Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

times greater risk for development of radiation-induced thyroid cancer than males,presumably because of hormonal influences on thyroid function. Persons of Jewishand North Mrican ancestry also appear to be at greater risk than the general population.The majority of radiation-induced thyroid neoplasms are well-differentiatedpapillary adenocarcinomas, with a lower percentage being of the follicular form.Because radiation-induced thyroid cancers do not usually include the anaplastic andmedullary types, the associated mortality rate is only approximately 5%.The latency period for benign nodules is 5 to 35 years, and for thyroid malignanciesit is 10 to 35 years. The dose-response data for thyroid cancer fit a linearpattern. Studies comparing the sources of radiation exposure indicate that, in general,internal irradiation from radioactive material such as iodine-131 is substantiallyless effective in producing cancer than the same dose delivered via externalirradiation.Irradiation of the thyroid may produce other responses, such as hypothyroidismand thyroiditis. Threshold estimates for adult populations range from 2 Gy(200 rad) for external irradiation to 50 Gy (5,000 rad) for internal (low-dose-rate)irradiation. Lower threshold estimates exist for children. Approximately 10% ofpersons with internal thyroid doses of200 to 300 Gy (20,000 to 30,000 rad) to thethyroid gland from radioactive iodine will develop symptoms of thyroiditis and/ora sore throat, and higher doses may result in thyroid ablation.The Chernobyl nuclear plant accident released large quantities of radioiodinewhich resulted in a substantial increase in the incidence of thyroid cancer amongchildren living in heavily contaminated regions. The increase so far has been almostentirely papillary carcinoma, and the incidence rate falls to normal levels amongchildren born more than 6 months after the accident. So far, no other cancers haveshown an increased incidence, and there was no identifiable increase in the incidenceof birth defects.For women exposed to low-level ionizing radiation, breast cancer is of major concernbecause of its high radiogenic incidence and high mortality rate. One out ofevery 8 women in the United States is at risk of developing breast cancer (approximately180,000 new cases of breast cancer occur each year in the United States),and approximately 1 out of every 30 women is at risk of dying from breast cancer.Some of the etiologic factors in the risk of breast cancer include age at first full-termpregnancy, family history of breast cancer, race, and estrogen levels in the blood.Women who have had no children or only one child are at greater risk for breastcancer than women who have had two or more children. In addition, reduced risksare associated with women who conceive earlier in life and who breast-feed for alonger period of time. Familial history of breast cancer can increase the risk twofoldto fourfold, with the magnitude of the risk increasing as the age at diagnosis in thefamily member decreases. There is also considerable racial and geographic variationin breast cancer risk. Several investigations have suggested that the presence of estrogenacting as a promoter is an important factor in the incidence and latency associatedwith spontaneous and radiation-induced breast cancer.The breast cancer risk for low-LET radiation appears to be age dependent,being approximately 50 times higher in the 15-year-old age group (approximately30/10,000 or 0.3% per year) after an exposure of 0.1 Gy (l0 rad) than in women