Tehran University of Medical Sciences Frontiers in Biomedical Technologies 2345-5837 0 0 2026 02 15 1507 1507 Masomeh Najafabadi Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran. Soheil Elmtalab Department of Medical Physics and Biomedical Engineering, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Arman Esmailzadeh Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran. Mostafa Alizade-Harakiyan Department of Medical Physics, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran Hamed Zamani d Department of Medical Physics, School of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Ahmad Shanei Department of Medical Physics, School of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran. Nadia Najafizadeh Department of Radiation Oncology, School of Medicine, Seyyed Al-Shohada Hospital, Isfahan University of Medical Sciences, Isfahan, Iran. Ali Yousefi Department of Medicinal Chemistry, School of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran. 2025 10 29 2026 02 07 Purpose: This study compares helical Tomotherapy in the supine position with three-dimensional Conformal Radiotherapy (3D-CRT) delivered in both prone and supine positions in pendulous left-breast cancer patients, aiming to achieve optimal target coverage while minimizing dose to Organs At Risk (OARs). Materials and Methods: Twenty non-metastatic patients with large pendulous left breasts received three separate treatment plans (3D-prone, 3D-supine, and Tomo-supine). Dose–Volume Histogram (DVH)-based indices, including Dmean, V5Gy, V10Gy, V20Gy, and V30Gy for the heart, ipsilateral lung, contralateral lung, and contralateral breast, as well as CI and HI for the PTV, were evaluated. Plans were generated using TIGRT (3D-CRT) and Accuray Precision® Tomotherapy TPS. Statistical analysis was performed using paired t-tests, and p < 0.05 was considered significant. Results: Tomotherapy achieved superior target conformity and homogeneity (higher CI and lower HI) and slightly increased PTV Dmean compared to 3D-CRT. However, it increased low- and intermediate-dose exposure to the ipsilateral lung and contralateral organs while significantly reducing high-dose volumes. The prone 3D-CRT approach demonstrated the lowest ipsilateral lung mean dose among all techniques. Conclusion: While tomotherapy provides excellent PTV coverage and dose uniformity, it increases low-dose exposure to OARs compared with 3D-CRT. Therefore, the choice of technique should be individualized based on clinical priorities, particularly in younger patients, where secondary cancer risk may be of concern. https://fbt.tums.ac.ir/index.php/fbt/article/view/1507 https://fbt.tums.ac.ir/index.php/fbt/article/download/1507/553