Is thyroid gland an organ at risk in breast cancer patients treated with locoregional radiotherapy Results of a pilot study

Background: Aim was to evaluate the dose distribution within the thyroid gland its association with hypothyroidism in breast cancer (BC) patients receiving supraclavicular (SC) radiation therapy (RT). Materials and Methods: Consecutive 40 BC patients with baseline normal thyroid function tests (TFTs), were randomized into two groups: (a) Adjuvant chest wall/breast with SC-RT (20 patients) and (b) control group (adjuvant chest wall/breast RT only); 20 patients. The thyroid gland was contoured for each patient. Each patients dose volume histogram (DVH), mean thyroid volume, the volume percentages of the thyroid absorbing respectively 5, 10, 20, 30, 40, and 50 Gy (V5, V10, V20, V30, V40, and V50), and D mean (average dose in whole volume of thyroid) were then estimated. TFTs were performed at the time of the last follow-up and compared. Results: Mean thyroid volume of cohort was 19.6 cm (3) (4.02-93.52) and D mean of thyroid gland in SC-RT and control group was 25.8 Gy (16.4-52.2) and 5.6 Gy (0.7-12.8), respectively. Median values of V5, V10, V20, V30, V40, and V50 were 54%, 51%, 42.8%, 30.8%, 27.8%, and 7.64%, respectively, in SC-RT as compared to control group (V5;4.9%, V10;2.4%, V20;1.75%, V301%, V40;0%, and V50;0%, respectively) with P < 0.0001. At 52 months, a majority of patients (90%) had a normal thyroid function whereas four patients (10%) had hypothyroidism; 3/20 (15%) patients in SC-RT and 1/20 (5%) in control group with P < 0.001. Significant prognostic factors were; SC-RT (P = 0.001), V30 above 50% (P = 0.001), and smaller thyroid volume (P = 0.03). Conclusion: The risk of hypothyroidism in BC patients after SC-RT depends on the thyroid gland volume and V30 > 50% and the risk can be minimized by thyroid gland shielding during RT.