Dosimetric Comparison of Various Spot Placement Techniques in Proton Pencil Beam Scanning

Abstract

Purpose: To present quantitative dosimetric evaluations of five proton pencil beam spot placement techniques.

Materials and methods: The spot placement techniques that were investigated include two grid-based (rectilinear grid and hexagonal grid, both commonly available in commercial planning systems) and three boundary-contoured (concentric contours, hybrid, and optimized) techniques. Treatment plans were created for two different target volumes, one spherical and one conical. An optimal set of planning parameters was defined for all treatment plans and the impact of spot placement techniques on the plan quality was evaluated in terms of lateral/distal dose falloff, normal tissue sparing, conformity and homogeneity of dose distributions, as well as total number of spots used.

Results: The results of this work highlight that for grid-based spot placement techniques, the dose conformity is dependent on target cross-sectional shape perpendicular to beam direction, which changes for each energy layer. This variable conformity problem is mitigated by using boundary contoured spot placement techniques. However, in the case of concentric contours, the conformity is improved but at the cost of decreased homogeneity inside the target. Hybrid and optimized spot placement techniques, which use contoured spots at the boundary and gridlike interior spot patterns, provide more uniform dose distributions inside the target volume while maintaining the improved dose conformity. The optimized spot placement technique improved target coverage, homogeneity of dose, and minimal number of spots. The dependence of these results on spot size is also presented for both target shapes.

Conclusion: This work illustrates that boundary-contoured spot placement techniques offer marked improvement in dosimetry metrics when compared to commercially available grid-based techniques for a range of proton scanned beam spot sizes.

Keywords: proton therapy; spot placement in proton pencil beam scanning.

Figure 1.

Dose distributions from all five spot placement techniques for spherical target volume using large spot size (row 1), spherical target volume using small spot size (row 2), conical target volume using large spot size (row 3), and conical target volume using small spot size (row 4). The beam is incident from the left (rows 1 and 2) and from the top (rows 3 and 4).

Figure 2.

Spatial distribution of spots using all available spot placement techniques for the conical target volume at an energy layer of 121.77 MeV shown as a blue stripe (a), using rectilinear grid (b), hexagonal grid (c), concentric contours (d), hybrid (e), and optimized (f).

Figure 3.

Dose profiles for the spherical target volume using all spot placement techniques. Lateral and central axis dose profiles using large spot size (a, b), and central axis depth dose profiles using small spot size (c, d).

Figure 4.

Dose profiles for the conical target volume using all spot placement techniques. Lateral and central axis dose profiles using large spot size (a, b), and central axis depth dose profiles using small spot size (c, d).