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

PURPOSE

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) between 10 and 15 Gy using intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) and examine its impact on the hypothyroidism risk.

METHODS

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 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.

RESULTS

Initial radiotherapy plans had a thyroid D of 18.4 ± 7.9 Gy (IMRT: 20.4 ± 8.8 Gy, VMAT: 16.2 ± 6.2 Gy). Replanning significantly reduced D 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 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 = 10 Gy) or 16.3% (D = 13.5 Gy).

CONCLUSION

Implementing a thyroid organ at risk (OAR) constraint D 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.

TRIAL REGISTRATION

Retrospectively registered.