S1849
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
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Radiotherapy treatments for glioblastoma in an MRI-guided linear accelerator compared to C-arm linear accelerator Carlos Ferrer 1 , Concepción Huertas 1 , Alonso La Rosa 2 , Beatriz Moreno 2 , Inmaculada Navarro 2 , Marcos Martínez 1 , Giorgia Yang 1 , Laura Zaragoza 2 , Moisés Sáez 1 1 Medical Physics, H.U. La Paz, Madrid, Spain. 2 Radiation Oncology, H.U. La Paz, Madrid, Spain Purpose/Objective: Glioblastoma multiforme (GBM) is a type of malignant, aggressive, and fast-growing brain tumor that originates in glial cells. Standard treatment includes surgery, followed by radiotherapy and chemotherapy. In comparison with C-arm linacs, MRI-guided radiation therapy linacs offer several advantages. These include superior tumor localization due to the enhanced tissue contrast provided by MRI compared to X-ray imaging and the possibility of performing adaptive treatments in addition to dose escalation. Furthermore, the adaptive capabilities of the MR-linacs enable the planned target volume to be recontoured and adapt the plan for each fraction.Radiotherapy treatments with an MR-linac could be advantageous, but only if the dosimetric outcome is comparable to that achieved with a C-arm linac, which allows the use of non-coplanar beams to spare Organs at Risk (OAR) and improve the dose conformation. This study compares the dosimetric outcomes of GMB treatments obtained with MR-linacs and C-arm linacs, and analyze whether treatments with the MRI Linac represent an improvement. Material/Methods: Four initial patients have been analyzed. Radiotherapy GBM treatments were prescribed to 60 Gy in 30 fractions and planned with Elekta Monaco TPS with the Step-and-Shoot technique for a 7 MV FFF Elekta MRI-linac with 1.5 T magnetic field. Same patients were planned with VMAT technique and a 6 MV Elekta Infinity C-arm linac which included a non-coplanar arc. PTV coverage, PTV volume changes, mean ad maximum doses to OARs, conformity, homogeneity, dose fall-off and treatment time were evaluated. Results: Averaged dosimetric differences between treatments are shown in Tables 1 and 2. Main differences are found for left lens, eye and optic nerve, which receive higher mean doses in the MR-linac plans. This also occurs for healthy brain or optic chiasm. Table 1. Averaged dosimetric results for the three patients analyzed.
As shown in Table 2, PTV coverage was similar, whereas MR-linac plans resulted more conformed, less MU and treatment time. C-arm linac plans were more homogeneous with better OAR sparing and dose fall- off. Table 2. Main parameters and indices analyzed.
Conclusion: Both types of linac plans were clinically acceptable. PTV coverage and OAR constraints were met. However, OAR sparing was improved with C-arm linac probably due to the non-coplanar arc, which extend the treatment time. MR-linac plans were more conformal, and the possibility to adapt the treatment makes them a good option for GMB radiotherpy
treatments. References:
Moats, E., Siddiqui, M. S., Parikh, P., & Snyder, K. C. (2024). Dosimetric comparison of glioblastoma radiotherapy treatment plans on a low-field MRI- guided linear accelerator compared to conventional C- arm linear accelerator. Medical Dosimetry, 49(3), 215- 221.Graham, J. A., Redler, G., Delozier, K. B., Yu, H. H. M., Oliver, D. E., & Rosenberg, S. A. (2022). Dosimetric feasibility of hippocampal avoidance whole brain radiotherapy with an MRI - guided linear accelerator. Journal of Applied Clinical Medical Physics, 23(6), e13587. Keywords: Glioblastoma, MR-linac, Adaptive radiotherapy
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Validation of an automated CBCT-based synthetic CT dose calculation software for breast cancer Jessica Prunaretty, Nicolas Mir, Céline Bourgier, David Azria, Pascal Fenoglietto Radiotherapy, Institut du Cancer de Montpellier, Montpellier, France
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