Combining Scanning Spot Weight Optimization with Ridge Filter Design for Single-Energy Proton Pencil Beam
Abstract
Purpose
To develop an optimization procedure of spot weight and ridge filter (RF) dimension for single-energy proton pencil beam and apply it to a water phantom with a square-shape target in the center (prescription dose 50 Gy).
Methods
This optimization process included three steps. (1) A dummy IMPT plan was created by replacing different energy layers in a real IMPT plan with single energy but inserting different numbers of equal-thickness PMMA slabs in the beam path. Every layer in this plan has the same spot map and the spot distance was determined by the beam width used. Doses for all beamlets was calculated with a fast Monte Carlo dose engine (FDC) and fed to the L-BFGS algorithm for spot weight optimization. (2) The optimized spot weights were converted to RF dimensions using it’s relation to the ladder area in the ridge filter. The summary of the spot weights at the same spot across different layers were assigned to a real IMPT plan with a single-energy layer. (3) The single-energy layer IMPT plan was reoptimized with the ridge filter in the beam path.
Results
For the water phantom, the single layer plan used 208.3 MeV beam with a spot distance of 5 mm. The ridge filter was composed of 484 pyramids, and each pyramid had a base size of 3.974´3.974 mm2, a total thickness of 66 mm and 22 ladders. The DVH indices for the target are 46.6 Gy (93.2%) for D98, 47.8 Gy (95.6%) for D95, 52.6 Gy (105.2 %) for D5, 53.3 Gy (106.6%) for D2. The Homogeneity Index is 1.1.
Conclusion
This procedure works for the water phantom according to the acceptable plan parameters. This procedure is promising for real patient application with flash beam.