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Abstract
Background/aim: To investigate the association between the thyroid dysfunction and thyroid radiation dose in regional nodal irradiation (RNI) using volumetric modulated arc therapy (VMAT) for breast cancer.

Patients and methods: We reviewed medical data of 67 patients with breast cancer who underwent curative surgery followed by adjuvant radiotherapy, including RNI using VMAT, between 2018 and 2021. All patients had normal thyroid functional test results, including thyroid stimulating hormone (TSH), T3, and free-T4. We defined subclinical hypothyroidism as increased TSH with or without decreased levels of free-T4 and T3 after the completion of VMAT. We calculated dose-volume histogram parameters (DVHPs), including the mean dose and relative thyroid volume receiving at least 10, 20, 30, and 40 Gy.

Results: The median follow-up time was 23.2 months. The 3-year locoregional failure-free survival, progression-free survival, and overall survival rates were 96.3%, 94.7%, and 96.2%, respectively. The mean thyroid dose was 21.4 Gy (range=11.5-29.4 Gy). Subclinical hypothyroidism was noted in 14 patients (20.9%) and the median time to the event was 4.1 months. Among the DVHPs, the relative volume receiving ≥20 Gy (V20Gy) was associated with subclinical hypothyroidism. The 2-year rates of subclinical hypothyroidism were 24.8% and 59.1% in patients with V20Gy ≤46.3% and >46.3%, respectively.

Conclusion: A significant proportion of patients with breast cancer developed subclinical hypothyroidism after undergoing VMAT for RNI. Our findings highlight the importance of considering the thyroid as an organ at risk for VMAT planning, and suggest that V20Gy could be a useful dose-volume constraint.

Affiliations

Hyeon Kang Koh # 1, Younghee Park # 2, Taeryool Koo 3, Kwang-Ho Cheong 4, Me Yeon Lee 4, Hae Jin Park 5, Kyoung Ju Kim 6, Soah Park 6, Taejin Han 7, Sei-Kwon Kang 7, Boram Ha 8, Jai-Woong Yoon 8, Me Young Kim 9, Hoonsik Bae 9
1Department of Radiation Oncology, Konkuk University School of Medicine and Konkuk University Medical Center, Seoul, Republic of Korea.
2Department of Radiation Oncology, Ewha Womans University Seoul Hospital, Ewha Womans University College of Medicine, Seoul, Republic of Korea.
3Department of Radiation Oncology, Hallym University Sacred Heart Hospital, Hallym University College of Medicine, Gyeonggi-do, Republic of Korea; kootaeryool@hallym.or.kr.
4Department of Radiation Oncology, Hallym University Sacred Heart Hospital, Hallym University College of Medicine, Gyeonggi-do, Republic of Korea.
5Department of Radiation Oncology, Hanyang University College of Medicine, Seoul, Republic of Korea.
6Department of Radiation Oncology, Hallym University Kangnam Sacred Heart Hospital, Hallym University College of Medicine, Seoul, Republic of Korea.
7Department of Radiation Oncology, Kangdong Sacred Heart Hospital, Seoul, Republic of Korea.
8Department of Radiation Oncology, Hallym University Dongtan Sacred Heart Hospital, Hallym University College of Medicine, Gyeonggi-do, Republic of Korea.
9Department of Radiation Oncology, Hallym University Chuncheon Sacred Heart Hospital, Hallym University College of Medicine, Gangwon-do, Republic of Korea.
#Contributed equally.