Dosimetric Validation of a System to Treat Moving Tumors Using Scanned Ion Beams That Are Synchronized With Anatomical Motion.

Michelle Lis,Athena Paz,Wayne Newhauser,Marco Donetti,Moritz Wolf,Timo Steinsberger,Christian Graeff

doi:10.3389/fonc.2021.712126

Abstract

PurposeThe purpose of this study was to validate the dosimetric performance of scanned ion beam deliveries with motion-synchronization to heterogenous targets.MethodsA 4D library of treatment plans, comprised of up to 10 3D sub-plans, was created with robust and conventional 4D optimization methods. Each sub-plan corresponded to one phase of periodic target motion. The plan libraries were delivered to a test phantom, comprising plastic slabs, dosimeters, and heterogenous phantoms. This phantom emulated range changes that occur when treating moving tumors. Similar treatment plans, but without motion synchronization, were also delivered to a test phantom with a stationary target and to a moving target; these were used to assess how the target motion degrades the quality of dose distributions and the extent to which motion synchronization can improve dosimetric quality. The accuracy of calculated dose distributions was verified by comparison with corresponding measurements. Comparisons utilized the gamma index analysis method. Plan quality was assessed based on conformity, dose coverage, overdose, and homogeneity values, each extracted from calculated dose distributions.ResultsHigh pass rates for the gamma index analysis confirmed that the methods used to calculate and reconstruct dose distributions were sufficiently accurate for the purposes of this study. Calculated and reconstructed dose distributions revealed that the motion-synchronized and static deliveries exhibited similar quality in terms of dose coverage, overdose, and homogeneity for all deliveries considered. Motion-synchronization substantially improved conformity in deliveries with moving targets. Importantly, measurements at multiple locations within the target also confirmed that the motion-synchronized delivery system satisfactorily compensated for changes in beam range caused by the phantom motion. Specifically, the overall planning and delivery approach achieved the desired dose distribution by avoiding range undershoots and overshoots caused by tumor motion.ConclusionsWe validated a dose delivery system that synchronizes the movement of the ion beam to that of a moving target in a test phantom. Measured and calculated dose distributions revealed that this system satisfactorily compensated for target motion in the presence of beam range changes due to target motion. The implication of this finding is that the prototype system is suitable for additional preclinical research studies, such as irregular anatomic motion.

Highlights

Proton and ion therapy provide conformal dose distributions for static targets, and in the past few decades, have emerged as a formidable alternative to photon therapy
This result confirms the suitability of the method for reconstructing dose distribution for the main purpose of this study, which is to assess the quality of dose distributions delivered by various techniques
The 10-phase multi-phase 4D (MP4D) approach produced hotspots in the clinical target volume (CTV) that were < 103% of the prescribed absorbed dose. These findings suggest that the 10phase MP4D approach provides good dosimetric quality that closely approaches the quality that was achieved for statictarget deliveries

Summary

Introduction

Proton and ion therapy provide conformal dose distributions for static targets, and in the past few decades, have emerged as a formidable alternative to photon therapy. Ion therapies have mainly been used to treat static tumors, including several in the regions of the head and neck region, cranium, retina, and the spine, with high conformity [1, 2], resulting in reduced toxicities and tumor recurrence [3]. Conformal treatments have been shown to be partially effective in reducing complications associated with radiotherapy of moving tumors, such as nonsmall cell lung cancer, including pneumonitis and cardiac complications [4, 5]. Contemporary ion beam therapies for thoracic tumors still have high complication rates and low survival rates [6]. Additional unmet clinical needs include shorter treatment times and streamlined patientspecific quality assurance procedures. It is imperative to develop treatment methods that can meet these clinical needs

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