Exhibit Hall | Forum 3
Purpose: To evaluate the long-term isocenter coincidence stability of a C-Arm linac-based stereotactic radiosurgery (SRS) program using a custom Winston-Lutz (W-L) test.
Methods: A custom plastic composite hidden target phantom was designed comprising a cuboid cranial shape, air cavities (sinus), higher density plastic inserts (bones), and a single, 6 mm steel ball at the planned isocenter. Daily W-L measurements were performed at four cardinal gantry angles and two couch rotations (+/- 90°) with gantry=180° after alignment of the phantom using CBCT imaging. Images were acquired with the electronic portal imaging device (EPID) at each gantry/couch position. A script was written and validated to assess the position of the cone image and ball shadow in each of the six EPID images. Data were collected for 4.5 years and evaluated.
Results: The W-L test was performed 970 instances during the 4.5 years of data acquisition. The average and standard deviations of the six W/L measurements were 0.28+/-0.16mm(G0C0), 0.40+/-0.12mm (G180C0), 0.22+/-0.13mm(G90C0), 0.26+/-0.11mm(G2700C0), 0.36+/-0.20mm(G180C90), and 0.46+/-0.14mm(G180C270). Measurements with couch rotations had larger distances but remained less than 1.00mm for all measurements. A drift in the couch position in 2019 required a mechanical adjustment of the couch support frame by 0.40 mm. Absolute and relative changes due to the couch frame adjustment were observed in the data following repairs. Average W-L distance decreased from 0.60mm to 0.23mm and W-L measurements exceeding 0.75 mm decreased from 59 occurrences to 12.
Conclusion: The long-term stability of the isocenter coincidence is reliable for a linac-based SRS program. A W-L pretreatment measurement can detect sub-mm positional variations and triage linac and couch mechanical adjustments.
Stereotactic Radiosurgery, Quality Assurance
TH- External Beam- Photons: Quality Assurance - Linear accelerator