Thyroid Avoidance in Breast Cancer Patients Irradiated to Supraclavicular Nodes: A Dosimetry Study
Alessandro Clivio,
Switzerland
PO-2043
Abstract
Thyroid Avoidance in Breast Cancer Patients Irradiated to Supraclavicular Nodes: A Dosimetry Study
Authors: Alessandro Clivio1, Daniel R. Zwahlen2, Sonja Koch2, Cezarina Negreanu2, Enrico Barletta2, Helmut Haerle2, Elena Hofmann2, Christoph Oehler2
1Zentrum für Radiotherapie Rüti Zürich-Ost-Linth AG, Radiation Oncology, Rüti ZH, Switzerland; 2Kantonsspital Winterthur, Radiation Oncology, Winterthur, Switzerland
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Purpose or Objective
Hypothyroidism (HT) is a common long-term sequela in women with breast cancer receiving radiotherapy (RT) to the supraclavicular nodes. The thyroid is often neglected in terms of dosimetry protection. The aim of this study was to investigate optimized IMRT and VMAT radiation treatment plans that spare the thyroid without compromising CTV/PTV coverage.
Material and Methods
Fourty-three consecutive women with breast cancer between January 2020 and April 2021 were included who received RT with 50 Gy / 25 fractions to the supraclavicular fossa that neglected the thyroid. IMRT and VMAT techniques were used in 23 and 20 patients, respectively. After contouring the thyroid new VMAT or IMRT plans were calculated with a mean RT dose (Dmean) constraint <10 – 12 Gy to the thyroid and compared with the original plans with regard to RT doses to the thyroid and other organ at risk (OAR) as well as CTV/PTV coverage. A Fisher's sign test was used to assess differences between original and new plans. The risk for hypothyroidism was calculated using a formula that was developed by the PENTEC initiative.
Results
The thyroid Dmean was 18.4 ± 7.9 Gy in all original plans (IMRT: 20.4 ± 8.8 Gy; VMAT: 16.2 ± 6.2 Gy) and only 7 patients (16%) had a Dmean below 10 Gy. Replanning resulted in a thyroid Dmean of 10.3 ± 4.5 Gy (-44%) (IMRT: 10.1 ± 4.1 Gy (-50%); VMAT: 10.5 ± 5.1 Gy (-35%)). The proportion of patients with a thyroid Dmean <10 Gy was improved to 47% (IMRT: 33.3%; VMAT: 61%). Coverage of the CTV and the PTV in the optimized plans was not compromised with a V95% of 99.7% and 96.6%, respectively. The doses to the OARs remained the same except for the spinal cord where D0.1cc increased from 10.4 ± 5.9 Gy to 11.7 ± 6.9 Gy. The calculated risk for hypothyroidism decreased significantly from 24.5 ± 11.9% to 13.3 ± 5.2% (p < 0.05). The risk for uncompensated HT was reduced from 5.4 ± 2.6% to 3.0 ± 1.2% whereas the risk for compensated HT was reduced from 19.0 ±9.2% to 10.4 ± 4.1%.
Conclusion
Thyroid avoidance for breast cancer patients receiving RT to the supraclavicular nodes is possible in half of the patients with a Dmean <10 Gy, particularly with VMAT planning. The calculated risk for overall HT and uncompensated HT can be reduced from 24.5% to 13.3% and from 5.4% to 3%, respectively.