A proposed method for linear accelerator photon beam steering using EPID

Abstract Beam steering is the process of calibrating the angle and translational position with which a linear accelerator's (linac's) electron beam strikes the x‐ray target with respect to the collimator rotation axis. The shape of the dose profile is highly dependent on accurate beam steering and is essential for ensuring correct delivery of the radiotherapy treatment plan. Traditional methods of beam steering utilize a scanning water tank phantom that makes the process user‐dependent. This study is the first to provide a methodology for both beam angle steering and beam translational position steering based on EPID imaging of the beam and does not require a phantom. Both the EPID‐based beam angle steering and beam translational steering methods described have been validated against IC Profiler measurement. Wide field symmetry agreement was found between the EPID and IC Profiler to within 0.06 ± 0.14% (1 SD) and 0.32 ± 0.11% (1 SD) for flattened and flattening‐filter‐free (FFF) beams, respectively. For a 1.1% change in symmetry measured by IC Profiler the EPID method agreed to within 0.23%. For beam translational position steering, the EPID method agreed with IC Profiler method to within 0.03 ± 0.05 mm (1 SD) at isocenter. The EPID‐based methods presented are quick to perform, simple, accurate and could easily be integrated with the linac, potentially via the MPC application. The methods have the potential to remove user variability and to standardize the process of beam steering throughout the radiotherapy community.


The EPID can be used with new linac designs such as the Varian
Halcyon, for which it is more difficult to use a tank because of space limitations.
Varian EPIDs have been used previously for beam profile QA. 6,7 However, these methods are based on flood field-corrected EPID images. One of the effects of the flood field is to remove any asymmetry in the beam at the time the flood field is taken. This means that prior to flood field calibration, steering of the beam needs to be performed with an alternate method and subsequent flood field-corrected EPID measurements can only be used as a constancy check. Hence, flood fieldcorrected EPID images are not appropriate for beam steering.
The EPID flood field calibration procedure involves irradiating the whole EPID detector panel. The flood field correction image can be separated into two components: pixel-to-pixel sensitivity variation (variation in pixel signal with uniform input), known henceforth as the Pixel Sensitivity Matrix (PSM); and response of the EPID to the nonuniformity of the beam horns (variation in pixel signal with equal sensitivity), known henceforth as the Beam Response. The Beam Response is exaggerated by the EPID compared to an ion chamber measurement due to the increased response of the EPID to low-energy photons. 8 The resulting exaggerated beam horns are advantageous for beam symmetry measurement as extra sensitivity is provided.
This study builds on research from different projects for a new application; linac photon beam steering using EPID, which has not previously been attempted. The methods presented have all been modified specific to this new application. Specifically, the work of Yaddanapudi et al. 9 provides a method of checking photon beam symmetry using PSM-corrected EPID images for linac acceptance testing purposes and the work of Bin Cai et al. 10 highlights the advantages of PSM-corrected EPID imaging for beam profile analysis. However, in our study, we use a different, simplified method of PSM correction, which makes the process more easily adoptable in a clinical setting. We also evaluate the method against the IC Profiler specifically for the beam angle steering application. Secondly, the work of Chojnowski et al. 11 provides an EPID-based method for checking linac focal spot alignment with collimator rotation axis, but this method can only be used for beam translational position steering when beam angle steering has been performed immediately prior. In Chojnowski's work, beam angle steering is assumed whereas in our study, we provide an EPID-based method of checking beam angle steering so that this assumption does not need to be made. We also further evaluate Chojnowski's method against an IC Profiler method. Thirdly, the work of Greer et al. 12 provides the base theory for determining the PSM (i.e., moving the EPID panel); however, a simplified version of Greer's method is used in this study, which is appropriate to the application and makes the method more easily adoptable.

2.A | Materials
All measurements in this study were performed on a single Varian TrueBeam 2.5 Stx linac with an aS1200 EPID. The aS1200 EPID utilizes a 43 × 43 cm 2 panel with a backscatter plate between the detection panel and positioning arm. The detector matrix is 1196 × 1190 pixels providing 0.34 mm resolution at isocenter. The backscatter plate removes EPID arm backscatter as a source of error in the measurements . The principles of this study should be applicable to any EPID panel. However, in this study, the methods have   been validated only on the Varian aS1200 EPID and there may be   additional considerations for other EPID panel types. For example, with the Varian aS1000 EPID panel, it is necessary to account for EPID arm backscatter that may affect the application of the PSM.
The Sun Nuclear IC Profiler is a 2D ion chamber array specifically designed for beam profile measurements. The IC Profiler utilizes ion chambers separated by 0.51 cm in both the in-plane and cross-plane directions, which allows for measurements of up to 32 × 32 cm 2 field size at isocenter. The IC Profiler has been recently characterized for beam angle steering by Gao et al. 2 who found agreement between the IC Profiler and water tank wide field symmetry to within 0.7% in 95% of cases measured. However, the study of Gao et al. did not investigate a method of beam translational position steering, although such a method using IC Profiler has been presented by Barnes and Greer 13 and which is used for comparison in this study.

2.B | Measurement methods
Beam angle steering can be assessed with dosimetric measurement at a minimum of two equidistant off-axis beam profile points. The aim of beam angle steering is to achieve equal measured signal at these points (i.e., beam horns of the same height). For beam angle steering in the Varian factory, the field size is set to maximum and the beam is steered using an ion chamber in a fixture that is attached to the collimator in the accessory tray slot. 14 With collimator rotation, this phantom places the ion chamber at equidistant off-axis points at 25% and 75% distance across the field. These points minimize the influence of beam translational position steering and jaw positioning on the measurement. In this project, the linac aS1200 EPID is used as the detector for dosimetric measurement of the equidistant off-axis measurement points. To isolate the Beam Response required to perform the symmetry measurement, the PSM at the measurement points must first be characterized and then removed from a wide field raw EPID image (i.e., not flood field corrected). The beam can then be angle steered so that the Beam Responses at the opposed off-axis points are equal.
The PSM at the measurement points is measured from EPID images of the same section of the beam that have been imaged with various lateral and longitudinal displacements of the EPID. The concept of imaging with EPID displacement forms the basis of the PSM method of Greer. 12 The concept is that the PSM can be isolated from the Beam Response by taking a series of images where the beam is kept constant (e.g., 5 × 5 cm 2 field at central axis), but imaged with different parts of the EPID panel and hence with different PSM. If an image taken with offset EPID is ratioed with an image where the EPID is centered, then the Beam Responses will cancel and the variation from unity in the ratio image will be due solely to differences in the PSM. For the beam steering application, the PSM need only be measured for the off-axis measurement points, and hence, the complete method of Greer is not required. The procedure for measuring the PSM at these points requires as input the appropriate current EPID flood field, a wide field image and a series of five images, one with EPID centered and a further image each with the EPID panel moved in each of the four directions to the off-axis measurement points. The flood field is exported from the TrueBeam console and to collect the rest of the data, a plan was created including the six required fields.
The first field was the wide field image and the remaining five fields The magnitude of the Beam Response at opposing points is then compared as a percentage deviation to provide a measure of wide field symmetry and hence accuracy of beam angle steering.

2.B.1 | Region-Of-Interest size dependence
A potential weakness of the PSM method is the potential for ROI size to influence the resulting measured Beam Response and PSM. To investigate this, a dataset was chosen and the analysis performed with both the standard ROI size of 7 × 7 pixels (1.6 × 1.6 mm 2 ) and a larger ROI of 45 × 45 pixels (10.1 × 10.1 mm 2 ). The percentage deviation for each point in the resulting Beam Response arrays was calculated.

2.B.2 | Comparison with IC Profiler
EPID measured wide field symmetry compared to IC Profiler measured The PSM at 10 cm off-axis, henceforth known as the PSM 10 , was measured for each available photon beam energy (6 MV, 10 MV, Response from wide field raw images. Symmetry was calculated for each beam in both in-plane and cross-plane. The IC Profiler was then used to measure the dose profiles at d max for the same beams using a 30 × 30 cm 2 field size. In document 60976, the International Electrotechnical Commission (IEC) defines beam symmetry as the maximum ratio of the higher to lower absorbed dose at any two positions symmetrical to the radiation beam axis and inside the flattened area. 15 The IEC definition of symmetry was used in this study to compare the EPID to IC Profiler results.

EPID measured symmetry sensitivity to beam angle steering
To test the sensitivity of the Beam Response to changes in beam angle steering, the EPID method was performed pre-and post beam angle steering of the 6 MV beam. The change in EPID measured symmetry was compared with the change in symmetry as measured by the IC Profiler.

2.C | Translational beam position steering using EPID compared to IC Profiler
The methodology presented so far only applies to beam angle steering.
However, to steer the beam correctly, a methodology is also required for beam translational position steering. If beam angle has been correctly steered, then misalignment of the focal spot with collimator axis is due to misaligned translational position steering. As such, once beam angle steering has been achieved using the methods already presented, then the method of Chojnowski et al. 11 can be used to align the focal spot via The departmental method of focal spot alignment using IC Profiler was presented in Barnes and Greer. 13 In this method, the focal spot alignment to collimator rotation axis is determined using the IC Profiler

2.D | EPID panel position reproducibility
The accuracy and reproducibility of the EPID panel positioning are essential for the reproducibility of the proposed EPID beam angle steering method. The setup uncertainty of the method will be dependent on how well the EPID reproduces its vertical, lateral, and longitudi-

3.A | ROI size dependence
The results of Table 1 show the variation in measured beam response at each of the measurement points when PSM 10 was calculated using a 7 × 7 pixel ROI compared to a 45 × 45 pixel ROI on the same dataset. The mean deviation was 0.14 ± 0.06% (1 SD).
3.B | EPID measured wide field symmetry compared to IC Profiler measured Table 2 compares the measured wide field symmetry between the EPID method and IC Profiler. The mean percentage difference between EPID and Profiler was measured at 0.19 ± 0.18% (1 SD). Table 3 shows the measured symmetry for the 6 MV beam before and after beam angle steering as well as the measured change with the IC Profiler and EPID methods. The cross-plane direction had the greatest measured change in symmetry at 1.10% using Profiler and 0.87% using EPID.

4.A | Region-Of-Interest size dependence
The results of Table 1 show that the Beam Response measurement is within 0.2% for a ROI size change from 7 × 7 pixels to 45 × 45 pixels. The change is symmetric about the central axis in both planes which indicates a change in the shape of the Beam Response. Since this variation is symmetric, then the wide field symmetry measurement will not be influenced.

4.B | EPID measured wide field symmetry compared to IC Profiler measured
The results of Table 2 show agreement in symmetry between the EPID method and IC Profiler to within 0.19 ± 0.18% (1 SD) across all four photon beams and both measurement planes. For conventional water tank methods, achieving wide field symmetry of 1% is often deemed acceptable. 4 Table 2 shows greater discrepancy in wide field symmetry measurement between EPID and IC Profiler for FFF beams compared to flattened beams. The mean FFF disagreement is 0.32% while the mean flattened beam disagreement is 0.06%. This is likely due to the greater dose gradient at the measurement points for the FFF beam compared to the flattened beam. This gradient means that setup errors in both IC Profiler and EPID will have an exaggerated influence. Varian FFF beams utilize the same electron beam (including beam steering settings) as the equivalent energy flattened beam. 16 Treatment centers whose beams are configured in this way could simply beam angle steer the flattened beam and then set the same steering settings for the corresponding FFF beam.

4.C | EPID panel position reproducibility
The EPID panel positioning results of Fig. 1 shows subpixel and hence submillimeter reproducibility, which are clinically insignificant.
However, the panel positioning reproducibility will be machine dependent and may vary over time. As such, this should be checked as part of the routine linac QA program with tolerances applied the same as that are applied to water tank detector positioning. 3

4.D | Proposed workflow
To ensure ongoing accurate beam steering, it is proposed that both the EPID-based wide field symmetry and focal spot alignment tests be A weakness in the current study is the need to use off-axis points only 10 cm from central axis. This is required because of the limitation in allowed EPID panel movement in the longitudinal direction for this linac type, which limits where the PSM measurement can occur. To minimize the influence of translational positional beam steering on the beam angle steering, it is also preferable to measure at 15 cm off-axis rather than 10 cm. Currently, the Sun Nuclear Daily QA3 device, which is commonly used for daily checks of beam symmetry, utilizes a 20 × 20 cm 2 field and symmetry assessment on two off-axis points 8 cm either side of central axis. This is comparable to the EPID measurements at 10 cm off-axis presented in this study.
A further weakness of the methods is that they currently only apply with the Varian aS1200 EPID. This is because of the backscatter plate, which is not included on earlier Varian EPID models and

ACKNOWLEDGMENTS
The authors thank Dennis Pomare for his invaluable input on Varian linac operations and his great understanding and insights into the beam steering process.

CONF LICT OF I NTEREST
The authors declare that they have no conflict of interest in this study.