The impact of electronic portal imaging device position error on 3D dose verification of volumetric modulated arc therapy

Abstract

Objective To analyze the impact of electronic portal imagingdevice(EPID) position error on three-dimensional dose verification of volumetric modulated arc therapy (VMAT). Methods Five Suremark SL-20 lead points were fixed on Elekta tray, and EPID images were collected in 0-360° rotation, one image per 5°. The position error relative to the accelerator was analyzed via Matlab. Then the images position error was corrected according to the analysis, and the 3D dose was reconstructed with the corrected images. The dose distributions of double arcs, clockwise arc(arc 1), and counterclockwise arc(arc 2) of 16 nasopharyngeal carcinoma patients' VMAT plan were evaluated by γ analysis, and the results of before and after position error correction were compared. Results Compared to 0° gantry angle, the error of source to the image distance (SID) was maximum (1.20 cm) when the gantry angle was 180°. On account of the SID change, the maximum error along the up-down (y) direction in the iso-center planar was 2.28 mm and the left-right (x) direction error was within ±0.5 mm. The 3D γ analyses of 16 nasopharyngeal carcinoma in VMAT plans were obviously increased after the position error along y was corrected. The double arcs, arc1 and arc 2 were increased by (4.12±1.67) % (t=- 9.86, P< 0.05), (3.47±1.64) % (t=- 8.46, P< 0.05) and (5.08±1.30) % (t=- 15.63, P< 0.05) in 5%/3 mm standard, respectively. However, in 3%/3 mm standard, γ value of the double arcs, arc 1 and arc 2 were increased by (7.63±2.24) % (t=- 13.63, P< 0.05), (6.03±2.07) % (t=- 11.66, P< 0.05), (9.17±2.23) % (t=- 16.41, P< 0.05), respectively. Since the EPID position error along x was corrected after y, the 3D γ analysis of reconstruction dose indicated that the average of the 5%/3 mm and 3%/3 mm γ value were increased by 0.23% and 0.24%, respectively. Conclusions EPID motion error along the gantry to table direction of the accelerator can't be ignored. When reconstruct dose based on EPID, a modification should be made for rebuilding more accurate patients’ 3D dose distribution. Key words: Electronic portal imaging device; Position error; Volumetric modulated arc therapy; 3D dose verification; γanalysis