Normalize the response of EPID in pursuit of linear accelerator dosimetry standardization

Abstract

Normalize the response of electronic portal imaging device (EPID) is the first step toward an EPID‐based standardization of Linear Accelerator (linac) dosimetry quality assurance. In this study, we described an approach to generate two‐dimensional (2D) pixel sensitivity maps (PSM) for EPIDs response normalization utilizing an alternative beam and dark‐field (ABDF) image acquisition technique and large overlapping field irradiations. The automated image acquisition was performed by XML‐controlled machine operation and the PSM was generated based on a recursive calculation algorithm for Varian linacs equipped with aS1000 and aS1200 imager panels. Cross‐comparisons of normalized beam profiles and 1.5%/1.5 mm 1D Gamma analysis was adopted to quantify the improvement of beam profile matching before and after PSM corrections. PSMs were derived for both photon (6, 10, 15 MV) and electron (6, 20 MeV) beams via proposed method. The PSM‐corrected images reproduced a horn‐shaped profile for photon beams and a relative uniform profiles for electrons. For dosimetrically matched linacs equipped with aS1000 panels, PSM‐corrected images showed increased 1D‐Gamma passing rates for all energies, with an average 10.5% improvement for crossline and 37% for inline beam profiles. Similar improvements in the phantom study were observed with a maximum improvement of 32% for 15 MV and 22% for 20 MeV. The PSM value showed no significant change for all energies over a 3‐month period. In conclusion, the proposed approach correct EPID response for both aS1000 and aS1200 panels. This strategy enables the possibility to standardize linac dosimetry QA and to benchmark linac performance utilizing EPID as the common detector.

A regression‐based approach to compute the pixels sensitivity map of linear accelerator portal imaging devices

“…Similar to Cai et al. 's 16 work, we found that the PSM was stable at the 1% level for measurements separated by 3 months. Cai reported energy/modality dependence for the PSM, but we found the PSM to be energy/modality independent for photons.…”

“…For fixed radiation fields, we also intercompare field measurements spanning a range of lateral offsets that could be attained by the imager to confirm position independence. This, in effect, cross‐validates our method with those of Greer et al 13 and Cai et al, 16 who inter‐related shifted EPID measurements to derive the PSM. For this series of measurements, the imager was placed at the isocenter plane and the jaws adjusted to produce long‐narrow beams (slits) on the EPID imager.…”

“…We found that single pixel misalignment in the imager shift would result in 1% PSM deviations in 6% of the EPID panel pixels; a 0.1% machine output variation between measurements would result in up‐to 5% pixel sensitivity errors; and that a 0.1% background offset (mimicking ghosting) could result in 5% errors in 85% of EPID pixels. Cai et al 16 . utilized sequential calibration field and dark‐field irradiations to minimize the effect of the ˜1% image ghosting and small (1 cm) imager offsets to minimize the change in scatter conditions inherent to the aS1000 imager (which has non‐uniform backscatter).…”

A proposed method for linear accelerator photon beam steering usingEPID

“…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.…”

Development of an Electronic Portal Imaging Device Dosimetry Method

“…One factor is that EPID is mounted on the linac by means of a support arm, and the low-energy scatter ray produced by the interaction between X-rays and the support arm may lead to asymmetry of the EPID image in the inline direction [13,[21][22][23][24][25]; relevant studies show that the effect may exceed 5% [26,27]. The other factor is that EPID requires dark field and flood field calibration to ensure that the response of each pixel is constant, which results in the off-axis response of EPID being different from the actual output of the linac [28,29]. Therefore, when EPID is used as a dosimetry tool, these two factors must be corrected before converting pixel values into dose values.…”