S2760
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
1 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD BARCELONA, BARCELONA, Spain. 2 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD MÁLAGA, MÁLAGA, Spain. 3 Servei de Física i Protecció Radiològica, Hospital Universitari Vall d'Hebron, Vall d'Hebron Barcelona Hospital Campus, BARCELONA, Spain Purpose/Objective: Comparison of non-flattened vs. flattened beams for ultra-hypofractionated breast radiotherapy Material/Methods: Twenty patients treated for left breast cancer were randomly selected. The prescription dose was 26 Gy to the breast volume (PTV26), with a simultaneous integrated boost of 29 Gy to the tumor bed (PTV29), in 5 sessions. Patients were treated using 6 MV flattened (6 MV FF) beams from a Varian TrueBeam. Plans were calculated using the Varian Eclipse v. 16.1 treatment planning system. The irradiation technique consisted of multiple fixed-gantry IMRT (Sliding Window) beams. All cases were replanned de novo by keeping the same beam arrangements but using 6 MV flattening filter- free (6 MV FFF) photons. All plans were verified using the PTW Octavius 4D system. A 3D gamma index analysis using the 3% global/2 mm criteria was performed. Three dose thresholds (10%, 50%, and 90%) were used.The dosimetric data shown in the Table were extracted from the Eclipse plans. A paired comparison (FFF vs. FF) was performed for the dosimetric data, monitor units (MUs), beam-on time (BOT), and gamma passing rate (GPR). For each metric, a two-tailed t-test was performed with a 0.05 significance level.The out-of-field dose was evaluated for the FFF and FF plans calculated by Eclipse for a mock breast case using the whole-body CT scan of a real patient. Both plans were then simulated using the PRIMO Monte Carlo software v. 0.3.64.1814. Estimated out-of-field doses from the simulations were compared (FFF vs. FF) along the in-plane axis from the inferior aspect of the breast PTV (Figure).
Results: The Table shows the differences in the analyzed metrics between both beams. All differences were dosimetrically negligible.On average, the 6 MV FFF beam allowed a 47% reduction in BOT with an increase of 22% in MUs with respect to the 6 MV FF beam (Table).Lower out-of-field dose was observed for the FFF plan than for the FF plan (Figure)All plans met the universal tolerance limit recommended by the AAPM Task Group No. 218 (Table).
Conclusion: Plans were dosimetrically similar regardless of the energy used. The FFF beams allowed a reduction in treatment time, which may contribute to patient comfort, especially for treatments planned with deep inspiration breath-hold. The trade-off is the increase in MUs associated with the FFF beams, although it does not seem related to an increase in out-of-field dose. Keywords: breast, non-flattened, flattened Digital Poster 149 COMPARATIVE ANALYSIS OF RADIOTHERAPY TARGET VOLUMES WITH PSMA PET-CT AND CEMRI IN HIGH-GRADE GLIOMAS Souvik Mondal 1 , Deepak Kumar Das 1 , Sandip Kumar Barik 1 , Saroj Kumar Das Majumdar 1 , Kanhaiyalal Agarwal 2 , Rabi Narayan Sahu 3 , Sudipta Mohakud 4 , Abhisek Mishra 5 , Soel Ahmed 1 , Anupam Muraleedharan 1 , Priyambat Dash 1 , Satabdi Mohaptara 1 , Swrangsri Mashahari 1 , Dillip Kumar Parida 1 1 Radiation Oncology, AIIMS BBSR, Bhubaneswar, India. 2 Nuclear Medicine, AIIMS BBSR, Bhubaneswar, India. 3 Neurosurgery, AIIMS BBSR, Bhubaneswar, India. 4 Radiodiagnosis, AIIMS BBSR, Bhubaneswar, India. 5 Community and Family Medicine, AIIMS BBSR, Bhubaneswar, India Purpose/Objective: This study aims to compare the radiotherapy target volumes defined by post-operative CEMRI brain and PSMA PET-CT in high-grade gliomas (glioblastoma, astrocytoma, oligodendroglioma, WHO CNS 5 classification). It evaluates difference in GTV_EDEMA,
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