Author: Courtney R. Buckey, Quan Chen, Suzanne J. Chungbin, Edward L. Clouser, Yi Rong, Jun Tan, Jennifer Yan, Xiang Sheng Yan 👨🔬
Affiliation: Mayo Clinic Arizona, Mayo Clinic 🌍
Purpose:
To identify planning techniques that consistently result in robust VMAT-TBI plans that allow for larger patient setup errors.
Methods:
Conventional planning method often results in poor junction robustness between VMAT-TBI fields, even with the feathering feature turned on. A setup error of 1.1 to 2.8 mm can result in 10% dose change in those plans. Various planning techniques were systematically explored on Eclipse TPS V15.6 to produce consistent and robust dose gradients in junction areas.
Results:
Successful techniques identified as: A) 40 cm horizontal field size for Head plan B) 0o collimator angle for Head plan to better control lens dose. C) 90o collimator angle for all other plans. D) MLC opening no more than 15cm for all upper body plans. After these plan techniques were used, the robustness of the junction area was consistently achieved at >4.9 mm setup error for 10% dose variation for all the 34 junction areas in 6 patients. All plans show significant dosimetric improvements compared to the conventional planning method, with V100% and D98% to target increased by 5.8% and 5.5% on average; and D0.03cc to target, Dmean to lung, and Dmean to kidney decreased by 11.1%, 6.0%, and 6.2% on average, respectively.
Conclusion:
A set of VMAT-TBI specific planning techniques were identified that resulted in consistently slow dose gradients capable of tolerating 4.9mm misalignment between field junctions. In addition to improved robustness, the quality of plans was also greatly improved.