A closer look at the conventional Winston-Lutz test: Analysis in terms of dose.

Aim: To investigate whether the target-isocenter deviations reported by a conventional Winston-Lutz (WL) test actually reflect the shifts of the measured prescription isodose line with respect to the target.

Background: A conventional WL test uses a metallic ball as a target that aims at several fields. But this test does not report information on the accuracy of the delivery in terms of dose.

Materials And Methods: A conventional WL test using a metallic pointer as a target (Pointer-WL test) has been recreated in the Eclipse treatment planning system over an acrylic phantom containing a radiochromic film (Dose-WL test). After Dose-WL test delivery, the shift of the 80% prescription isodose line with respect to the target center (d80%-center) was measured using film dosimetry. The Pointer-WL and Dose-WL tests were performed in 10 different sessions. The isocenter deviation reported by the Pointer-WL test was compared to the d80%-center vector, according to the three patient's directions (Left-Right or LR; Anterior-Posterior or AP; and Superior-Inferior or SI).

Results: The deviations (mean ± SD) found for the Dose-WL tests (LR: 0.5 ± 0.4 mm; AP: 0.5 ± 0.4 mm; SI: 0.6 ± 0.2 mm) were in most cases less than 1 mm, and they were significantly smaller (all  < 0.05) than the maximum deviations reported by the Pointer-WL tests (LR: 1.3 ± 0.3 mm; AP: 1.2 ± 0.4 mm; SI: 1.1 ± 0.3 mm).

Conclusions: The Dose-WL test described in this study allows estimating the spatial accuracy of the prescription isodose line.