4D strategies for lung tumors treated with hypofractionated scanning proton beam therapy: Dosimetric impact and robustness to interplay effects.

PURPOSE

To investigate the impact of four-dimensional robust optimization (4DRO) on dose delivered to lung cancer patients in pencil beam scanning proton therapy.

METHODS AND MATERIALS

2 strategies were compared for 20 lung cancer patients, using a different number of breathing phases of the reconstructed 4D computed tomography (CT) included in the plan optimization problem. In the restricted approach combined with gating, only 3 phases close to reference end-exhale were considered instead of the whole breathing cycle. The prescribed dose was 60 Gy(RBE) in 10 fractions. Target coverage (D98%) and dose to healthy tissues were evaluated using Wilcoxon signed-rank test. To assess the robustness against interfractional anatomical and respiratory variations, the optimized plans were recalculated on re-evaluation 4DCTs. To compare the sensitivity of both strategies to interplay effects, we implemented an end-to-end test with a home-made heterogeneous moving phantom and ionization chambers measurements. Robustly optimized plans with prescription doses of 6 Gy(RBE) were delivered in different dynamic conditions.

RESULTS

Both 4D robustly optimized plans reached the same target coverage (p = 0.56), while a statistically significant decrease of the homolateral lung dose was observed using the restricted approach (p < 0.0001). Plan recalculations within 15 days from the treatment simulation showed the same robustness of target D98% against interfractional variations (p = 0.48), with an average decrease of approximately 3 Gy(RBE). Phantom measurements confirmed the delivery accuracy of the restricted approach (mean dose deviations <5%). Higher deviations were found for ungated full 4DRO and larger motion amplitude.

CONCLUSION

The restricted approach combined with gating improved normal tissue sparing and was shown to be more robust to single fraction deliveries and large motion amplitude.