Abstract
Purpose: Long term variation of ExacTrac image isocenter‐to‐room lasers on a Novalis machine were analyzed and the congruence between radiation isocenter and ExacTrac kV‐image isocenter was evaluated by using an in‐ house Winston‐Lutz (WL) based MatLab program as well as ExacTrac WL‐ images. Methods: Over 100 daily verification data of ExacTrac isocenter phantom as well as of WL phantom were analyzed to find out the long term trend of ExacTrac image isocenter‐to‐room lasers. Radiation isocenter was also evaluated using the pre‐radiosurgery WL Test performed with ten angle combinations of gantry/couch. The GafChromic film was analyzed with an in‐house MatLab program to compute the vertical‐, longitudinal‐, and lateral‐coordinates of the radiation isocenter‐to‐room lasers by combining the isocenter displacement of every angle combination. By comparing the WL radiation isocenter‐to‐room lasers with ExacTrac kV‐image isocenter‐ to‐room lasers, the 3D displacement between radiation isocenter and ExacTrac WL‐image isocenter was determined. Results: The vertical‐, longitudinal‐, and lateral‐displacements of the ExacTrac image isocenter‐to‐ room lasers for the isocenter phantom were 0.35±0.16mm, 0.07±0.15mm, and −0.03±0.10mm, respectively. The displacements for the WL phantom were 0.43±0.18mm, 0.28±0.20mm, and −0.12±0.25mm, respectively. The average displacements of WL phantom from the radiation isocenter in the vertical‐, longitudinal‐, and lateral‐coordinates were 0.51±0.19mm, 0.12±0.33mm, and −0.11±0.24mm, respectively. Conclusions: We evaluated the congruence between radiation isocenter and ExacTrac kV‐image isocenter using an in‐house WL‐based MatLab program. The average discrepancy was well within 0.6 mm in all directions.
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