S524
Clinical - Haemotology
ESTRO 2026
Purpose/Objective: This study provides a practical guide for implementing the Extended Cone-Beam Computed Tomography (CBCTExtended) technique for patient positioning verification in Total Body Irradiation (TBI). It also compares its accuracy and reproducibility with the conventional CBCT-based verification method currently used in clinical routine. Material/Methods: Positioning verifications were analyzed for ten TBI patients treated with a TrueBeam linear accelerator using Varian Eclipse v16.1 as the treatment planning system (TPS).In the conventional approach, a CBCT (or orthogonal images) is acquired at each treatment isocenter. The displacement corrections (matching) for the lateral, vertical, and longitudinal axes are recorded separately, and the mean offset for each axis is applied manually across all isocenters during treatment.In the CBCTExtended approach, consecutive CBCTs are acquired across isocenters sharing the same lateral and vertical coordinates, varying only the longitudinal position. In the TPS configuration, the Delta coordinates (lateral, vertical, longitudinal) are assigned automatically to the first isocenter containing the CBCT field, while subsequent isocenters have all Delta values set to zero except for the longitudinal offset—equal to the inter-isocenter distance (see Figure 1). A merged CBCT sequence is then generated (Figure 2), allowing a single global image registration and a unified couch shift correction applied only longitudinally.To evaluate performance, maximum deviations from the mean displacement for each axis using the conventional method were compared with those obtained using the CBCTExtended technique.
Conclusion: IMPT delivered with BH technique for mediastinal lymphoma is safe and clinically effective, achieving excellent oncologic outcomes with minimal acute and late toxicities. Despite the limited sample size, these findings support its potential role as a safe treatment option for patients with long-term survival expectancy and where simultaneous dose minimization to heart, lungs and breast tissue is difficult to achieve with photons. References: 1. Technical challenges in the treatment of mediastinal lymphomas by proton pencil beam scanning and deep inspiration breath-hold (Roberto Righetto, Francesco Fracchiolla, Lamberto Widesott, Stefano Lorentini, Francesco Dionisi, Barbara Rombi, Daniele Scartoni, Sabina Vennarini, Marco Schwarz, Paolo Farace) Keywords: Mediastinal lymphoma, proton therapy Digital Poster 4921 Extended CBCT in Total Body Irradiation (TBI): Practical Guide and Comparison with the Conventional Technique ABEL RODRIGUEZ ARANDA 1 , ADRIAN ANDRADES 2 1 Medical-Physics, Lanzarote University Hospital, Arrecife, Spain. 2 Raiotherapy, Las Palmas University Hospital, Las Palmas, Spain
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