Author: Peter Balter, Thomas Alexander Cuthbert, Weiliang Du, Song Gao, Nicholas Zacharopoulos 👨🔬
Affiliation: Aktina Medical Corp, The University of Texas MD Anderson Cancer Center 🌍
Purpose: The purpose of this study is to evaluate the accuracy and reproducibility of a commercial product for verifying the size, location, and coincidence of physical isocenter (P_iso) and radiation isocenter (R_iso) on c-arm linear accelerators (LINAC).
Methods: The isoPoint system utilizes optical tracking combined with automated analysis of EPID images to provide a more deterministic and sensitive Winston-Lutz (WL) test. The system measures the collimator axes of rotation at various gantry angles to determine the P_iso. It then uses this data, along with optical guidance, to accurately position a radiopaque ball (BB) at the P_iso. Fifteen images of the BB are acquired with the EPID at various gantry, collimator, and couch angles. The isoPoint software then does a 3D WL analysis to determine the LINAC geometric errors as well as the location of the R_iso relative to the P_iso. The isocenter error Z-matrix was used to characterize beam axis-to-target errors across combinations of measured gantry and couch positions. We characterized a LINAC using isoPoint three times, each with an independent setup. Additionally, we conducted one test on five other LINACs. The results were analyzed for comparison.
Results: The results of three independent isoPoint tests on one LINAC are reported as average and standard deviation. The radius of P_iso is 0.28±0.005mm, the maximum distance of collimator rotation axis to the P_iso is 0.28±0.005mm, the maximum couch rotation axis to the P_iso is 0.12±0.024mm. The radius of R_iso is 0.24±0.005mm, the maximum couch rotation axis to the R_iso is 0.17±0.025mm, the maximum Z-matrix value is 0.63±0.014mm. The distance between P_iso and R_iso is 0.25±0.016mm. The isoPoint tests on other LINACs are similar.
Conclusion: The isoPoint system gave highly consistent data on LINAC isocenters in an automated, non-user dependent, manner at higher precision than manually positioned BB based WL tests.