S211
Clinical - Breast
ESTRO 2026
infraspinatus and subscapularis muscles. This was mainly the case for patients receiving axillary radiotherapy to levels I–IV (n = 65) or III–IV only (n = 31). In patients treated to levels I–II only (n = 8), advantages were limited to the teres major and minor muscles, whereas in patients without axillary radiotherapy (n = 24), only the teres major muscle benefited. In the ALTJ region, Δ Dmin showed a significant reduction of 1.5 Gy overall; The largest dose reductions by PT ( Δ Dmin = 10.7 Gy, Δ Dmean = 13.0 Gy) were seen in patients receiving levels III–IV or no axillary radiotherapy.
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Dosimetric Comparison of the Shoulder Region Between Paired Photon and Proton Plans in Breast Cancer Patients Yuqin Liang 1 , Karolien Verhoeven 1 , Gloria Vilches- Freixas 1 , Sophie C. J. Bosma 2 , Richard Canters 1 , Femke E. Froklage 3 , Ruud Houben 1 , John H. Maduro 4 , Jelle Overbosch 5 , Joan J. Penninkhof 3 , Margriet G. A. Sattler 3,6 , Janine M. Simons 3 , Nanna M. Sijtsema 4 , Rozemarijn Vliegenthart 5 , Anne P. G. Crijns 4 , Sofia Rivera 7 , Liesbeth J. Boersma 1 1 Department of Radiation Oncology (Maastro), GROW- Research Institute for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands. 2 Department of Radiation Oncology, Leiden University Medical Center, Leiden, Netherlands. 3 Department of Radiotherapy, Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, Netherlands. 4 Department of Radiation Oncology, University Medical Center Groningen, University of Groningen, Groningen, Netherlands. 5 Department of Radiology, University Medical Center Groningen, University of Groningen, Groningen, Netherlands. 6 Holland PTC, Holland PTC, Delft, Netherlands. 7 Radiation Oncology Department, Gustave Roussy, Villejuif, France Purpose/Objective: Adjuvant locoregional radiotherapy reduces the risk of locoregional recurrence in breast cancer (BC) but may cause arm and shoulder symptoms (AS). In the literature, dose to the shoulder region has been related to lymphedema and impaired function 1, whereas dose to the axillary-lateral thoracic vessel juncture (ALTJ) region is mainly associated with lymphedema 2. Since proton therapy (PT) is known to reduce dose to normal tissues compared with photon therapy (XRT), we aimed to compare the dosimetry of the shoulder region between XRT and PT in BC patients. Material/Methods: All 128 BC patients who underwent XRT-PT plan comparison at one institution (2019–2024) were eligible. Ipsilateral shoulder muscles and ALTJ regions were auto-delineated with a Mirada atlas; ALTJ contours were manually corrected. Median (with interquartile range) values were calculated for each dosimetric parameter (Dmean, Dmin, V15Gy), and the difference in median values between the XRT and PT groups ( Δ = XRT − PT) was tested using the Wilcoxon signed-rank test. Results: PT significantly reduced shoulder muscle doses compared with XRT, showing consistently lower doses ( Δ Dmean ≥ 10 Gy, Δ V15Gy ≥ 10%, see Table and Figure) for the teres major, teres minor, supraspinatus,
Conclusion: Compared with XRT, PT results in significant dosimetric advantages for five major shoulder muscles, with the largest benefit in patients with level III-IV irradiation; in patients without radiotherapy to level I-II, PT resulted also in lower doses to the ALTJ region. Further research is needed to determine whether these reductions translate into clinically meaningful benefits. References: 1. Johansen, S., Fosså, K., Nesvold, I.L., et al., Arm and shoulder morbidity following surgery and radiotherapy for breast cancer. Acta Oncologica 53. 521-529 (2014). https://doi.org/10.3109/0284186X.2014.8805122. Gross, J.P., Lynch, C.M., Flores, A.M., et al., Determining the Organ at Risk for Lymphedema After Regional Nodal Irradiation in Breast Cancer. International Journal of Radiation Oncology*Biology*Physics 105. 649-658 (2019).
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