S1944
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
accelerator (LINAC) using intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT), and CyberKnife (CK) using step-and- shoot IMRT. Material/Methods: Fifteen patients with left-sided breast cancer treated with APBI were retrospectively analysed. Exhale phase planning CT scans were acquired with 1.25 mm slice thickness in the supine position. The gross tumour volume (GTV) was defined as the tumour bed with visible seroma surrounded by surgical clips. The clinical target volume (CTV) was created by expanding the GTV by 20 mm minus the surgical-free margin. The planning target volume (PTV) was generated from the CTV by 2 mm expansion for CK, and by 4 mm for conventional LINAC. All treatment plans were optimized by using 6 MV-FFF photon beams. LINAC- based plans were created retrospectively with IMRT and VMAT techniques, while clinically accepted CK plans used step-and-shoot IMRT with fiducial-based Synchrony motion tracking method and kV imaging. The prescribed dose was 3 × 7.45 Gy for each case. The PTV coverage was evaluated using the V100% parameters. According to our institution’s SBRT clinical protocol, DVH parameters of organs at risk (OARs) such as skin, ribs, heart, ipsilateral lung, ipsilateral breast-PTV, contralateral lung and contralateral breast were analysed. Results: The PTV V100% values were 97.7% for both LINAC and CK, according to the same normalisation. As a consequence of the chosen motion control approach on CK, the average size of PTV was smaller with CK (67.3 cm3) than with LINAC (93.1 cm3). Near maximum dose values for the heart, ipsilateral lung, skin and ribs are shown in Figure1. Contralateral lung irradiation could be reduced by using LINAC-IMRT. CK resulted in smaller V50% values for the ipsilateral breast-PTV than LINAC-VMAT, and LINAC-IMRT, 9.18%, 12.88%, and 15.18%, respectively.
Conclusion: Full-course dose accumulation is recommended for more stable CTV dosimetry assessment. More critically, the accumulation method significantly underestimates superficial dose coverage for patients with a higher superficial volume ratio. This finding suggests that current DIBH protocols, particularly those using IDENTIGY™-SGRT/Halcyon, may require plan optimization or stricter motion management for thin/superficial breast targets to ensure dose coverage integrity at the skin surface. Keywords: Breast cancer 、 deep inspiration breath hold 、 SGRT Dosimetric comparison of left-sided accelerated partial breast irradiation (APBI) performed with conventional linear accelerator and CyberKnife Enik ő Koszta 1,2 , Mátyás Csoboth-Róbert 3 , Tibor Major 1,4 , Zoltán Takácsi-Nagy 1,4 , Csilla Pesznyák 1,5 , Gábor Stelczer 1,5 1 Center of Radiotherapy, National Institute of Oncology, Budapest, Hungary. 2 Doctoral School of Physical Sciences, Budapest University of Technology and Economics, Budapest, Hungary. 3 Faculty of Mechanical Engineering, Budapest University of Technology and Economics, Budapest, Hungary. 4 Department of Radiotherapy, Semmelweis University, Budapest, Hungary. 5 Institute of Nuclear Techniques, Budapest University of Technology and Economics, Budapest, Hungary Purpose/Objective: Stereotactic body radiation therapy (SBRT) is an accurate and precise treatment method with steep dose gradient and high dose conformity. The aim of our study was to compare the dosimetry of APBI treatments performed with conventional linear Digital Poster 3695
Conclusion: APBI with 6 MV-FFF photon beams provides excellent target coverage and effective OAR sparing for left- sided breast cancer. Compared to LINAC, the irradiated volume could be reduced and OARs sparing could be improved by CK using Synchrony fiducial respiratory motion tracking. Among LINAC-based methods, VMAT provided improved dose distribution
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