Effect of dose constraint on the thyroid gland during locoregional intensity‐modulated radiotherapy in breast cancer patients

The aim of the present study was to compare radiation dose received by thyroid gland using different radiotherapy (RT) techniques with or without thyroid dose constraint (DC) for breast cancer patients. Computerized tomography (CT) image sets for 10 patients with breast cancer were selected. All patients were treated originally with opposite tangential field‐in field (FinF) for the chest wall and anteroposterior fields for the ipsilateral supraclavicular field. The thyroid gland was not contoured on the CT images at the time of the original scheduled treatment. Four new treatment plans were created for each patient, including intensity‐modulated radiotherapy (IMRT) and helical tomotherapy (HT) plans with thyroid DC exclusion and inclusion (IMRT(DC(−)), IMRT(DC(+)), HT(DC(−)), and HT(DC(+)), respectively). Thyroid DCs were used to create acceptable dose limits to avoid hypothyroidism as follows: percentage of thyroid volume exceeding 30 Gy less than 50% (V (30) < 50%) and mean dose of thyroid (TD(mean)) ≤ 21 Gy. Dose‐volume histograms (DVHs) for TD(mean) and percentages of thyroid volume exceeding 10, 20, 30, 40, and 50 Gy (V (10), V (20), V (30), V (40), and V (50), respectively) were also analyzed. The D(mean) of the FinF, IMRT(DC(−)), HT(DC(−)), IMRT(DC(+)) and HT(DC(+)) plans were 30.56 ± 5.38 Gy, 25.56 ± 6.66 Gy, 27.48 ± 4.16 Gy, 18.57 ± 2.14 Gy, and 17.34 ± 2.70 Gy, respectively. Median V(30) values were 55%, 33%, 36%, 18%, and 17%, for FinF, IMRT(DC(−)), HT(DC(−)), IMRT(DC(+)), and HT(DC(+)), respectively. Differences between treatment plans with or without DC with respect to D (mean) and V (30) values were statistically significant (P < 0.05). When thyroid DC during breast cancer RT was applied to IMRT and HT, the TD(mean) and V (30) values significantly decreased. Therefore, recognition of the thyroid as an organ at risk (OAR) and the use of DCs during IMRT and HT planning to minimize radiation dose and thyroid volume exposure are recommended.