Demonstration of an enhanced dosing pattern for debulking large and bulky unresectable tumors via differential hole-size spatially fractionated radiotherapy.

Abstract

Purpose/objective: We propose a novel lattice deployment for spatially fractionated radiotherapy (SFRT) treatments. In this approach, a larger diameter high-dose sphere is centrally placed in the bulky tumor mass and surrounded by smaller diameter high-dose spheres.

Materials/methods: Thirty SFRT patients (10 head and neck [HN], 10 abdominal/pelvis, and 10 chest/lung cases) treated with an MLC-based crossfire method were retrospectively analyzed. Eleven differential hole-size lattice patterns were benchmarked against the clinically delivered SFRT plans (1 cm diameter cylinders, 2 cm spacing) and the standard uniform lattice pattern (1.5 cm diameter spheres, 3 cm spacing). These patterns varied in core diameter (C: 2-4 cm), spacing (S: 2-4 cm), and peripheral diameter (P: 1-2 cm). In addition to peak-to-valley-dose ratio (PVDR), tumor dose metrics (D_50%, V_50%, D_mean)_, D_max to nearby critical organs, and ablative dose (V_75%/V_50% and V_15Gy) were evaluated.

Results: 10 out of 11 differential hole-size patterns showed increases in D_50%, D_mean, and V_50% compared to the standard lattice pattern. One pattern (C = 3 cm, S = 2 cm, P = 1.5 cm) outperformed the clinical SFRT plans in D_50% (Δ = 1.8 Gy, p = 0.003; Δ = 2.0 Gy, p = 0.015; Δ = 0.9 Gy, p = 0.045), D_mean (Δ = 1.6 Gy, p = 0.003; Δ = 1.7 Gy, p = 0.021; Δ = 0.7 Gy, p = 0.042), and V_50% (Δ = 20.4%, p < 0.001; Δ = 16.6%, p = 0.008; Δ = 10.3%, p = 0.079) for the HN, abdominal/pelvis, and chest/lung SFRT patients, respectively. This pattern also demonstrated average increases to D_5% D_10%, D_90% across all 30 patients compared to both benchmarked patterns. However, this pattern showed reduced PVDR compared to the clinical and standard SFRT plans but still achieved a ratio > 3.0. All differential hole-size patterns demonstrated decreases in D_max to critical organs compared to the clinical SFRT plans. Moreover, compared to the clinical SFRT and the standard lattice plans, 9 out of 11 differential hole-size patterns demonstrated increases in V_75%/V_50% and V_15Gy.

Conclusion: All differential hole-size SFRT replans were clinically acceptable, with C = 3 cm, S = 2 cm, and P = 1.5 cm providing the optimal setting for select tumors. Differential lattice patterns enhanced the ablative dose to the bulky tumors while restricting the maximum dose to adjacent critical organs.