30. Multifield optimization in protontherapy: An analysis of the loss in robustness and the advantages of robust optimization

• Published in: Physica medica Vol. 56; pp. 80 - 81
• Main Authors: Barbareschi, A., Algranati, C., Schwarz, M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.12.2018
• ISSN: 1120-1797; 1724-191X
• DOI: 10.1016/j.ejmp.2018.04.040

The conformality of protontherapy treatment plans can be improved using Multi Field Optimization (MFO), a treatment technique where a homogeneous dose distribution to the target is achieved with inhomogeneous fields. Such plans are more susceptible to uncertainties and errors (are less robust). Aim of our study is to verify the degree of robustness of MFO plans for five patients considered suitable for MFO planning, representing the main treatment sites treated at our center (brain, head and neck, spine). To establish how to consider uncertainties during MFO planning, we compared PTV-based planning uncertainties considered by expanding the treatment volume) with the robust optimization (rMFO plans) provided by RayStation TPS. Our robustness analysis consisted in simulating 100 possible scenarios for each plan, each one representing an isocenter shift (from a gaussian distribution where the standard deviation was half the value of the maximum shift considered) and a variation in the Housenfield Units of the planning CT (+3%, −3%). We obtained a set of 100 Dose Volume Histogram lines from which we could evaluate robustness parameters such as the median and the worst-case value for specific dose and volume indexes, and conformality parameters such as the dose difference achieved between the CTV and the dose-limiting healthy organ. As a benchmark of plan robustness, we first calculated robustness parameters of 10 treatment plans that had previously been used for treatment. If robustness parameters of our MFO plans were in accordance with such values, the loss in robustness would be negligible. The robustness analysis of our rMFO plans shows a target coverage in the worst-case scenario of a D95 between 89% and 60% of the prescribed dose, and our benchmark values are of 90% – 65%. We choose to represent D95 instead of D99 because for all patients the healthy organ to be spared was inside the treatment volume. Our analysis demonstrates that for MFO plans the PTV is no longer useful, as the robust optimization provides more robust plans, increasing robustness on healthy organs (see Table 1). Robustness parameters of rMFO plans are in good accordance with our benchmark values.