Thyroid Dysfunction Following Supraclavicular Irradiation in the Management of Carcinoma of the Breast

Radiation induced thyroid dysfunction is usually underestimated in patients with breast cancer who had supraclavicular irradiation (RT). In the present study, a total of 28 patients with breast cancer received supraclavicular RT were evaluated focusing on radiation dose-volume factors in relation to thyroid function. Thyroid function tests, including serum thyroid stimulating hormone, free thyroxine, free triiodothyronine, were analyzed prior to RT and 3, 6, 9, 12, 18 and 24 months after RT. Based on each patient’s dose volume histogram (DVH), total volume of the thyroid, mean radiation dose the thyroid and percentages of thyroid volume which received radiation doses 10-50 Gy (V10-V50) were considered for statistical analysis. The median follow-up time was 25 months (range, 12.3-36 months). Of 28 patients 6 (21%) were diagnosed with hypothyroidism (HT). The median time to the development of HT was 9 months (range: 3-18 months). Mean thyroid dose was 31 Gy (19-48 Gy) and mean thyroid volume was 32 cc (12-64 cc). We found that V20 (OR= 10, 95% CI= 1.15-86.88, p= 0.05), V30 (OR= 10, 95% CI= 1.15-86.88, p= 0.05) and V40 (OR= 21, 95% CI= 1.61-273.34, p= 0.02) and mean thyroid dose ≥36 Gy (OR= 10, 95% CI= 1.15-86.88) (p= 0.05), had a significant impact on development of HT. Moreover, significant elevation was observed in mean TSH level between baseline (1.85±1.47 mIU/L) and at 6 months (3.80±7.42 mIU/L), (p= 0.003). Supraclavicular RT in patients with breast cancer appear to amplify the risk of HT. We believe further investigations in larger cohort are required to confirm our results.