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

Alessandro Clivio; Daniel R Zwahlen; Sonja Koch; Cezarina Negreanu; Enrico Barletta; Helmut Haerle; Elena Hofmann; Christoph Oehler

doi:10.1007/s00066-024-02321-8

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

Strahlenther Onkol. 2024 Nov 25. doi: 10.1007/s00066-024-02321-8. Online ahead of print.

Authors

Alessandro Clivio¹, Daniel R Zwahlen^{1

2}, Sonja Koch², Cezarina Negreanu², Enrico Barletta^{1

2}, Helmut Haerle², Elena Hofmann², Christoph Oehler^{3

4}

Affiliations

¹ Zentrum für Radiotherapie Rüti Zürich-Ost-Linth AG, Rüti, ZH, Switzerland.
² Department of Radiation Oncology, Hospital Winterthur, Winterthur, Switzerland.
³ Zentrum für Radiotherapie Rüti Zürich-Ost-Linth AG, Rüti, ZH, Switzerland. Christoph.Oehler@ksw.ch.
⁴ Department of Radiation Oncology, Hospital Winterthur, Winterthur, Switzerland. Christoph.Oehler@ksw.ch.

PMID: 39585411
DOI: 10.1007/s00066-024-02321-8

Abstract

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_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.

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_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.

Results: 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).

Conclusion: 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.

Trial registration: Retrospectively registered.

Keywords: Breast radiotherapy; Dose constraint; Long-term survivors in breast cancer; Organ at risk (OAR); Risk of hypothyroidism.