ESTRO 2024 - Abstract Book

S3643

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

[3] Landelijk Indicatie Protocol Protonentherapie (Versie 2.2) (LIPPv2.2) Hoofd-Halstumoren, Nederslandse Vereniging voor Radiotherapie en Oncologie, 2019

2220

Poster Discussion

Robust optimization of the GTV is able to reduce normal tissue irradiation in lung SBRT

Thomas L Fink 1,2 , Charlotte Kristiansen 1 , Torben S Hansen 1 , Torben F Hansen 1,2 , Rune S Thing 1

1 Department of Oncology, Lillebaelt Hospital, University Hospital of Southern Denmark, Vejle, Denmark. 2 Institute for Regional Health Research, University of Southern Denmark, Odense, Denmark

Purpose/Objective:

When treating lung tumors with stereotactic body radiation therapy in free breathing, sufficient margins have to be added to ensure delivery of the intended dose, but the concept of creating an ITV and adding an extra margin to form a PTV might not create the optimal treatment volume. The concept of robust optimization is widely used for proton therapy planning, but has not gained much ground in photon therapy planning. We examined whether robust optimization of the GTV without creating an ITV or PTV could reduce the normal tissue irradiation compared to our standard treatment planning using ITV and PTV.

Material/Methods:

Forty patients were included in this retrospective dose evaluation study. All patients were treated with lung SBRT with either 66 Gy in 3 fractions or 45 Gy in 3 fractions prescribed to the GTV. The dose prescribed to the PTV was 67% of the GTV dose (45 or 30 Gy, respectively). All patients went through our standard treatment planning: First, a 4D-CT was performed. Treatment was planned on the mid-position respiratory phase. A GTV was contoured on all 10 phases and summed to an ITV before adding a 5mm isotropic margin to create a PTV. For tumors located near the thoracic wall, risk adapted treatment planning ensured that the thoracic wall did not receive more than 45 Gy (aiming for D0.05ccm < 35 Gy). We have no limitations for the maximum dose inside the GTV. Treatment planning was performed in RayStation 11B (RaySearch Laboratories, Stockholm, Sweden) with two partial arcs delivering dose through the diseased lung only.

All patients received our standard SBRT treatment, which was delivered as VMAT on Elekta linacs equipped with the Agility MLC, using a 10 MV FFF beam (Elekta AB, Stockholm, Sweden).

Subsequently a plan using robust optimization was made for each patient. The same delineations of GTV and OAR’s were used and no ITV or PTV were used. Plans were created with robust optimization and then evaluated with the same isocenter position shift of 5mm in each direction. In total 14 scenarios were optimized and evaluated. Additionally, the extreme phases of respiration were included in the optimization and evaluation.

The two plans were compared for each patient for differences in the following parameters:

Plan Monitor Units,

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• Irradiated isodose volumes from 5 to 66 Gy,