Abstract
PurposeThe aim of this work is to describe the clinical implementation of respiratory‐gated spot‐scanning proton therapy (SSPT) for the treatment of thoracic and abdominal moving targets. The experience of our institution is summarized, from initial acceptance and commissioning tests to the development of standard clinical operating procedures for simulation, motion assessment, motion mitigation, treatment planning, and gated SSPT treatment delivery.Materials and methodsA custom respiratory gating interface incorporating the Real‐Time Position Management System (RPM, Varian Medical Systems, Inc., Palo Alto, CA, USA) was developed in‐house for our synchrotron‐based delivery system. To assess gating performance, a motion phantom and radiochromic films were used to compare gated vs nongated delivery. Site‐specific treatment planning protocols and conservative motion cutoffs were developed, allowing for free‐breathing (FB), breath‐holding (BH), or phase‐gating (Ph‐G). Room usage efficiency of BH and Ph‐G treatments was retrospectively evaluated using beam delivery data retrieved from our record and verify system and DICOM files from patient‐specific quality assurance (QA) procedures.ResultsMore than 70 patients were treated using active motion management between the launch of our motion mitigation program in October 2015 and the end date of data collection of this study in January 2018. During acceptance procedures, we found that overall system latency is clinically‐suitable for Ph‐G. Regarding room usage efficiency, the average number of energy layers delivered per minute was <10 for Ph‐G, 10‐15 for BH and ≥15 for FB, making Ph‐G the slowest treatment modality. When comparing to continuous delivery measured during pretreatment QA procedures, the median values of BH treatment time were extended from 6.6 to 9.3 min (+48%). Ph‐G treatments were extended from 7.3 to 13.0 min (+82%).ConclusionsActive motion management has been crucial to the overall success of our SSPT program. Nevertheless, our conservative approach has come with an efficiency cost that is more noticeable in Ph‐G treatments and should be considered in decision‐making.
Highlights
A four‐dimensional computed tomography (4D‐CT) study obtained at time of treatment simulation, used to approximate motion throughout treatment, is the current gold standard for motion assessment for spot-scanning proton therapy (SSPT)
Using the 4D-CT to define a geometric uncertainty margin for targets and organ at risks (OARs) is typically not adequate for SSPT given that (a) protons are extremely sensitive to heterogeneities in their path[12] and (b) tumor motion in the context of an energetically- and spatially sequential SSPT delivery can result in dosimetric patterns of constructive and destructive interference — so called “motion interplay”
After developing and testing an in‐house hardware solution for motion management, we subsequently established what we view as conservative decision‐ making criteria to ensure dosimetrically acceptable outcomes when dealing with moving targets
Summary
A four‐dimensional computed tomography (4D‐CT) study obtained at time of treatment simulation, used to approximate motion throughout treatment, is the current gold standard for motion assessment for spot-scanning proton therapy (SSPT). Motion interplay in SSPT can occur both as a consequence of target motion (perpendicular to beam direction) and as a consequence of WET changes along the beam path associated with motion — that is, volumetric interplay.[7] The overall magnitude of the associated dose perturbation depends on the target motion dynamics, spot delivery/ timing parameters, and their mutual degree of synchronization. Other related motion management strategies that can mitigate interplay include: aligning the preferential directionality of spot scanning with the principle (cranial‐caudal) motion direction; the use of fractionation[11]; and reduction of target motion through actively gated beam delivery or some form of mechanical restrictions such as compression.[12,13] Lastly, not all SSPT optimization approaches are equivalent in terms of motion robustness. Per‐field dose distributions derived from Multi‐field Optimization (MFO) tend to be more heterogeneous than SFO fields and are, more sensitive to anatomical variation, inclusive of motion‐related variation
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