Efficient quality assurance method with automated data acquisition of a single phantom setup to determine radiation and imaging isocenter congruence

Abstract

We developed a quality assurance (QA) method to determine the isocenter congruence of Optical Surface Monitoring System (OSMS, Varian, CA, USA), kilovoltage (kV), and megavoltage (MV) imaging, and the radiation isocenter using a single setup of the OSMS phantom for frameless Stereotactic Radiosurgery (SRS) treatment. After aligning the phantom to the OSMS isocenter, a cone-beam computed tomography (CBCT) of the phantom was acquired and registered to a computed tomography (CT) scan of the phantom to determine the CBCT isocenter. Without moving the phantom, MV and kV images were simultaneously acquired at four gantry angles to localize MV and kV isocenters. Then, Winston-Lutz (W-L) test images of the central BB in the phantom were acquired to analyze the radiation isocenter. The gantry and couch were automatically controlled using the TrueBeam Developer Mode during MV, kV, and W-L image acquisition. All the images were acquired weekly for 17 weeks to track the congruence of all the imaging modalities' isocenter in six-dimensional (6D) translations and rotations, and the radiation isocenter in three-dimensional (3D) translations. The shifts of isocenters of all imaging modalities and the radiation isocenter from the OSMS isocenter were within 0.2 mm and 0.2° on average over 17 weeks. The maximum discrepancy between OSMS and other imaging modalities or radiation isocenters was 0.8 mm and 0.3°. However, systematic shifts of radiation isocenter anteriorly and laterally relative to the OSMS isocenter were observed. The measured discrepancies were consistent from week-to-week except for two weeks when the isocenter discrepancies of 0.8 mm were noted due to drifts of the OSMS isocenter. Once recalibration was performed on OSMS, the discrepancy was reduced to 0.3 mm and 0.2°.By performing the proposed QA on a weekly basis, the isocenter congruencies of multiple imaging systems and radiation isocenter were validated for a linear accelerator.

Keywords: automation; imaging quality assurance; optical imaging.

Figure 1

(a) Coordinates of the phantom and locations of five embedded alumina ceramic BBs. The coordinate system of the phantom is noted with red arrows in 3D. (b) OSMS cube phantom (15.0 × 15.0 × 15.0 cm³) placed on a leveling platform. The surface of the phantom is opaque in order to be visible to OSMS. 3D, three‐dimensional; OSMS, optical surface monitoring system.

Figure 2

Mean, standard deviation (error bars) and maximum/minimum (plus signs) of the discrepancies of the isocenter of MV (circle), kV (square), CBCT (star), and radiation (diamond) with respect to the OSMS isocenter are shown in (a) VRT (mm), (b) LNG (mm), (c) LAT (mm) and (d) pitch (°), (e) roll (°), f) Rtn (°) over 17 weeks. All the isocenters agree within 0.2 mm and 0.2° on average over that period. CBCT, cone‐beam computed tomography; OSMS, optical surface monitoring system.

Figure 3

Trend of the discrepancies from the OSMS isocenter to the MV (circle), kV (square), CBCT (star), and radiation (diamond) isocenters measured in (a) VRT (mm), (b) LNG (mm), (c) LAT (mm) and (d) pitch (°), (e) roll (°), (f) Rtn (°) over 17 weeks. Drastic drifts or fluctuations of the isocenters are not evident in the data taken from weeks 1 to 13, but fluctuation is shown in weeks 14 and 15. Monthly calibration on OSMS was performed before data acquisition in week 16. Isocenter discrepancies determined from the data taken in weeks 16 and 17 were similar to those from data taken before week 13. CBCT, cone‐beam computed tomography; OSMS, optical surface monitoring system.