Robust optimization in intensity-modulated proton therapy to account for anatomy changes in lung cancer patients

Abstract

Background and purpose: Robust optimization for IMPT takes setup and range uncertainties into account during plan optimization. However, anatomical changes were not prospectively included. The purpose of this study was to examine robustness and dose variation due to setup uncertainty and anatomical change in IMPT of lung cancer.

Material and methods: Plans were generated with multi-field optimization based on planning target volume (MFO-PTV) and worst-case robust optimization (MFO-RO) on simulation computed tomography scans (CT0) for nine patients. Robustness was evaluated on the CT0 by computing the standard deviation of DVH (SD-DVH). Dose variations calculated on weekly CTs were compared with SD-DVH. Equivalent uniform dose (EUD) change from the original plan on weekly dose was also calculated for both plans.

Results: SD-DVH and dose variation on weekly CTs were both significantly lower in the MFO-RO plans than in the MFO-PTV plans for targets, lungs, and the esophagus (p<0.05). When comparing EUD for ITV between weekly and planned dose distributions, three patients and 28% of repeated CTs for MFO-RO plans, and six patients and 44% of repeated CTs for MFO-PTV plans, respectively, showed an EUD change of >5%.

Conclusions: RO in IMPT reduces the dose variation due to setup uncertainty and anatomy changes during treatment compared with PTV-based planning. However, dose variation could still be substantial; repeated imaging and adaptive planning as needed are highly recommended for IMPT of lung tumors.

Keywords: Adaptive planning; IMPT; Robust optimization; Robustness evaluation.

Fig. 1

Examples of the multi-field optimized plan (MFO-PTV, left) and the robust-optimized MFO (MFO-RO, right) plan on the (a) original computed tomography (CT) scans and on the (b) verification CT scans. Note that owing to the anatomy changes, the prescription line was broken, and dose to the esophagus and the spinal cord was elevated on the repeated CT scan for the MFO-PTV plan (b, left), but the dose was maintained on the repeated CT scan for the MFO-RO plan (b, right).

Fig. 2

Nominal dose-volume histograms (DVHs; thick lines), standard deviation (SD)-DVHs (shaded areas, ±2σ), and weekly DVHs (thin lines) for (a) the multi-field optimized plan (MFO-PTV) and (b) the robust-optimized MFO (MFO-RO) plan for a patient. ITV, internal target volume.

Fig. 3

(a) Standard deviation of the dose variation (DV_SD-DVH) and weekly dose variation (DV_weekly) for the multi-field optimized plan (MFO-PTV) and the robust-optimized MFO (MFO-RO) plan evaluated for each patient. (b) Equivalent uniform dose (EUD) variation for the internal target volume (ITV) for different patients (shown in different colors) on the MFO-RO plans (solid lines) and MFO-PTV plans (dashed lines) as a function of time. X indicates adaptive planning.