S2278
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Material/Methods: A simulation framework was designed to evaluate, fraction by fraction, the CTV motion vs a proposed PTV margin and decision support threshold. The simulation was driven by retrospective data from
imaging. An online decision-support strategy enhanced workflow efficiency by minimizing the frequency of additional positional adaptations, while the derived margins ensured adequate CTV coverage. Keywords: MR-Linac, PTV, margin
previously treated patients:186 recorded displacements dP → V between planning and verification images quantified using 3D rigid
registration 193 intrafraction motion traces derived from 2D cine MR images, acquired at 5Hz in three orthogonal planes, quantified by 2D rigid image registration. Simulation settings included a proposed PTV margin and a decision threshold distance dTH for triggering a secondary ATP. A smaller dTH increases ATP frequency but allows for smaller margins. Constraints were:Secondary ATP frequency < 10%CTV inside PTV for ≥ 90% of fractionsEach simulated fraction followed these steps (see Figure 1): Sample a dP → VIf dP → V > dTH, perform ATP and therefore set dP → V = 0. Sample CTV motion trace and offset it by dP → VCheck if CTV motion smaller than the PTV margin (95% of the treatment time)For each margin and dTHcombination, 10000 fractions were simulated. An iterative procedure increased dTH and margin until ATP frequency and success rate met criteria.
Results: The simulation results showed that an isotropic dTH resulted in 7% frequency of secondary ATP. The required PTV margin for this dTH was (3, 3.5), (5, 5), (5, 5) mm in the L-R, A-P, S-I directions respectively. The new clinical PTV margins were chosen as (4, 4), (5, 5), (5, 5) mm to keep margins at integer mm and be symmetrical in L-R. An isotropic dTH7% secondary ATP frequency. The required PTV margin for this threshold were:L-R: (3, 3.5) mmA-P: (5, 5) mmS-I: (5, 5) mmFinal clinical margins were set to (4, 4), (5, 5), (5, 5) mm to maintain integer values and symmetry in L-R. Conclusion: Reduced PTV margins for ultra-hypo-fractionated prostate MR-Linac treatments were established based on intrafraction motion analysis using cine MR
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