Author: Cambridge L Bui-Nguyen, Alexander S. Nguyen, Liqiang (Lee) Tao 👨🔬
Affiliation: Varian AOS @Epic Care, UC Berkeley 🌍
Purpose: To quantity variations in Winston Lutz (WL) test results due to different energies used in SBRT, SRT or SRS.
Methods: A WL phantom with a 6.4 mm diameter center radio-opaque sphere is used in all the tests. The phantom is first CT scanned and exported to the treatment planning system to contour the external and centered sphere structures for IGRT. Four treatment plans, each correspond to a different energy: 6MV, 10MV, 6FFF and 10FFF, are created to deliver radiations with all beams at 100 SAD, isocenter at the center of the sphere, and 3x3 field size. The Winston Lutz beam configurations for each plan are as followed
Gantry Collimator Couch
180 45 0
90 270 0
270 90 0
0 0 0
0 0 270
0 0 90
The phantom is first setup on the table with laser and cone beamed. Just like a regular IGRT treatment session, the cone beam image is then registered with a referenced image set to derive a set of shifts in 6D. The shifts are then applied to move the phantom to its final position. WL beams for each energy are delivered and the resulting EPID images collected for offline analysis using a commercially available software.
Results: Using 6MV flatten beam as baseline, WL maximum and total deltas between imaging and radiation isocenters from the other energies generally agree within +- 0.3mm.
Conclusion: There are small but significant variations in radiation isocenter positions when measured using the WL test. Since TG-142 recommended 1 mm tolerance for imaging and radiation isocenter congruence, we suggest that single energy WL using 6MV is considered adequate if the maximum and total deltas are less than 0.7 mm. If deviation is greater than 0.7mm then further WL testing with other energies are warranted.