Clinical implementation of respiratory-gated spot-scanning proton therapy: An efficiency analysis of active motion management

Abstract

Purpose: The 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 methods: A 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.

Results: More 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%).

Conclusions: Active 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.

Keywords: breath-holding; motion management; phase-gating; proton therapy; spot-scanning.

Figure 1

Respiratory gating interface system diagram. The interface provides health logic (i.e., ready) signals to the real‐time position management system (RPM) software via the Varian “Gating Switch Box,” and in turn, our interface forwards gating (beam‐on/beam‐off) logic signals it receives from the RPM (via the same Gating Switch Box) to the synchrotron delivery/control system.

Figure 2

Simplified decision‐making scheme for patients treated with respiratory‐gated spot‐scanning proton therapy. In addition, repainted delivery is used in cases that have ≥5 mm residual motion and, typically these plans are created with 2–4 fields.

Figure 3

General decision‐making scheme for IGRT using our standard stereoscopic kV imaging system. X‐Ray imaging always starts at our defined “setup” couch angle (270°); verification x‐ray imaging is typically performed at the actual treated couch angles. Issues with localization encountered during treatment may require us to return to couch angle 270°.

Figure 4

(a) Number of patients treated under three different motion management strategies involving the real‐time position management system, namely breath‐hold, phase‐gating, and coached shallow breathing. (b) Distribution of sites treated using these types of motion management.

Figure 5

Film analysis of gated delivery test with a motion phantom. The position of the BBs (drawn “dots”) was used for alignment. The white line indicates the location where 1D profiles were extracted — to avoid interference with the drawn dots.

Figure 6

Number of repaints per spot normalized to the total number of unique spot positions. Energy layers were broken down in two groups — distal (1/3) and proximal (2/3). The histograms were created using DICOM RTPlan files of treatments were “Max‐MU” based rescanning was used.

Figure 7

Delivery time data retrieved from our Record & Verify system for patients treated using some form of motion management. The box plots show a comparison of treatment efficiency for sites treated with three different modalities: free‐breathing + repainting (FB), breath‐hold (BH), and phase‐based gating. The boxplots present the data distribution as follows: maximum, 75% quartile, median, 25% quartile and minimum. Outliers are plotted as single markers.

Figure 8

Comparison of treatment time and continuous delivery mode (QA) using boxplots as previously described (Fig. 7). The sites in the phase‐gated bin for various sites include: liver, esophagus, lung, pancreas, and bile‐duct. The results are presented in two formats: (a) contrasting absolute time of each delivery mode and (b) ratios between treatment time and continuous delivery.