The results from performing this study with both sets of machines at our two centers are very promising. As part of our monthly QA program to ensure the alignment of the light field with the radiation field per TG 40/142, physicists at our respective centers spend a considerable amount of time, setting XV films on the couch, developing the films, and then analyzing the results using RIT or other analysis software. Indeed this has been a cumbersome process; involving multiple films if adjustments to the field sizes, or the light and radiation field are needed. This test tool obviates the necessity to shoot multiple films, as the results are almost instantly visible, thereby enabling a quicker quality control process. The introduction of deliberate errors of various magnitudes and directions as described above is easily visualized on the EPID images, thereby enabling the physicist to make the necessary corrective changes in real time.
The double exposure technique provided a better approach for visualization of the congruence. It is easy to count the visible markers in the image. However, for PortalVision (PV) the contrast between the two images was not significant, partly because only 1 MU was used for each exposure. Nonetheless, using the intensity profile the edge detection algorithm was able to detect the radiation field and it coincided with the light field as shown by the tungsten markers. Contrary to the PV, the double exposure technique with the iViewGT showed a significant contrast between the two images and hence the agreement could be evaluated without the need for edge detection (Figure 7).
With the wide availability of EPIDs and also with most departments going filmless, this QA tool provides a very convenient way to check radiation and light field congruence. It is inexpensive easy to use, quick to setup and the results are instantaneous. Most of the available tools have only one field size. Our QA tool has 4 different field sizes making it possible to test a variety of field sizes.
Previous studies have already demonstrated the feasibility of EPID for QA [19, 20] and especially light field and radiation congruence [4–6, 11]. Dunscombe et al.  and Luchka et al.  have previously explored light and radiation congruence with EPID. However, they used video based system that has poorer image resolution compared to the amorphous silicon detector reported herein. Imaging systems based on amorphous silicon produce images with a higher detective quantum efficiency (DQE) than fluoroscopic imaging devices because much more optical photons are detected .
Also, other test tools have been developed and tested by different researchers [4–6, 11]. However, these test tools have different limitations and are not widely available. We have evaluated a test tool here that is commercially available. Also we have not used any in-house software for analysis making it possible for any center with an EPID to purchase the tool and be able to apply the technique.
The CR plate provides another approach to verify the light/radiation congruence for centers that do not have EPID. CR has a few advantages over traditional films. For example, the possibility of digital post processing of CR overcomes poor tissue contrast which was the major limitation of conventional radiographic portal film . Other advantages of CR include: It is economical because of low running costs compared to film, a low initial investment compared to the EPID and possibility of using one CR system for several therapy units. Secondly the similarity of photostimulable phosphor plate to film gives it an edge over the EPID which is also limited by its radiosensitive electronics . Finally in this digital age, another advantage of using CR or EPID for light radiation field congruence is the ability to store images online and have permanent record.
Other researchers such as Peace et al.  and Soh et al.  have demonstrated the successful use of CR for LINAC QA. Peace et al.  designed a test tool for light field radiation congruence by embedding a 1 mm diameter lead wires in a Perspex. They used a single technique method and found that the lead wires enclosing the field in the cross-plane could not be distinguished very clearly from the unexposed part of the image. They then suggested the contrast of the lead wire in the image could be improved by performing a double exposure. We used a double exposure technique and found a good contrast with the tungsten markers. To check the coincidence between light field and radiation field, Soh et al.  placed four coins at each of the four edges of a square light field size (15 cm × 15 cm). They found the CR could be used even though their approach was very simplistic.
One of the main limitations that have been put forward when using visual assessments is that the process is extremely subjective. Also, that the results could depend on the viewing conditions, monitor performance and observer experience . With this simple QA tool, setting a baseline, contrast, the process can be easily reproduced and the subjectivity is eliminated and also the tungsten markers have a large contrast compared to the acrylic and thus would be very visible under different conditions.
Our study has its limitations. For example, we did not use the tool to test the radiation/light match over a period of time. However, there is no reason to believe that slight changes in the performance of the EPID over a period of time should affect the results. It is expected that with proper calibration of the EPID, the QA tool could be used at any time. Also the QA tool is not designed to fit on the LINAC head and hence to test in different gantry angles can be problematic. But then since it is only a plate and the EPID rotate with the gantry, the QA tool can easily be adjusted to the appropriate gantry angle.