Technical Note: Optimization of spot and trimmer position during dynamically collimated proton therapy

Abstract

Purpose: To demonstrate a novel theoretical optimization design which considers beam spot and trimmer positioning in addition to beamlet weighting for dynamically collimated proton therapy (DCPT) treatments. Prior to this, the previous methods of plan optimization used to study this emerging technology relied upon an intuitive selection criterion to fix the trimmers blades for a uniform grid of beam spots before determining the individual beamlet weights. To evaluate the potential benefit from this new optimization design, a treatment planning optimization study was performed in order to compare the algorithm’s functionality against the existing methods of plan optimization. Materials and methods: A direct parameter optimization (DPO) method was developed to determine beam spot and trimmer positions cohesively with beamlet weighting for DCPT treatment plans. Gradients were numerically determined from applying small adjustments to the aforementioned parameters and quantifying the resulting impact on an objective function. This technique was compared to the conventional trimmer selection algorithm (TSA) which does not optimize spot position concurrently with trimmer position. Both planning methods were used to optimize a set of brain treatment plans, and the resulting dose distributions were compared with dose–volume histogram quantities in addition to target coverage, homogeneity, and conformity metrics. Results: An overall improvement to the target conformity and healthy tissue sparing was achieved with DPO over TSA while maintaining an equivalent planning target volume (PTV) coverage index for the three brain patients evaluated in this study. On average, the conformity index improved by 5.5% when utilizing DPO. A similar improvement in reducing the dose to several organs at risk was also noted. Conclusion: Both the TSA and DPO planning methods can achieve highly conformal treatments with the dynamic collimation system (DCS) technology. However, an improvement in the target conformity and healthy tissue sparing was achieved by simultaneously optimizing beam spot position, trimmer location, and beamlet weights using DPO in comparison to the TSA technique.

Innovations and the Use of Collimators in the Delivery of Pencil Beam Scanning Proton Therapy

“…This is apparent when considering how PBS systems can scan faster that the translation speed of the collimator, introducing treatment time penalties. Spot and collimator placement algorithms have been developed to minimize collimator travel and treatment times while maintaining coverage [ 49 , 50 ].…”

Proton therapy needs further technological development to fulfill the promise of becoming a superior treatment modality (compared to photon therapy)

“… 17 , 18 , 19 , 20 , 21 In preclinical studies, utilization of collimation systems have been shown to provide a substantial improvement in high‐dose conformity when treating both brain and head and neck cancers. 22 , 23 , 24 These dosimetric advantages are far superior to traditional aperture‐based collimation approaches 25 , 26 , 27 while still yielding robust treatment plans. 28 Arc geometries have also recently been proposed as an alternative method to improve high‐dose conformity of proton therapy.…”

Treatment planning for proton therapy: what is needed in the next 10 years?

“…It has been shown that by taking the borders of the treated volume into account, the lateral penumbra can be effectively and significantly reduced. 78,79 This is achieved without increasing the number of spots (which would increase the treatment time) and without inclusion of the spot position in the iterative optimization process 80 (which could significantly increase the calculation time). An optimal dose distribution can never be achieved by starting with a suboptimal positioning of the spots.…”