ESTRO 2024 - Abstract Book

S4320

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

corresponding calculations in a clinical treatment planning system (TPS). In the TPS, the translational motion was emulated as multiple isocenter shifts [3].

Results:

The dominant translational motion was observed in cranio-caudal (prostate:[-5.9, +8.4]mm; LN:[-9.9;+11.0]mm) and anterior-posterior direction (prostate:[-5.6; +6.9]mm; LN:[-9.6; +11.0]mm). The pitch was the largest observed rotation (prostate: [-22.5; +25.2]deg; LN: [-3.9; +5.5]deg). For single fractions, the prostate CTV ΔD 99.5% range was [-16.2,+2.5]% and the LN CTV ΔD 98% range [-19.8, +1.2]%-points. After averaging over all imaged fractions, these decreased to [-3.0, +1.7]%-points for the prostate CTV and [-3.1, +0.9]%-points for the LN CTV (Figure 1).

The geometrical errors for both prostate and LN mainly occurred along a cranial-anterior axis with high correlation between longitudinal and vertical displacements (Figure 2). For the prostate, even small caudal-posterior displacements decreased the CTV D 99.5% due to a steep dose gradient towards the rectum (Figure 2a, bottom left quadrant). Displacements in the opposite anterior-cranial direction in some cases slightly increased D 99.5% due to CTV motion into a high dose rim near the PTV surface (created by the TPS to increase the dose gradient) or motion away from a low dose region planned to protect the rectum (Figure 2a, top right quadrant). This asymmetry in the dosimetric impact of motion directions was not observed for the LN (Figure 2b).