ESTRO 2023 - Abstract Book

S1608

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ESTRO 2023

procedures), but a reduction in terms of dose variation was found for bladder and rectum when adapted plan was used: the whole data are reported in figure 1.

Conclusion The results of this preliminary analysis show that performing daily adaptation in prostate leads to a dosimetric benefit with a statistically significant gain in PTV coverage (V95%), with similar risk in terms of toxicity for surrounding OARs, where similar dose values were observed.

PO-1866 Dose adaptive radiotherapy: Compensating for target motion without PTV margins

E. van der Bijl 1 , J. Sonke 2 , P. Remeijer 2 , U. van der Heide 2 , T. Janssen 2

1 Radboudumc, Department of Radiation Oncology, Nijmegen, The Netherlands; 2 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The introduction of daily online plan adaptation is motivated by its potential to reduce planning target volume (PTV) margins. However, daily online adaptation also allows the dose distribution to be adjusted every treatment fraction to compensate for geometrical inaccuracies in the previous fractions. In this work, we studied the potential benefit of daily dose adaptation as an alternative for PTV margins. Materials and Methods We considered two adaptation strategies, where we assume that the accumulated target dose so far is known and that random ( σ ) and systematic ( Σ ) Gaussian geometric errors of the population are known. In both strategies the first N-1 fractions are planned with 0 mm margin, while the last fraction has an isotropic margin of 2.5 √ ( Σ ^2+ σ ^2 ) to ensure geometric coverage during the last fraction. In the first strategy we accumulated the dose after N-1 fractions to determine the required (inhomogeneous and anisotropic) dose distribution in the last fraction, to arrive at the desired dose coverage of the CTV. The second strategy updates the planned dose after each fraction. As a proof of principle, we considered an idealized spherically symmetric target and dose distribution characterized by a Gaussian penumbra σ p=3.2 mm and we simulated N=5 fraction treatments based on Gaussian geometric errors characterized with random error σ ={0.5, 1.0, 1.5, 2.0} mm and systematic error Σ = 1 mm in all directions. We implemented dose accumulation by a subdivision of the CTV in 24 segments with equal solid angles, the local planned dose in each sector is calculated from the minimal accumulated dose in this segment. We assessed the accumulated target coverage, target inhomogeneity and mean dose in a 1 cm ring around the CTV, and compared this with the classical situation with a fixed PTV margin and a homogeneous prescription dose to the PTV.

Results