S2709
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
Is 2D-kV stereoscopic imaging reliable for patient setup in head and neck cancer radiotherapy? Sophie Boisbouvier 1,2 , Mathurine Basset 3 , Sylvie Chabaud 3 , Séverine Metzger 3 , Marie-Claude Biston 4,5 1 Radiation oncology department, Centre Léon Bérard, Lyon, France. 2 UMR Inserm 1052 CNRS 5286, Centre de Recherche en Cancérologie de Lyon, Lyon, France. 3 Direction of clinical research and innovation, Centre Léon Bérard, Lyon, France. 4 Medical physics department, Centre Léon Bérard, Lyon, France. 5 CREATIS, CNRS UMR5220, Inserm U1044, INSA-Lyon, Université Lyon 1, Lyon, France Purpose/Objective: Daily 3D imaging is recommended in head-and-neck (HN) radiotherapy to monitor setup errors and internal anatomical changes despite the added dose 1–2. Feasibility of using 2D-kV imaging instead of cone- beam CT (CBCT) for selected treatment fractions was assessed to reduce dose, time and control intrafraction position. Material/Methods: Patients aged>18, requiring intensity-modulated radiotherapy to oropharynx, oral cavity, hypopharynx or larynx with bilateral irradiation of the lymph node could be included in this prospective single-centre study (NCT04670991). Immobilisation, CT scan, delineation and treatment planning followed the department protocol 3. At each radiotherapy session, 2D-kV images were acquired and compared to the reference images using a ROI encompassing the high- risk CTV (CTVHR) and the adjacent part of the spine. Translational and rotational shifts were applied before CBCT acquisition. Using similar ROI, CBCT was registered to CT scan using an automatic bone registration (R1) followed by a soft tissue registration on the CTVHR+5mm (R2)4. Manual adjustments were allowed (R3). Final shifts were applied, and beam delivery was monitored with 2D-kV if residual corrections after CBCT were <2mm and <2°. Data collection included shifts based on R1, R2 and R3. Primary endpoint is the 3D vector of absolute residual positioning error observed with CBCT reference imaging, after repositioning performed with ExacTrac®. Secondary endpoints include: rate of discrepencies >2mm or 2°. Mean intrafractional variations observed with ExacTrac®.Residual positionning errors and intra faction variability were explored using an analysis of variance including cohort, subject and session factors. Results: Between 2021 and 2024, 44 patients treated with radiotherapy were enrolled in this study: 15 oropharynx, 15 oral cavity and 14 hypopharynx/larynx. Out of 1456 radiotherapy session, 1303 were analysable. Depending on the
imaging in patients treated with the bra. Material/Methods:
Between March and September 2025, 6 patients with large and/or pendulous breasts undergoing adjuvant radiotherapy after breast-conserving surgery were enrolled. A bra size was selected according to chest circumference and cup volume. All were treated according to the FAST-Forward protocol (26 Gy/5 fractions), delivered Monday through Friday and managed by the same team of radiation therapists (RTTs) to minimize setup variability. Daily CBCTs were acquired before and after treatment delivery. Automatic registration was performed using a grey- value algorithm within a region of interest encompassing the whole breast, sternum, and ribs, with manual adjustment by the radiation oncologist when needed. Translational (lateral - X, longitudinal - Y, vertical - Z) and rotational (pitch, roll, yaw) shifts were recorded for each fraction. Additionally, nipple-to- pectoral distance (NPD), medio-lateral thickness (MLT) and cranio-caudal extension (CCE) were measured on each CBCT and compared with those in the simulation CT to evaluate geometric consistency throughout treatment. Results: The bra sizes ranged from 3 to 11. Across 30 treatment sessions, no CBCT acquisition had to be repeated due to incorrect breast positioning within the bra. Mean translational shifts in pre-treatment CBCTs were 1.2 ± 4.1 mm (X), –1.1 ± 4.7 mm (Y), and –1.3 ± 2.0 mm (Z); post-treatment values were 0.9 ± 1.0 mm (X), – 0.7 ± 1.1 mm (Y), and –2.2 ± 2.3 mm (Z), indicating minimal intrafraction motion. Mean rotational deviations were below 1° across all axes, confirming high setup stability. Daily NPD, MLT and CCE measured on CBCTs were compared with those obtained from the simulation CT, showing consistent geometry with mean differences of 3,0 ± 1,9 mm, 7,9 ± 4,0 mm and 6,0 ± 3,8 mm, respectively, and no significant day-to- day variation. Conclusion: The use of a radiotherapy-dedicated bra provided excellent setup reproducibility and stability throughout the treatment course. The device effectively ensured consistent breast geometry across fractions, reinforcing its value in patients with large and/or pendulous breasts. The involvement of a dedicated RTT team highly contributed in minimizing setup variability. Recruitment to reach the planned sample size (n=20) is ongoing for confirmatory analysis. Keywords: Breast cancer, immobilization device, CBCT
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