A novel staggered lattice SFRT approach for the management of nonresponding bulky tumors: A dosimetric study.

We propose a novel implementation of spatially fractionated radiotherapy (SFRT), in which 2 SFRT treatments are delivered with staggered high- and low-dose regions. Thirty previously treated patients with SFRT (10 head and neck, 10 chest/lung, and 10 pelvis/abdominal) were replanned using this staggered scheme. Six different staggered schemes were investigated. The first treatment was planned via either a standard lattice deployment or a maximized-sphere placement method. For the second treatment, 3 approaches were employed to ensure the staggering of the high- and low-dose regions by creating a structure that is a Boolean of the original gross tumor volume (GTV) minus various isodose volumes of the first SFRT treatment (30%, 40%, and 50%). This new structure was used for lattice deployment for the second treatment. All replans utilized 1.5 cm diameter spheres with center-to-center spacing of at least 3√2 cm. Plan quality was assessed based on biological effective dose (BED) delivered to the GTV (D5%, D10%, D50%, D90%, and D95%). Normal tissue sparing was assessed using D2cm (maximum dose 2 cm away in any direction from the GTV) and the maximum BED to nearby critical structures. All replans were benchmarked against the clinically delivered MLC-based 3D-conformal SFRT plans. Out of 6 treatment schemes, this novel treatment scheme demonstrated a statistical increase in all GTV metrics, with treatment scheme #3 utilizing the 50% isodose volume producing the best quality plans in terms of BED D5% (Δ mean = 9.59 Gy, p-value < 0.001), D10% (Δ mean = 8.07 Gy, p-value < 0.001), Dmean (Δ mean = 8.69 Gy, p-value < 0.001), D50% (Δ mean = 8.28 Gy, p-value < 0.001), D90% (Δ mean = 8.77 Gy, p-value < 0.001), and D95% (Δ mean = 7.77 Gy, p-value < 0.001) compared to the original clinical plans. All treatment schemes produced a statistically significant decrease in D2cm compared to the clinical plans. However, for head and neck patients, the maximum BED to the spinal cord increased significantly compared to the clinical plan. In contrast, patients with chest/lung and pelvic tumors observed no statistically significant increases in maximum doses to nearby critical organs. Especially for patients with pelvic tumors, we observed an average decrease in the maximum BED to these critical organs for most treatment schemes. This novel staggered treatment scheme demonstrated enhanced GTV dose metrics in terms of BED, suggesting potential for improved clinical outcomes and tumor debulking. However, this scheme may not apply to all patients, especially those with head and neck SFRT cases. We recommend using this scheme for pelvic patients with tumors that are radioresistant and/or rapidly growing bulky masses.