Purpose: Pencil beam scanning (PBS) proton therapy has been considered as a potential modality for the 3D form of spatially fractionated radiation therapy known as lattice therapy, however, no practical solutions have been introduced in the clinic. We developed a robust proton lattice planning method using multiple fields and evaluated its dosimetric characteristics to clinically acceptable photon lattice treatment.
Methods: Two orthogonal beams were used to deliver dose to 1cm-diameter vertices with 3.2-3.5cm spacing inside the gross target volume (GTV). The primary beam (PB) and robust complementary beam (RCB) delivered 67% and 33% of the prescribed dose to each vertex, respectively. Robust optimization using 5mm setup and 3.5% range uncertainties was assigned only to the RCB. Proton lattice treatment plans were generated on seven patients previously treated with photon lattice therapy. The number and total volume of vertices, peak-to-valley dose ratios (PVDR) defined as D90/D10 of GTV, and volume of low dose to GTV have been used for the plan comparisons.
Results: Compared to proton lattice plans without robust optimization, RCB improved the robustness of plans due to the position and range uncertainties. In comparison to photon plans, proton plans have ~40% more vertices and higher PVDR (10.5±2.5 vs. 2.5±0.9, p<0.005). As there is a greater number of vertices, a greater volume of vertices was observed in GTV. (18.2±25.9cc vs 12.2±14.5cc). Additionally, proton lattice plans show more conformal dose with less low dose to GTV volume, V30%: 60.9±7.2% vs. 81.6±13.9% and V10%: 88.3±4.5% vs. 98.6±3.6% (p<0.01).
Conclusion: This proton lattice planning strategy which uses a pair of primary and robust complimentary beams can generate better plan quality, a greater number of vertices, and higher PVDR than clinically acceptable photon lattice plans. Further study on the patient specific quality assurance is required before it can be utilized in the clinic.
TH- External Beam- Particle/high LET therapy: Proton therapy – Development (new technology and techniques)