A Modular System for Treating Moving Anatomical Targets With Scanned Ion Beams at Multiple Facilities: Pre-Clinical Testing for Quality and Safety of Beam Delivery

Management of Motion and Anatomical Variations in Charged Particle Therapy: Past, Present, and Into the Future

The major aim of radiation therapy is to provide curative or palliative treatment to cancerous malignancies while minimizing damage to healthy tissues. Charged particle radiotherapy utilizing carbon ions or protons is uniquely suited for this task due to its ability to achieve highly conformal dose distributions around the tumor volume. For these treatment modalities, uncertainties in the localization of patient anatomy due to inter- and intra-fractional motion present a heightened risk of undesired dose delivery. A diverse range of mitigation strategies have been developed and clinically implemented in various disease sites to monitor and correct for patient motion, but much work remains. This review provides an overview of current clinical practices for inter and intra-fractional motion management in charged particle therapy, including motion control, current imaging and motion tracking modalities, as well as treatment planning and delivery techniques. We also cover progress to date on emerging technologies including particle-based radiography imaging, novel treatment delivery methods such as tumor tracking and FLASH, and artificial intelligence and discuss their potential impact towards improving or increasing the challenge of motion mitigation in charged particle therapy.

Preliminary tests of dosimetric quality and projected therapeutic outcomes of multi-phase 4D radiotherapy with proton and carbon ion beams

Objective. The purpose of this study was to perform preliminary pre-clinical tests to compare the dosimetric quality of two approaches to treating moving tumors with ion beams: synchronously delivering the beam with the motion of a moving planning target volume (PTV) using the recently developed multi-phase 4D dose delivery (MP4D) approach, and asynchronously delivering the ion beam to a motion-encompassing internal tumor volume (ITV) combined with rescanning.Approach. We created 4D optimized treatment plans with proton and carbon ion beams for two patients who had previously received treatment for non-small cell lung cancer. For each patient, we created several treatment plans, using approaches with and without motion mitigation: MP4D, ITV with rescanning, static deliveries to a stationary PTV, and deliveries to a moving tumor without motion compensation. Two sets of plans were optimized with margins or robust uncertainty scenarios. Each treatment plan was delivered using a recently-developed motion-synchronized dose delivery system (M-DDS); dose distributions in water were compared to measurements using gamma index analysis to confirm the accuracy of the calculations. Reconstructed dose distributions on the patient CT were analyzed to assess the dosimetric quality of the deliveries (conformity, uniformity, tumor coverage, and extent of hotspots).Main results. Gamma index analysis pass rates confirmed the accuracy of dose calculations. Dose coverage was >95% for all static and MP4D treatments. The best conformity and the lowest lung doses were achieved with MP4D deliveries. Robust optimization led to higher lung doses compared to conventional optimization for ITV deliveries, but not for MP4D deliveries.Significance. We compared dosimetric quality for two approaches to treating moving tumors with ion beams. Our findings suggest that the MP4D approach, using an M-DDS, provides conformal motion mitigation, with full target coverage and lower OAR doses.

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

Purpose

The purpose of this study was to validate the dosimetric performance of scanned ion beam deliveries with motion-synchronization to heterogenous targets.

Methods

A 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.

Results

High 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.

Conclusions

We 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.