Spatially fractionated stereotactic body radiation therapy (Lattice) for large tumors

Abstract

Purpose: Stereotactic body radiation therapy (SBRT) has demonstrated clinical benefits for patients with metastatic and/or unresectable cancer. Technical considerations of treatment delivery and nearby organs at risk can limit the use of SBRT in large tumors or those in unfavorable locations. Spatially fractionated radiation therapy (SFRT) may address this limitation because this technique can deliver high-dose radiation to discrete subvolume vertices inside a tumor target while restricting the remainder of the target to a safer lower dose. Indeed, SFRT, such as GRID, has been used to treat large tumors with reported dramatic tumor response and minimal side effects. Lattice is a modern approach to SFRT delivered with arc-based therapy, which may allow for safe, high-quality SBRT for large and/or deep tumors.

Methods and materials: Herein, we report the results of a dosimetry and quality assurance feasibility study of Lattice SBRT in 11 patients with 12 tumor targets, each ≥10 cm in an axial dimension. Prior computed tomography simulation scans were used to generate volumetric modulated arc therapy Lattice SBRT plans that were then delivered on clinically available Linacs. Quality assurance testing included external portal imaging device and ion chamber analyses.

Results: All generated plans met the standard SBRT dose constraints, such as those from the American Association of Physicists in Medicine Task Group 101. Additionally, we provide a step-by-step approach to generate and deliver Lattice SBRT plans using commercially available treatment technology.

Conclusions: Lattice SBRT is currently being tested in a prospective trial for patients with metastatic cancer who need palliation of large tumors (NCT04553471, NCT04133415).

Figure 1

(A) Geometric representation of sphere placement. Yellow dots represent the 1.5 cm diameter planning target volume (PTV)_6670 target vertices, and dotted line vertices represent the transposed target vertices from adjacent planes. Axial planes where vertices are placed are separated by 3 cm in plane. Within a plan, vertices are separated by 6 cm center to center (4.5 cm edge to edge) in orthogonal axes, and 3√2 cm along the diagonal. (B) Axial computed tomography slices of a target with the yellow outlined target vertices (PTV_6670) in each plane, red gross tumor volume_2000, and green PTV_2000. Magenta arrows denote cropped vertices in PTV_6670 that extend outside of the gross tumor volume_2000. (C) Dose distribution after volumetric modulated arc therapy planning for the target with blue representing 20 Gy and red 66.7 Gy. (A color version of this figure is available at https://doi.org/10.1016/j.adro.2020.100639.)