THYROID NODULARITY AND CANCER AMONG CHERNOBYL CLEANUP WORKERS FROM ESTONIA

Thyroid examinations, including palpation, ultrasound and, selectively , fine-needle aspiration biopsy, were conducted on nearly 2,000 Cherno byl cleanup workers from Estonia to evaluate the occurrence of thyroid cancer and nodular thyroid disease among men with protracted exposure to ionizing radiation. The examinations were conducted in four cities in Estonia during March-April 1995, 9 years after the reactor acciden t. The study population was selected from a predefined cohort of 4,833 cleanup workers from Estonia under surveillance for cancer incidence. These men had been sent to Chernobyl between 1986 and 1991 to entomb the damaged reactor, remove radioactive debris and perform related cle anup activities. A total of 2,997 men were invited for thyroid screeni ng and 1,984 (66%) were examined. Estimates of radiation dose from ext ernal sources were obtained from military or other institutional recor ds, and details about service dates and types of work performed while at Chernobyl were obtained from a self-administered questionnaire. Blo od samples were collected for assay of chromosomal translocations in c irculating lymphocytes and loss of expression of the glycophorin A (GP A) gene in erythrocytes. The primary outcome measure was the presence or absence of thyroid nodules as determined by the ultrasound examinat ion. Of the screened workers, 1,247 (63%) were sent to Chernobyl in 19 86, including 603 (30%) sent in April or May, soon after the accident. Workers served at Chernobyl for an average of 3 months. The average a ge was 32 years at the time of arrival at Chernobyl and 40 years at th e time of thyroid examination. The mean documented radiation dose from external sources was 10.8 cGy. Biological indicators of exposure show ed low correlations with documented dose, but did not indicate that th e mean dose for the population was higher than the average documented dose. Ultrasound examinations revealed thyroid nodules in 201 individu als (10.2%). The prevalence of nodules increased with age at examinati on, but no significant associations were observed with recorded dose, date of first duty at Chernobyl, duration of service at Chernobyl, bui lding the sarcophagus or working on the roof of neighboring buildings or close to the damaged reactor. Nodularity showed a nonsignificant (p ((1)) = 0.10) positive association with the proportion of lymphocytes with chromosome translocations, but associations with the frequency of variant erythrocytes in the GPA assay were weak and unstable (p((1)) greater than or equal to 0.46). The majority of fine-needle biopsies t aken on 77 study participants indicated benign nodular disease. Howeve r, two cases of papillary carcinoma and three benign follicular neopla sms were identified and referred for treatment. Both men with thyroid cancer had been sent to Chernobyl in May of 1986, when the potential f or exposure to radioactive iodines was greatest. Chernobyl cleanup wor kers from Estonia did not experience a markedly increased risk of nodu lar thyroid disease associated with exposure to external radiation. Po ssible reasons for the apparent absence of effect include low radiatio n doses, the protracted nature of the exposure, errors in dose measure ment, low sensitivity of the adult thyroid gland or the insufficient p assage of time for a radiation effect to be expressed. (C) 1997 by Rad iation Research Society.