Assessing radiation and light field congruence with a video based electronic portal imaging device

Abstract

Projected light fields are used on treatment simulators and teletherapy treatment units to delineate the size and position of the radiation beam. Any discrepancy between these fields will lead to a systematic field placement error, with possibly serious implications with regard to the accuracy of the delivered dose distribution in the patient. Conventionally, film has been used for regular quality control tests of light and radiation field congruence, but this is a time consuming method and is not suitable for daily checks. A new method is described that uses a specially designed test phantom, a video-based electronic portal imaging device and a personal computer to test for radiation and light field congruence on treatment accelerators. This method consists of aligning the test phantom in the light field of a treatment linac and acquiring an electronic portal image. A computer program then automatically analyzes the image and determines the degree of congruence between the two fields. The final result of the test is a go, warning, or no go decision depending on the extent of misalignment between the light and radiation fields. Two algorithms were tested for reproducibility (<0.4 mm), sensitivity to noise (<0.2 mm), and positional accuracy (<0.4 mm) and are shown to give results comparable to the conventional film method. Daily testing of field congruence over a period of 84 days demonstrated differences in the results determined by the two algorithms of less than 0.1 +/- 0.2 mm (standard deviation) at 6 MV and 0.22 +/- 0.13 mm at 23 MV. Routine testing is possible as the effort and time required are minimal, and the test can be performed during daily routine start-up procedures.

A simple quality assurance test tool for the visual verification of light and radiation field congruent using electronic portal images device and computed radiography

“…Dunscombe et al [4] and Luchka et al [5] 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.…”

“…So far, several test tools and appropriate software have been developed to quantitatively study light and radiation field congruence using EPIDS [4-6,11]. The use of some of the approaches is dependent on the availability of custom designed software used in a stand-alone fashion [5] or incorporated into the commercial EPID system [6]. However, these approaches involve potential cost and also in some cases access to the EPID software.…”

Guide to clinical use of electronic portal imaging

“…The EPID has also been put to use for quality assurance of treatment machines 86 and of treatment techniques, such as radiosurgery 87 and dynamic wedge and MLC therapy 88 , 89 . In each case, the EPID has allowed more precise, quantitative results to be obtained with much less effort than would have been achievable using conventional QA tools.…”

Utilizing an electronic portal imaging device to monitor light and radiation field congruence

“…However, the use of film is time‐consuming, due to film processing, and its analysis can be subjective unless the film is digitized. Although methods to assess light and radiation field congruence with an EPID have been previously explored by Luchka et al 4 . and Dunscombe et al ., 5 the current study was performed using an amorphous silicon detector, which has superior image resolution compared to the video‐based systems reported previously.…”