Technical Note: Comprehensive performance tests of the first clinical real-time motion tracking and compensation system using MLC and jaws

Abstract

Purpose: To evaluate the performance of the first clinical real-time motion tracking and compensation system using multileaf collimator (MLC) and jaws during helical tomotherapy delivery.

Methods: Appropriate mechanical and dosimetry tests were performed on the first clinical real-time motion tracking system (Synchrony on Radixact, Accuray Inc) recently installed in our institution. kV radiography dose was measured by CTDIw using a pencil chamber. Changes of beam characteristics with jaw offset and MLC leaf shift were evaluated. Various dosimeters and phantoms including A1SL ion chamber (Standard Imaging), Gafchromic EBT3 films (Ashland), TomoPhantom (Med Cal), ArcCheck (Sun Nuclear), Delta4 (ScandiDos), with fiducial or high contrast inserts, placed on two dynamical motion platforms (CIRS dynamic motion-CIRS, Hexamotion-ScandiDos), were used to assess the dosimetric accuracy of the available Synchrony modalities: fiducial tracking with nonrespiratory motion (FNR), fiducial tracking with respiratory modeling (FR), and fiducial free (e.g., lung tumor tracking) with respiratory modeling (FFR). Motion detection accuracy of a tracking target, defined as the difference between the predicted and instructed target positions, was evaluated with the root mean square (RMS). The dose accuracy of motion compensation was evaluated by verifying the dose output constancy and by comparing measured and planned (predicted) three-dimensional (3D) dose distributions based on gamma analysis.

Results: The measured CTDIw for a single radiograph with a 120 kVp and 1.6 mAs protocol was 0.084 mGy, implying a low imaging dose of 8.4 mGy for a typical Synchrony motion tracking fraction with 100 radiographs. The dosimetric effect of the jaw swing or MLC leaf shift was minimal on depth dose (<0.5%) and was <2% on both beam profile width and output for typical motions. The motion detection accuracies, that is, RMS, were 0.84, 1.13, and 0.48 mm for FNR, FR, and FFR, respectively, well within the 1.5 mm recommended tolerance. Dose constancy with Synchrony was found to be within 2%. The gamma passing rates of 3D dose measurements for a variety of Synchrony plans were well within the acceptable level.

Conclusions: The motion tracking and compensation using kV radiography, MLC shifting, and jaw swing during helical tomotherapy delivery was tested to be mechanically and dosimetrically accurate for clinical use.

Keywords: motion tracking; synchrony; tomotherapy.

Fig. 1

Phantom setup for CTDI measurements. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 2

(a) Setup for longitudinal profile scans with blocks of rectangular virtual water. (b) Setup for ion chamber measurement. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 3

Delivery setup with Delta4 Phantom + on a HexaMotion six‐dimensional motion management system. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 4

Variation of jaw offset peak factors (JOPFs) (a) and jaw offset width factors (JOWFs) (b) with depths. JOPF/JOWF are the ratios of the profile peak/width value when the jaw is shifted to that when the jaw is centered. (c) Calculated and measured LSPF values at 15 and 100 mm depths for 1 cm jaw with −10 mm lateral offset. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 5

Comparison of instructed and predicted tracking target positions in X, Y, and Z, together with the distribution of three‐dimensional distance between instructed and predicted target positions for the fiducial tracking. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 6

Comparison of instructed and predicted tracking target positions in X, Y, and Z, together with the distribution of three‐dimensional distance between instructed and predicted target positions for the lung tumor tracking. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 7

The measured jaw center and predicted target Y positions from the FFR plan delivery. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 8

Comparison of IEC X (a) and IEC Y (b) profiles measured with films in the following deliveries: Synchrony with ±10 mm target motion, non‐Synchrony, non‐Synchrony with ±10 mm target motion, Synchrony with ±10 mm target motion but the surrogate with a phase shift of 25%, Synchrony ±10 mm target motion but the surrogate with ±15 mm motion. Big motions were in Y direction. [Color figure can be viewed at wileyonlinelibrary.com]

Fig. 9

Comparisons of two‐dimensional dose and profile distributions measured with films for the FNR plan deliveries. (a) No motion compensation vs no motion, (b) Motion compensated vs no motion, (c) X profiles, and (d): Y profiles. The profiles were for fiducial tracking with ±10 mm irregular target motion, non‐Synchrony and non‐Synchrony with ±10 mm irregular target motion. Location of profiles is indicated with horizontal solid green (X) and vertical dot dashed cyan (Y) lines on top. [Color figure can be viewed at wileyonlinelibrary.com]