An investigation into the robustness of dynamically collimated proton therapy treatments

To investigate the dosimetric robustness of dynamically collimated proton therapy (DCPT) treatment plans delivered using a dynamic collimation system (DCS) with respect to random uncertainties in beam spot and collimator position as well as systematic offsets in the DCS mounting alignment. This work also demonstrates a technique that can increase plan robustness while preserving target conformity.

Variability in beam spot and collimator positioning can result in changes to a beamlet's dose distribution and incident fluence. The robustness of the DCPT treatment plans was evaluated for three intracranial treatment sites by modeling treatment variability as normally distributed random variables with standard deviations reflecting a clinical system. The simulated treatment plans were then recalculated and compared against their nominal, idealized dose distribution among several trials. It was hypothesized that a plan's robustness to these delivery variables could be reduced by restricting a trimmer's placement toward a beamlet's central axis during collimation.

By introducing a minimum trimmer offset of 1.5 mm, the variation of the planning target volume (PTV) D_95% coverage was reduced to within 2% of the prescribed dose. The treatment plans with trimmers that were placed within 0.5 mm of a collimated beamlet's central axis resulted in the greatest healthy tissue sparing but deviations as high as 11.4% to the PTV D_95% were observed. The nominal conformity of these treatment plans utilizing the 1.5 mm trimmer offset was also well maintained. For each treatment plan studied, the 90% conformity index remained within 6.25% of the conformity index achieved without a minimum trimmer offset, and the D_50% of surrounding healthy tissue increased by no more than 3.1 Gy relative to a plan without a trimmer offset.

While DCPT can offer a significant reduction in healthy tissue irradiation, the results from this work indicate that special care must be taken to ensure proper PTV coverage amid uncertainties associated with this new treatment modality. A simple approach utilizing a minimum trimmer offset was able to preserve the majority of the target conformity and healthy tissue sparing the DCS technology affords while minimizing the uncertainties in this treatment approach.