ESTRO 2024 - Abstract Book

S4722

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

protons and compare the techniques in terms of organs at risk sparing. Secondly, our purpose was to investigate the potential benefits (if present) of a cardiac gated delivery for proton plans.

Material/Methods:

Eleven consecutive patients received two Deep Inspiration Breath Hold (DIBH) ECG-gated CT scans with and without contrast medium. The field of view (FOV) of the gated images was restricted to the heart only, to minimize the dose delivered during the radiological examination. We reconstructed two cardiac phases at 30% and 80% of the cardiac R-R cycle: the systolic and diastolic phase respectively. In addition, one DIBH scan of the whole thorax was acquired for treatment planning, without cardiac gating. We obtained two synthetic whole thorax and gated CTs as a result of a postprocessing operation consisting in the merging of the whole thorax scan with the gated images. The contouring was performed as follows: an invasive non-fluoroscopic intracardiac mapping, performed during a previous catheter ablation procedure, was used to define the target (CTV), then contoured in each cardiac phase. Intra and extracardiac organs at risk (OAR) were defined for both the cardiac phases, based on recent consensus guidelines [1]. The ITV, which is the result of the Boolean union of the diastolic and systolic phase CTVs, was contoured as the target for the photon optimization. Next, we computed two robustly optimized proton plans: the first one, which simulated a gated delivery scenario, was based on the diastolic phase only, and the second, which included the systolic phase as an additional CT in the robust optimization, simulated a non-gated delivery situation. Photon plans were simulated with the Elekta Monaco TPS, using VMAT with optimized arc aperture. Proton planning was performed using the RayStation TPS, with ±3mm of setup error and 3% of range uncertainty.

Results:

Non-gated proton plans granted a significant reduction in both the maximum and mean dose to cardiac OARs, with comparable target coverage. The implantable cardioverter-defibrillators did not interfere with the planning. Table 1 displays the mean differences (over the eleven patients) between photons and proton plans of the listed cardiac structures.

PHOTONS – PROTONS (NO GATING) Dmean (Gy[RBE]) Dmax (Gy[RBE])

Ascending Aorta

1.39

1.75

Descending Aorta

1.44

4.30

Aortic Arch

0.09

0.16

LAD

0.69

-0.42

LMCA

0.90

0.74

Circumflex Coronary

1.75

2.23

Left Atrium

1.91

1.27

Right Atrium

2.27

4.40

Superior Vena Cava

0.38

1.11

Inferior Vena Cava

4.10

6.32

Aortic valve

2.12

-0.36

Mitralic Valve

2.48

1.82

Tricuspid Valve

0.16

0.81