Applying a Proton Procedure of Optimizing Spot Weight and Ridge Filter to Single-Energy Carbon Pencil Beam
Abstract
Purpose
To apply a proton procedure of optimizing spot weight and Ridge Filter (RF) to single-energy carbon pencil beam and evaluate it in a water phantom with a square-shape target in the center (prescription dose 50 Gy).
Methods
A 3-step optimization procedure to optimize plan spot weight and RF dimension was developed for proton pencil beam recently. The procedure was based on a fast Monte Carlo dose engine (FDC) and L-BFGS algorithm. The 3 steps included (1) obtaining spot weight and RF dimension information from optimizing a dummy IMPT plan; (2) Converting spot weights of the optimized dummy IMPT plan to RF dimensions and spot weights for a real IMPT plan with a single-energy layer; and (3) reoptimizing the single layer IMPT plan with the ridge filter in the beam path. We applied this procedure to single-energy carbon pencil beam.
Results
For the water phantom, the single layer plan used 404.5 MeV beam with a spot distance of 4 mm. The ridge filter was composed of 729 pyramids, and each pyramid had a base size of 3.826´3.826 mm2, a total thickness of 66 mm and 22 ladders. The DVH indices for the target are 40.1 Gy (80.0%) for D98, 45.2 Gy (90.4%) for D95, 55.4 Gy (111 %) for D5, 57.0 Gy (114%) for D2. The Homogeneity Index is 1.2.
Conclusion
For single-energy carbon beam, this optimization procedure does not work as good as proton beam according to plan parameters. One of the reasons is that carbon beams are narrow in lateral direction which requires small spot distance and small size of pyramids in the procedure. It causes the conversion from spot weight to RF less accurate. We expected plan improvement for shallower target for which lower carbon energy will be used and lower energy has wider beam.