Room: Exhibit Hall | Forum 7
Purpose: For ensuring safe treatments with proton therapy delivered by pencil beam scanning (PBS) in moving targets, it is essential to evaluate the robustness of treatment plans against numerous uncertainties. For this purpose, we have developed a tool enabling realistic and comprehensive robustness evaluation, including against variable breathing motion patterns.
Methods: In order to assess the robustness of the PBS plan, multiple scenarios of treatment uncertainties are randomly sampled from probability distributions reported in the literature. Each scenario, representing a possible full treatment realization, is composed of several fractions. Therefore, to simulate both systematic and random uncertainties, a realistic daily 4D image that includes the effect of setup errors, uncertainty in the stopping powers, and variation in the motion amplitude, is generated by manipulating the planning 4DCT series.Dose distributions are calculated on each generated 4DCT using the fast Monte Carlo code MCsquare with a statistical uncertainty of 1%. Our 4D dose calculation algorithm enables the simulation of the interplay effects with variable motion periods. All fractions were accumulated for each scenario.
Results: In this example, a treatment plan for a lung tumor case was generated using the 4D robust optimizer MIROpt. Its robustness was then verified using the presented method. A minimum of 300 scenarios was necessary to reach an acceptable level of statistical uncertainty on the DVH-band. The test revealed a sufficient plan robustness against treatment uncertainties typically reported in the literature, except when the interplay effect is included (D95 band ranges from 89% to 96% of prescribed dose).
Conclusion: A realistic, and statistically sound method of treatment plan robustness verification is proposed. The presented uncertainty models are not specific to protons and can also be applied to photon treatments. Moreover, the generated 4DCTs can be directly used as uncertainty scenarios to feed robust optimizers in commercial TPS.
Funding Support, Disclosures, and Conflict of Interest: Kevin Souris is supported by a research grant from Ion Beam Application (IBA s.a., Louvain-la-Neuve, Belgium).