Practical Implementation of Lattice-Based Spatially Fractionated Radiotherapy for Large Tumors on an MR-Linac
Abstract
Purpose
Spatially fractionated radiotherapy (SFRT) using lattice dose distributions enables delivery of ablative, non-uniform doses to heterogeneous large tumors while sparing surrounding normal tissues. Implementation on MRI-guided linear accelerators (MR-Linac) introduces additional planning and delivery challenges related to beam geometry and adaptive workflows. This study demonstrates a practical lattice-based SFRT planning strategy for large tumors on an MR-Linac.
Methods
Two patients with large soft-tissue tumors (PTV volumes: 485.3 and 969.8 cc) were planned on an Elekta Unity MR-Linac using the Monaco treatment planning system. Lattice vertices were generated within each GTV using MIM software with a 0.5 cm exclusion margin from the tumor boundary. Spherical vertices (1.5 cm diameter) were placed with 3.0 cm center-to-center spacing and prescribed 66.7 Gy in 5 fractions (PTV_High). To maintain peak confinement and suppress valley dose, avoidance rings were incorporated during IMRT optimization. PTV_Low was defined with a 0.2 cm margin from the GTV and prescribed 20 Gy in 5 fractions. Target coverage, peak-to-valley dose, and plan complexity were evaluated. Patient-specific QA was performed using an MR-compatible ArcCHECK.
Results
All plans achieved adequate target coverage, with PTV V95% ≥ 95%. Lattice boost coverage was maintained, with >95% of the PTV_High receiving at least 95% of the prescribed boost dose. The largest plan required 199 segments from nine static beams and 12,654 MU, corresponding to an estimated beam-on time of approximately 45 minutes. Distinct peak–valley dose separation was preserved. MR-compatible ArcCHECK QA demonstrated acceptable deliverability, with gamma passing rates exceeding institutional thresholds.
Conclusion
This work demonstrates the feasibility and deliverability of lattice-based SFRT for large, heterogeneous tumors on an MR-Linac using a reproducible planning strategy. The approach maintains clinically acceptable target coverage and low OAR doses while enabling personalized MR-guided lattice planning. These results support further clinical evaluation and optimization of MR-guided lattice SFRT.