A novel lattice technique via adaptive spot assignment and LET optimization based proton arc therapy.

PURPOSE: Lattice radiation therapy is an innovative three-dimensional implementation of spatially fractionated radiation therapy. This study aimed to develop a novel lattice-based proton arc therapy (PAT) technique to deliver a highly modulated peak-valley spatial dose distribution and a modulated linear energy transfer (LET) distribution. METHODS AND MATERIALS: We introduced a PAT lattice algorithm that optimizes three key components, including energy-layer selection, spot assignment, and LET optimization (PATLESL). A simulated annealing algorithm was used to select the optimal energy layers. Meanwhile, an adaptive spot assignment strategy was implemented using a spot-sparsity optimization algorithm based on the primal-dual active set with continuation to achieve an improved peak-to-valley dose ratio (PVDR). Subsequently, LET optimization was achieved using the alternating direction method of multipliers with a minimum monitor unit constraint. Eighteen patients were selected to evaluate plan quality and delivery efficiency compared with the lattice technique based on the previously reported spot-scanning proton arc therapy energy sequence optimization algorithm (PATseq). Feasibility and dosimetric accuracy were further validated using clinical phantom measurements. RESULTS: Compared with PATseq, the PATLESL plan exhibited improved dose and LET distributions with higher delivery efficiency. More specifically, PATLESL achieved steeper dose gradients, improving the PVDR from 16.90 ± 18.04 to 41.85 ± 29.69 (p < 0.01), shortening the beam delivery time by 37.43%, and increasing the target spot coverage by 44.12% while reducing the spot number by 93.78%. It also increased the mean LET in the LTV from 2.84 ± 0.38 to 3.59 ± 0.26 keV/μm (p < 0.01), on average. Meanwhile, the phantom measurement confirmed high agreement between the planned and delivered results. CONCLUSIONS: A novel PAT lattice technique using adaptive spot assignment and LET optimization could deliver an enhanced peak-valley spatial dose distribution. Experimental validation on a clinical proton system confirmed deliverability and dosimetric accuracy, which may facilitate future clinical translation of proton lattice radiotherapy.