Robust Plan Analysis

Plans were then analyzed for robustness to determine the effect of the tested perturbations on target coverage and dose to OARs. Robust analysis is different from robust optimization in that it is performed after the treatment plan is optimized by using the Eclipse treatment planning software, but before the treatment plan is approved by the treating physician. Robust optimization is an algorithm in which the “worst-case” dose distribution is used to compute an objective function for a given iteration. Iterations include different combinations of isocenter shifting (±2 mm in the anterior-posterior, superior-inferior, and lateral directions) and scaling the stopping power ratios by ±3.5% [12]. Robust optimization cannot currently be performed by using Eclipse, and we do not yet routinely implement this process for patients with head and neck cancer. Instead, we use the process of robust analysis to assist the treating physician in recognizing the potential effects of setup and stopping power uncertainties on target coverage as well as dose to OARs.

During the robust analysis process, dose-volume histograms (DVHs) are created for the treatment plan as optimized in Eclipse as well as after applying several perturbations to the plan, including isocenter shifts ±3 mm in the anterior-posterior, superior-inferior, and lateral positions as well as stopping power ratio scaling of ±3.5%. Figures 1 and 2 show 2 examples of composite DVHs generated from the robust analysis process. To evaluate the robustness of the plan, the treating physician then evaluates the DVHs for target volumes as well as OARs under all combinations of perturbations tested including isocenter shifts and stopping power ratios. The treatment plan is ultimately deemed acceptable if 95% of the target volume is covered by 95% of the prescription dose and OARs do not exceed their tolerances even in the “worst-case” scenario. As always, sometimes clinical judgment must be used by the treating physician if compromises must be made regarding dose to OARs to improve target coverage or if compromises to target coverage must be made to spare OARs.

Dose-volume histogram generated to display the results of robust analysis performed on the multifield optimization intensity-modulated proton therapy treatment plan generated for a patient with adenoid cystic carcinoma of the right maxillary sinus (patient 1). The solid lines indicate the primary treatment plan and the dotted lines demonstrate the different perturbations tested during robust analysis (±3-mm isocenter shift and ±3.5% stopping power ratio). There was adequate target coverage of CTV1, which included the tumor bed plus a margin to encompass areas at high risk for recurrence; and CTV2, which included the entire operative bed and other areas at intermediate risk. The right eye and optic nerves were the closest organs at risk, and the dose to these structures in all perturbations studied was deemed acceptable for treatment. Abbreviation: CTV, clinical target volume.