Assessment of Exposure
The ethical committees approved the investigation. Information on exposures was assessed by a questionnaire sent to both cases and controls. If the answers were unclear, a nurse trained for this purpose supplemented the answers over the telephone. A written protocol was used during the telephone interviews. The questionnaires were blinded to case or control status. After that, we scrutinized all answers to be sure to obtain uniform assessment of exposure for all cases and controls. If the quality of the answers was judged to fulfill our criteria for assessment of exposure, the information was coded and registered for statistical analysis. Otherwise, the questionnaire was returned to the nurse for additional telephone interview.
Regarding medical x-ray examinations, we asked for the anatomic area that had been investigated, the years of the investigations, and the total number of x-ray examinations. Subjects who had worked as physicians were asked about radiology work.
For cellular phones, use while working and during leisure time, years of use, and mean number of minutes of daily use were assessed. Cumulative use in hours was calculated. Furthermore, we asked for information regarding use of a hands-free device with an earpiece and use in a car with fixed antenna, both taken as unexposed. In one question, we asked which ear was most frequently used during cellular telephone calls. Also, type of telephone, NMT (analogue) or GSM (digital) system, was assessed.
Exposure to different agents, eg, brand or chemical names, working conditions, and years and number of days per year of exposure were assessed. Since most low-calorie drinks contain aspartame,[5] information on drinking habits was obtained. Years of intake, times per day or week, and amount of drink each time were assessed for calculation of total dose.
Interviews and coding of the answers were made without knowing whether the person was a case or a control. All exposures within 1 year before diagnosis were disregarded. The same year was used for both cases and matched controls. Also, total exposure of less than 1 day was dismissed.
After informed consent, copies of x-ray films for tumor diagnosis and histopathologic reports were requested for the cases. Based on these copies, we judged whether the tumor was a new diagnosis or a recurrent disease, determined the anatomic localization of the tumor, and assessed histopathologic diagnosis.
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