Author: Junsheng Cao, Khushdeep Singh 👨🔬
Affiliation: Overlook Medical Center 🌍
Purpose: Eclipse v18 introduces Enhanced Leaf Modeling for MLCs. This study investigates the MLC off-axis characteristics using portal dosimetry
Methods:
MLC characteristics were analyzed for three TrueBeam 6MV energies by generating central and off-axis (12.5 cm from the central axis) treatment plans with small MLC fields (0.6 cm, 1 cm, and 2 cm wide) using Eclipse v16 and v18 algorithms. QA plans were evaluated in portal dosimetry under following three modes, applying a gamma criterion of 1% dose difference and 1 mm DTA:
Delivered and planned dose distributions were matched for the central axis QA plans. The shifts were then applied to the off-axis plans, followed by an absolute dose comparison.
Dose matching is done similar to the first mode. Dose distributions in delivered fields were normalized to planned dose values in their respective field centers. The relative normalization minimizes the effect of the beam profile difference in beam model and delivery.
Planned and delivered dose distribution were matched for each field separately, analyzed using relative dose normalization. This approach aimed to reduce machine-related MLC positioning offsets at different positions.
Results:
For the three analysis modes, the average relative differences in gamma passing rates (30% dose threshold) between central and off-axis plans in v18 were 9%, 5%, and 3%. For v16, the differences were 13%, 9%, and 6%. When using a 10% dose threshold, the average differences were 17%, 13%, and 12% for v18, and 12%, 9%, and 5% for v16.
Conclusion:
Eclipse v18 demonstrated superior off-axis gamma passing rates compared to v16 for 30% dose thresholds. Conversely, v16 exhibited superior performance at 10% thresholds. These results might be attributed to variations in low-dose handling in portal dosimetry or limitations in v18's MLC screw cutout modeling at off-axis. Further investigations employing alternative QA systems are planned to validate these observations.