Background: Many studies have shown an association between thyroid disorders and breast cancer. Among them, diffusely increased thyroid uptake of F18-fluorodeoxyglucose (F-18 FDG) has been associated with chronic lymphocytic thyroiditis. There is little or no information regarding the relationship between treatment of breast cancer and the development of new diffuse thyroid F-18 FDG uptake. Here, we report the frequency of this occurrence and some of the factors associated with it.
Methods: We performed a retrospective review of the positron emission tomography (PET) computed tomography (CT) database and identified 290 women who had underwent PET-CTs both before and after breast surgery. Of these, 246 were enrolled in this study and 44 were excluded, because before breast surgery they had either a history of prior thyroid disorders including thyroid cancer or abnormal serum free thyroxine and thyroid-stimulating hormone, or other malignancies. Tests for serum antithyroid antibodies were not available before thyroid surgery in all patients. There were 62 patients who did have antithyroid antibody tests before surgery and 27 were positive. They were not excluded from the study.
Results: Diffuse thyroidal uptake on PET-CT developed in 23 (9.3%) patients during the follow-up period (median=21.1 months; range=3.1-47.3 months). Age (≥55 years; hazard ratio [HR]=0.15; 95% confidence interval [CI]: 0.03-0.65; p=0.012), bilaterality of breast cancer (HR=3.87; 95% CI: 1.02-14.62; p=0.046), and postsurgical radiotherapy (HR=3.06; 95% CI: 1.03-9.16; p=0.045) showed independent association with new thyroid FDG uptake in multivariate analysis. All patients with thyroid FDG uptake met at least one of some criteria related to chronic thyroditis (positive for antithyroid antibodies or cytology findings or ultrasonography findings of Hashimoto's thyroiditis) and were associated with development of hypothyroidism (52.2% vs. 4.5% in patients without uptake; p<0.001).
Conclusions: Radiotherapy in breast cancer may be an independent predictive factor for the development of new diffuse thyroidal uptake in PET-CT associated with thyroid dysfunction during follow-up. The development of diffuse F-18 FDG uptake is a marker for thyroid abnormalities. Younger women who have radiotherapy after surgery for breast cancer, in particular, are candidates for close follow-up of their thyroid function.