out of 3,441 of the early workers (80%; 3% of the overall cohort) provided information on medical history (cancer and many non-cancer outcomes), job-specific work history (i.e. type of facility, frequency performed various procedures (diagnostic x-rays, fluoroscopy), lead apron and shield use, and other work practices), risk factors (cigarette smoking, weight by age, weight gain by anatomic site, intensity of physical activity by age, sun exposure and sunburns by age), and residential history for use in estimating ambient UVR exposure. In 2004, the questionnaire was mailed to 98,253 technologists who began working in 1950 or later; 58,628 (60%; 58% of the overall cohort) responded. The mail questionnaire included the same questions as the telephone interview, except that employment information was queried for the longest job in each decade rather than for each individual job. To increase overall response to the Third Survey, an abbreviated (12-page) version of the questionnaire that included fewer non-cancer outcomes and non-occupational risk factors was sent with a $1.00 bill to 39,625 persistent non-responders, of whom 12,252 (31%; 12% of the overall cohort) responded.

The Fourth Survey was conducted during 2012-2014 and involved four separate questionnaire modules. During 2012-2013, a 20-page general questionnaire was mailed to 93,787 living and located technologists who completed at least one of the first two questionnaires; 51,547 (55%) participated, providing updated information on incident cancers and other serious diseases, general work history factors, personal medical radiation exposures, and known and suspected cancer and other disease risk factors for use in improving estimates of occupational and personal medical doses and evaluating disease risks. In 2014, an abbreviated version of the general questionnaire that asked about fewer outcomes and risk factors was completed by 7,040 of 40,481 (17%) persistent non-responders, bringing the overall response to the general questionnaire to 58,587 (62%). Detailed work history modules were administered to subgroups of technologists who reported working with higher energy nuclear medicine procedures (NM module) and higher dose fluoroscopically-guided procedures (FG module). These modules captured calendar-specific data on specific types and frequencies of diagnostic and therapeutic procedures performed, related work practices (e.g. preparing the radiopharmaceutical, eluting the generator), and radiation safety measures employed (e.g. apron use, shield use, distance from patient) for use in improving occupational radiation dose estimates for individual technologists. The NM module was completed by 6,222 of 9,421 (66%) technologists who worked with nuclear medicine procedures and the FG module was completed by 14,647 of 21,748 (67%) technologists who worked with fluoroscopically-guided procedures.

Occupational radiation dose estimation

Estimates of absorbed dose from occupational radiation exposure to each organ for each technologist were derived for each year worked from 1916-1997 by converting badge dose measurements or estimates to the organ dose for each technologist. Before 1960, when individual badge dose data were not available, badge doses were estimated from historical period-specific distributions of badge dose measurements derived from the literature. From 1960-1997, individual badge doses were used directly or estimated from a population badge dose distribution derived from 921,134 individual annual badge readings from 79,959 participants. Information from three questionnaires on work history practices, including facility type where the technologist worked, number of years worked, and types of radiologic procedures performed were used to individualize badge doses when individual badge measurements were not available. All badge dose measurements and estimates were converted to organ absorbed dose in a systematic fashion using dosimetric factors (Simon 2011) that reflected changes over time in x-ray imaging technology, data from the third questionnaire on self-reported use of lead aprons across three decades (<1980, 1980-1989, >=1990), literature data on lead apron thickness, and calculations of x-ray transmission through the aprons (Simon 2011). Statistical methods were used to account for dose uncertainty. Complete details on the dosimetry system used for the USRT cohort have been published previously (Simon 2011; Simon et al 2014).

Ultraviolet Radiation

Indirect measurements for ultraviolet (UV) radiation exposure were developed to examine the relationship with skin and other cancers in the USRT study. City and state of residence at different ages (<13, 13-19, 20-39, 40-64, >=65) across the lifetime were collected in self-administered questionnaires. The residences were linked to satellite daily measurements of UVR to estimate annual average residential UV radiation exposures. Daily residential UV estimates based on these satellite data were strongly related to personal UV measurements in a sample of 124 USRT cohort members who agreed to wear a measurement badge (dosimeter) for a week. Other indirect information collected in self-administered questionnaires that is related to UV exposure and sun sensitivity included time spent outdoors during summer months, history of sunburns, skin, hair, and eye color, and propensity to sunburn. The USRT has also collected information on some common, potentially photosensitizing medications (e.g., medications that cause the patient's skin to be more sensitive to sunburn) including NSAIDs, prescription diuretics, oral contraceptives, and menopausal hormone therapy. While basal and squamous cell carcinoma of the skin are not systematically captured by cancer registries, the USRT study is one of very few nationwide U.S. cohorts that has prospectively collected information on these very common UV-related skin cancers.

Personal medical radiation exposure and cancer risk in the USRT

We collected questionnaire-derived information on personal medical diagnostic procedures over each technologist's lifetime in the baseline (first or second questionnaires). The objective was to follow up the cohort from the baseline questionnaire to evaluate cancer risks in relation to personal medical diagnostic procedures. The details of the methods used and findings from an assessment of thyroid cancer risk associated with radiation doses from personal diagnostic examinations among technologists followed during the period 1983-2014 is described in Little et al 2018.

References:

Little et al. Assessment of thyroid cancer risk associated with radiation dose from personal diagnostic examinations in a cohort study of US radiologic technologists, followed 1983-2014. BMJ Open 2018 May. DOI: 10.1136/bmjopen-2018-021536.

Simon SL. Organ-specific external dose coefficients and protective apron transmission factors for historical dose reconstruction for medical personnel. Health Phys 2011. DOI: 10.1097/HP.0b013e318204a60a.

Simon SL et al. Radiation organ doses received by U.S. radiologic technologists: Methods and findings. Radiat Res 2014 Nov. DOI: 10.1667/RR13542.1.