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  4. Online-Adaptive Proton Therapy: Assessing Accuracy of Robust Dose Restoration in Lung Patients

Online-Adaptive Proton Therapy: Assessing Accuracy of Robust Dose Restoration in Lung Patients

E Borderias Villarroel1*, K Bernatowicz1 , X Geets1 , E Sterpin1,2 , (1) Universite Catholique de Louvain, Brussels, (2) Katholieke Universiteit Leuven, Leuven

Presentations

(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: Density changes in the patient may alter planned proton ranges and compromise plan quality. To reduce proton range uncertainties, dose restoration (DR) uses isodose contours generated from the initial dose and associated weighted objectives to reoptimize the plan and reproduce the initial dose in repeated CTs. The objective of this work was to test the performance of DR in lung cancer patients.

Methods: Data included a planning CT and two repeated 4D-CTs (1rCT, 2rCT) for fourteen patients. The prescribed dose to target was Dp=66 Gy (33x2 Gy). Robust optimization on the CTV was performed in RayStation, with setup errors of 5 mm, range errors of 3%, and three phases of the respiratory cycle (end-exhale, end-inhale, and MidP). Robust dose restoration using the same robustness parameters as in planning was performed in the two series of repeated 4D-CTs.

Results: The evaluation of initial plans on repeated CTs showed large dose distortions, which were substantially reduced after restoration. Looking at the failure of the CTV coverage criteria D95%>95%Dp (D95%>62,7 Gy) in the MidP CT or inhale or exhale phases, 36% (5/14) and 69% (9/13) of the cases needed to be restored in the first (1rCT) and the second (2rCT) repeated 4D-CTs, respectively. After restoration, the mean D95(nominal) is increased from 63.51±1.14 to 65.02±0.39 Gy, and from 53.16±14.55 to 65.05±0.34 Gy for the two repeated 4D-CT series, respectively. Both mean D95 values after restoration are very close to the mean D95 in the planned doses (65.38 ± 0.37 Gy).

Conclusion: Restoring clinically-approved dose distribution on repeated CTs does not require new ROI segmentation and is compatible with an online adaptive workflow. With DR, we can accurately reproduce the initial dose, despite density changes, ensuring stable DVH-based parameters. Hot spots and underdosage in the CTV can be corrected by implementing DR.

Keywords

Protons, Lung

Taxonomy

TH- External Beam- Particle therapy: Proton therapy - adaptive therapy

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