Thyroid cancer studies on etiology and prognosis

Abstract: Thyroid cancer constitutes about 1% of all malignant tumours and the incidence is increasing in Sweden. It is rare in children before the age of 10. During puberty the female to male ratio increases to be two to three times more common in females. The ratio remains constant until menopause and thereafter declines. The etiology of this gender-dependent incidence difference is unclear. Ionizing radiation is the only well-established risk factor for the disease, while the impact of other etiological factors is not clear.A retrospective study based upon medical records of 218 females and 91 males with papillary, mixed or follicular types of thyroid cancer was conducted. Prognostic factors were compared by multivariate analysis using Cox's semiparametric hazard model. Differences in prognosis between women and men were found. There was a higher relapse rate and mortality risk among men. Distant metastasis, age >50 years, regional lymph node metastasis, low or moderate differentiation, and tumour related symptoms at diagnosis were also independent factors related to increased tumour mortality risk.A population-based case-control study including 180 cases and 360 controls was carried out to identify risk factors for thyroid cancer. Information on exposure was obtained by mailed questionnaires. The first part of the study investigated connections between medical ionizing radiation and thyroid cancer. The results showed that diagnostic X rays were a significant risk factor for papillary thyroid cancer in women between 20 and 50 years at diagnosis. Exposure to iodine-131 caused no increased risk for thyroid cancer. The result supports that external radiotherapy is a risk factor for thyroid cancer in women.The second part of the case-control study dealt with occupation and different exposures. Work with diagnostic X-ray investigations and work as a lineman was associated with thyroid cancer. Exposure to impregnating agents increased the risk.The third part of this study showed that one pregnancy increased the risk for papillary thyroid cancer. A medical history of asthma or allergy decreased the risk.Another case-control study using medical records as the source for assessment of exposure gave a non significantly increased risk for thyroid cancer in patients who had been treated with external radiotherapy including the thyroid gland.