S2197
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Category: Physics: Intra-fraction motion management and real-time adaptive radiotherapy Digital Poster 20 Evaluation of the intra-fraction prostate shifts during online adaptive SBRT JUAN-FRANCISCO CALVO-ORTEGA 1,2 , CORAL LAOSA- BELLO 1 , JOSÉ TORICES-CABALLERO 1 , SANDRA MORAGUES-FEMENIA 1 1 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD BARCELONA, BARCELONA, Spain. 2 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD, MÁLAGA, Spain Purpose/Objective: To evaluate the intra-fraction prostate motion during the online adaptive SBRT procedure used in our department, and the validity of our current CTV-PTV margin. Material/Methods: SBRT (5 x 7.25 Gy) for localized prostate cancer is adapted on every treatment fraction following the CBCT-based online adaptive procedure (Figure 1) developed by our group.1 Forty treated patients were randomly selected. Rectal spacer or implanted fiducial markers were not used. The planning target volume (PTV) was created by expanding the CTV (prostate) with margins of 2 mm in left-right direction, and 4 mm in anterior-posterior and superior-inferior directions. SBRT plans were designed with the Eclipse v. 16.1 treatment planning system, using a single VMAT with 6MV FFF photons from a Varian TrueBeam linac. As shown in Figure 1, a CBCT2 scan was acquired before delivering the adapted plan created on the CBCT1 scan. A post-treatment CBCT3 scan was acquired for intra-fraction prostate motion analysis. A total of 200 CBCT2/CBCT3 pairs were collected. Two types of CBCT2/CBCT3 registrations were performed. First, a 6D rigid registration focused on the pelvis bones was performed. Then, a manual CBCT2/CBCT3 registration focused on the prostate gland was done. From both registrations, the intra-fraction prostate motion (displacements and rotations) was calculated as the difference between the CBCT3 and CBCT2 images. In addition, the time elapsed between the CBCT2 and CBCT3 images was recorded for each fraction.
Results: Figure 2 shows the prostate shifts observed during 200 fractions. All detected displacements were < 3 mm, except an outlier anterior displacement of 10.2 mm.The detected intra-fraction prostate displacements (mean ± standard deviation) were: (0.0 ± 0.3) mm, (-0.1 ± 0.9) mm and (0.1 ± 0.4) mm in the left-right, anterior-posterior and superior-inferior directions, respectively. The mean 3D vector was (0.5 ± 0.5) mm.The detected intra-fraction pitch, swivel and roll rotations (mean ± standard deviation) of the prostate were: (0.1 ± 0.3) deg, (0.0 ± 0.2) deg and (0.0 ± 0.2) deg, respectively.The 99th percentiles of prostate absolute displacements were 0.8 mm in left-right, 2.3 mm in anterior-posterior, and 2.1 mm in superior- inferior directions. The time elapsed between the CBCT2 and CBCT3 images was (6.1 ± 1.1) minutes.
Conclusion: Small intra-fraction prostate shifts were observed during the analyzed SBRT fractions. Sub-millimeter mean displacements were noted, as already reported by other authors.1 Clinical CTV-PTV margins currently used in our clinical practice seem adequate. References: 1Radiat Oncol . 2014 Nov 28:9:262. Keywords: SBRT, intra-fraction, prostate
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