Radiation and Cancer Physics

PD 12 - Physics 6 - Poster Discussion - Adaptive Planning/Delivery and Motion

1103 - Comprehensive Evaluation for 4D Scanned Proton Plans

Tuesday, October 23
3:09 PM - 3:15 PM
Location: Room 217 C/D

Comprehensive Evaluation for 4D Scanned Proton Plans
C. O. Ribeiro1, A. Meijers1, E. W. Korevaar1, J. A. Langendijk2, and A. Knopf1; 1Department of Radiation Oncology, University Medical Center Groningen, University of Groningen, Groningen, Netherlands, 2University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, Netherlands

Purpose/Objective(s): Due to the high conformity achievable with pencil beam scanned proton therapy (PBS-PT), and consequently greater clinical benefits when compared to conventional radiotherapy, moving targets are future indications for this modality. However, those tumors are still not widely treated at PBS-PT facilities since pronounced differences between planned and delivered doses to the target can occur. Effects that can influence the robustness of 4D PBS-PT treatment plans are: machine uncertainties, setup and range errors, anatomic variations throughout treatment, and interplay effects, which occur due to the interference of the time structure of treatment delivery and target motion. The aim of this proof of concept study is the development and application of a method that realistically evaluates 4D PBS-PT deliveries.

Materials/Methods: By using scripting capabilities of the treatment planning system, our method evaluates 4D PBS-PT plans taking into account: (1) setup and range errors (by shifting the isocenter and applying a 3 % scaling to CT intensity values: 14 scenarios representing 14 possible treatment courses are simulated); and (2) machine uncertainties, (3) anatomy changes, (4) breathing motion, and (5) interplay effects (by calculating subplan doses on particular 4DCT phases and performing 4D dose accumulation in the planning 4DCT and in repeated 4DCTs). The subplans are split from the nominal plan using a dedicated script that retrieves information from the delivery machine log file (spot position, dose, and energy and the absolute time of delivery). Robustly optimized plans using five times layered rescanning for a lung (non-small cell lung cancer stage III) and an esophageal cancer patient (with a nominal dose of 60 Gy and 41.40 Gy to the CTV respectively) were created. These plans were delivered in dry runs to obtain log files at our facility. For both patients, a planning 4DCT and five weekly repeated 4DCT scans were available. By using the obtained log files and all available 4DCT scans, both 4D PBS-PT plans were evaluated using the described method. To evaluate robustness, the voxel-wise minimum dose distribution of the CTV (minimum dose obtained from all the scenarios in each voxel of this structure) was analysed in the averaged planning CT.

Results: Differences between nominal and voxel-wise minimum dose distributions were shown. V95(CTV) dropped from 100 % (nominal) to 99.94 % and 97.66 % for the lung and esophageal cancer patients respectively, when all 4D PBS-PT delivery disturbing effects were considered.

Conclusion: We developed a comprehensive method for robustness analysis of 4D PBS-PT plans. The power of such evaluation was demonstrated in lung and esophageal cancer patients, and this study is currently being extended for a broader patient population. In clinical practice, this method will trigger robustness settings for plan optimization, and also the selection and application of motion mitigation techniques.

Author Disclosure: C.O. Ribeiro: None. A. Meijers: None. E.W. Korevaar: None. J.A. Langendijk: Honoraria; IBA. Consultant; IBA. Research collaboration with IBA; IBA. Research collaboration with RaySearch; RaySearch. Research collaboration with MIRADA; MIRADA. A. Knopf: None.

Cassia Ribeiro, MSc

Disclosure:
No relationships to disclose.

Biography:
Cássia O. Ribeiro, MSc
PhD researcher
Department of Radiation Oncology, University Medical Center Groningen, University of Groningen, The Netherlands
Fields of research: Assessment of dosimetric errors induced by deformable image registration methods in 4D pencil beam scanned proton treatment planning for liver tumors; Motion management for the treatment of moving tumours with pencil beam scanned proton therapy.

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