Accuracy of TomoEDGE dynamic jaw field widths

Abstract Dynamic jaw delivery on the TomoTherapy H‐series platform, entitled TomoEDGE™, is an effective tool to decrease the patient dose along the superior and inferior edges of the treatment target. The aperture of the TomoTherapy jaws, that is, field width (FW), defines the longitudinal dose profile. A consistent FW dose profile is an important quantity for accurate and reproducible dose delivery in TomoTherapy. To date, no evaluation has been made of the accuracy and precision of the dose profiles produced by dynamic jaws. This study aims to provide a long‐term evaluation of the dynamic jaw FW dose profiles obtained on TomoTherapy utilizing the TomoTherapy Quality Assurance procedure (TQA). A total of 840 dose profiles were measured during 84 TQA procedures, performed over a 2‐yr period. The full width at half maximum (FWHM) and constancy of the FW dose profile measurements were analyzed and compared with the tolerances proposed by AAPM Task Group 148 (TG‐148) and those used by the manufacturer. The FWHM evaluation showed that the FWs > 2.0 cm respect the TG‐148 tolerance of 1%, while the asymmetric FWs ≤ 2.0 cm were outside the limit in 17.3% of measurements. Constancy results evaluated along the full profiles showed that 95.2% of measurements were within 3% of the baseline for symmetric FWs and 94.8% of measurements were within 4% of the baseline for asymmetric FWs. In conclusion, the analysis confirms the accuracy and precision of TomoEDGE™ technology in jaw positioning. This study has identified the potential to establish an appropriate QA tolerance for the asymmetric FWs used in dynamic jaw movement. Finally, the clinical significance of the observed discrepancies should be studied further to understand the dosimetric effect on patient treatments.

reducing treatment beam-on times. In TomoEDGE™, the front and back secondary collimators, known as jaws, move individually in the Y-axis direction (ie, longitudinal couch direction) as defined by the International Electrotechnical Commission (IEC) coordinate system (IEC Y). A load-side encoder for each jaw provides independent absolute position and velocity feedback. Thus, a sliding-window open-and-close jaw movement is performed at the superior and inferior aspects of the treatment target during irradiation. The aperture of the front and back jaws defines the longitudinal beam dose profile, which TomoTherapy also calls a field width (FW). Dynamic jaw motion requires the introduction of a new beam model with additional FWs for accurate dose calculation. In fact, the TomoEDGE™ beam model uses 10 longitudinal beam profiles, four symmetric and six asymmetric, while the original static jaw beam model required only three symmetric profiles (Fig. 1). Consistent FW profiles, symmetric and asymmetric, are necessary for accurate and reproducible dynamic dose delivery. It has been shown that the full width at half maximum (FWHM) of the FW profiles was an important variable directly linked to the delivered dose during TomoTherapy treatments. 2 The AAPM task group on quality assurance of TomoTherapy notes the importance of measured FWHMs and recommends a careful monitoring of the parameter to ensure agreement with the beam model. 3 To date, clinical advantages of TomoEDGE™ have been studied and reported, demonstrating a decrease in dose penumbra longitudinal to the target. 4,5 Furthermore, reduction in clinical treatment times with the use of TomoEDGE™ has been reported in multiple studies. 4,6 However, no studies have evaluated the TomoEDGE™ technology concerning jaw positioning accuracy and precision.
The aim of this work was to evaluate the performance of the dynamic jaws by measuring all 10 FWs commissioned on TomoEDGE™ on an approximate weekly basis for a 2-yr period. A comparison of FWHM was used to estimate the accuracy and precision of jaw positioning, while beam profile constancy was measured to increase the evaluation sensitivity to changes in beam shape. Ultimately, these data provide a useful perspective for a critical assessment of both the tolerance values indicated by the AAPM TG-148 report on quality assurance of TomoTherapy and those used by the manufacturer.

| MATERIALS AND METHODS
This study was performed on a TomoHDA machine and used the TQA Field Width Dynamic Jaws procedure, which sequentially automates open-field measurement of all 10 longitudinal FW profiles (see example in Fig. 2). An Exradin A1sl ion chamber (IC) (Standard Imaging, Middleton, WI) was inserted into a solid water slab phantom at a depth of 1.5 cm while the phantom was placed at a source-to-surface distance of 85 cm. The couch height was adjusted within the bore to ensure alignment of the solid water slab with the green lasers and compensate for sag. The same ion chamber and solid water slabs were used throughout the study to reduce the intrinsic variability in measurements. The longitudinal profiles were acquired with a couch speed of 1 mm/s at a sample rate of 100 ms using a TomoElectrometer in conjunction with TEMS software (Accuray Inc., Madison, WI). The procedure was run 84 times, collecting 840 FW profiles, spanning a 2-yr period from June 2013 to June 2015.
Throughout the period of the study, machine QA related to mechanical alignment and beam parameters was performed rigorously in accordance with the frequency recommended in TG-148.
Matlab software was used to analyze the measured FW dose profile data and to compare with those of the TomoEDGE™ beam model reference profiles, that is, the "gold standard" (GS), provided by the manufacturer. Measured data were normalized to their maximum values and the FWHM was calculated for each profile. Specifically, longitudinal positions of half-maximum values were interpolated linearly between the 0.1 mm measurement steps to increase sensitivity of the FWHM estimate. The comparison of the FWHM results was interpreted using the 1% tolerance limit recommended in the AAPM TG-148 report.
In addition, measured FW datasets were resampled using the same y-positions as GS reference data for beam profile constancy calculation. Beam profile constancy of the FWs was evaluated as described in Section II.A of TG-142 report on quality assurance of medical accelerators 7 using the GS profiles as the baseline for comparison, that is, where MP i and GSP i are off-axis ratios at measured and GS refer-

| RESULTS
An example of a FWHM timeline trend is shown in Fig. 4 were unable to consistently satisfy the AAPM TG-148 1% recommendation in one quarter of the measurements (Fig. 5).   T A B L E 2 Field width constancy (%). Results for the beam profile constancy (mean ± std) of the profile measurements with the gold standard reference FW profiles as the baseline. Full profile constancy was evaluated between the 5% profile values and peak constancy was evaluated between the 95% values.