Implementation of VMAT-based lattice spatially fractionated radiation therapy (SFRT) across linac platforms: Practical considerations for QA and motion management.

BACKGROUND

Spatially fractionated radiation therapy (SFRT) offers a method to treat bulky, radioresistant tumors while sparing normal tissues. Modern volumetric modulated arc therapy (VMAT) delivery enables conformal lattice SFRT plans, but practical guidance for implementation across different linear accelerators is limited.

PURPOSE: This study presents a structured clinical workflow for implementing lattice SFRT across Edge and Halcyon linacs, with emphasis on quality assurance (QA) and motion management.

METHODS

A systematic workflow was developed using Eclipse TPS (Acuros v16) with 6 MV flattening filter-free (FFF) beams and applied to Edge and Halcyon linacs. The implementation progressed from 28 preliminary phantom and test plans to 12 representative clinical test plans across head and neck, pelvis, and extremity sites, and 6 lung plans evaluating respiratory motion effects with and without the SDX breath-hold system. Two prescription strategies were investigated: (a) a single-fraction lattice regimen delivering 18 Gy to vertices with valley doses < 6 Gy, and (b) a simultaneous integrated boost (SIB) approach delivering 66.7 Gy to vertices with 20 Gy in 5 fractions to the entire target. Dosimetric accuracy and deliverability were evaluated using Delta4, Electronic Portal Imaging Device (EPID), and film measurements.

RESULTS

VMAT-based lattice SFRT plans were successfully delivered on both linacs with consistent dosimetric performance. Point-based peak-to-valley dose ratios (PVDRs) ranged from 2.8 to 3.4 across the two platforms for both dose prescriptions. Delta4 and EPID gamma pass rates exceeded 90% (3%/1 mm), and film dosimetry agreed within ∼5% uncertainty. Comparable outcomes between Edge and Halcyon support cross-platform reproducibility. In the evaluated lung cases, motion on the order of 5 mm was associated with changes in organ-at-risk (OAR) doses, highlighting the need for motion management.

CONCLUSIONS

This structured framework demonstrates that lattice SFRT can be safely implemented across different linac platforms. While dosimetric outcomes are institution-dependent, the method provides practical guidance for clinical adoption.