ESTRO 2023 - Abstract Book

S1815

Digital Posters

ESTRO 2023

1 Zentrum für Radiotherapie Rüti Zürich-Ost-Linth AG, Radiation Oncology, Rüti ZH, Switzerland; 2 Kantonsspital Winterthur, Radiation Oncology, Winterthur, Switzerland 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. Materials 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. S. Dicuonzo 1 , M. Zaffaroni 1 , K.M.S.A. Amin 1,2 , G. Ronci 3,4 , M. D’Arcangelo 3,4 , M.G. Vincini 1 , R. Cambria 3 , R. Luraschi 3 , D.P. Rojas 1 , M.A. Zerella 1 , F. Cattani 5 , R. Orecchia 6 , M.C. Leonardi 7 , B.A. Jereczek-Fossa 1,8 1 European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy; 2 University of Milan, Department of Oncology and Hemato-oncology, Milan, Italy; 3 European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 4 University of Milan, __, Milan, Italy; 5 European Institute of Oncology IRCCS , Unit of Medical Physics, Milan, Italy; 6 European Institute of Oncology IRCCS, Scientific Directorate , Milan, Italy; 7 European Institute of Oncology IRCCS, Division of Radiotherapy, Milan, Italy; 8 University of Milan, Department of Oncology and Hemato-oncology , Milan, Italy Purpose or Objective The aim of this analysis was to evaluate the quality of planned dose distribution in two groups of breast cancer patients treated at the same Institution with hypofractionated intensity-modulated radiotherapy (IMRT) delivered using Helical TomoTherapy®. Materials and Methods Two groups of patients (half treated between 2012 and 2015 and half treated between 2019 and 2020) with implant-based immediate breast reconstruction (IBR) who received post mastectomy RT to the chest wall (CW) and to the infra/supraclavicular nodal region (SVC) using a 15-fraction schedule (2.67 Gy/fraction). The RT treatment was delivered using the TomoTherapy Hi-Art System (Tomotherapy® System, Accuray Incorporated, Sunnyvale, CA) in helical mode. Dose distribution was evaluated according to dosimetric indices extracted from dose-volume histograms (DVHs) selected from those employed in the clinical routine. A quantitative scoring tool, adapted from the one used by Motwani et al., was employed to retrospectively compare the quality of the planned dose distribution between the sub-cohorts of the two treatment groups in terms of both target coverage and sparing of organs at risk (OARs). Results A total of 240 patients were included in the analysis. The distribution of total scores resulting from the summation of points and the distribution of planning objectives for planning target volume (PTV) coverage and for OAR sparing are reported in Table 1a . In particular, the percentage of optimal plans increased from 70.8% (2012-2015) to 79.2% (2019-2020), while the percentage of compromised plans decreased from 10.8% to 5.8%. Median values of dosimetric indexes, as well as the percentage of plans satisfying each planning objectives/constraint, both for the individual organs and target volumes, are reported in Table 1b. Table 1. Recap of the plans’ scores (a) and summary of the planning objective constraints (b) PO-2044 Plan quality comparison at 5 years in 2 cohorts of breast cancer patients treated with Tomotherapy®

a)