Thyroid avoidance in treatment planning for breast cancer patients irradiated to the supraclavicular nodes

Hypothyroidism affects up to 21% of women with breast cancer after supraclavicular node irradiation. The PENTEC (pediatric normal tissue effects in the clinic) initiative highlighted the need to minimize the thyroid dose, albeit without giving a specific constraint. This study aimed to define a reasonable target thyroid mean dose (D_mean) between 10 and 15 Gy using intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) and examine its impact on the hypothyroidism risk.

Forty-three breast cancer patients with supraclavicular irradiation neglecting the thyroid in terms of dose protection were included from 01/2020 to 04/2021. An IMRT or VMAT technique was used in 23 and 20 patients, respectively. Replanning aimed for a thyroid D_mean of 10 Gy. IMRT plans still exceeding 10 Gy were converted into VMAT plans. Fisher's sign test compared original and revised plans and the hypothyroidism risk was calculated.

Initial radiotherapy plans had a thyroid D_mean of 18.4 ± 7.9 Gy (IMRT: 20.4 ± 8.8 Gy, VMAT: 16.2 ± 6.2 Gy). Replanning significantly reduced D_mean to 10.3 ± 4.5 Gy (-44%) overall (IMRT: -50%, VMAT: -35%), with 56% achieving ≤ 10 Gy (IMRT: 33.3%, VMAT: 61%). Furthermore, an IMRT to VMAT conversion yielded a thyroid D_mean of 9.2 ± 3.5 Gy, with 74.4% of patients ≤ 10 Gy, albeit at the cost of higher doses to the contralateral breast. Clinical and planning target volume (CTV/PTV) coverage remained uncompromised. The calculated hypothyroidism risk significantly decreased from 24.5% to 13.3% (D_mean = 10 Gy) or 16.3% (D_mean = 13.5 Gy).

Implementing a thyroid organ at risk (OAR) constraint D_mean of 13.5 Gy was feasible in 88% of patients without compromising other OARs and CTV/PTV coverage, and resulted in a 33-46% reduction of the hypothyroidism risk.

Retrospectively registered.