Mini‐lattice radiation therapy: A treatment planning approach to miniaturize spatially fractionated lattice radiation therapy using a clinical linear accelerator

BACKGROUND: Spatially fractionated radiation therapy (SFRT) is a technique that delivers heterogenous dose distributions consisting of alternating regions of high dose "peaks" and low dose "valleys". Current delivery methods for SFRT using megavoltage x-rays usually treat large and bulky tumors with brass grid or volumetric modulated arc therapy (VMAT) lattice techniques. The size and spacing of high dose regions in these approaches are typically on the order of centimeters. However, multiple studies have suggested that decreasing these dimensions may improve the therapeutic ratio. Furthermore, a more compact approach to SFRT would allow for a greater number of high dose regions within the tumor, as well as application to smaller and more irregularly shaped targets thereby increasing the number of patients that could benefit from SFRT. PURPOSE: This study describes the commissioning and first patient treatment using mini-lattice radiation therapy (MLRT). MLRT uses a clinical linear accelerator and decreases the size and spacing of standard lattice SFRT by using individual multileaf collimator (MLC) leaves to deliver 5 mm wide high dose regions. METHODS: MLRT plans were created in the Varian Eclipse treatment planning system for a Varian Truebeam equipped with Millennium 120 MLCs. MLRT uses 6 MV Flattening Filter Free high dose rate and the width of individual MLCs to define 5 mm by 5 mm openings separated by closed MLCs to deliver alternating opened and blocked regions. Dynamic conformal arcs were used to conform MLCs to 4 mm spherical mini-lattice structures in the gross tumor volume (GTV). A MLRT-specific beam model was commissioned to accurately model the small MLRT fields. Film measurements were performed to assess the accuracy of MLRT plans calculations. Plans for seven treatment sites in different parts of the body for retrospective patient candidates were created with varying numbers of mini-lattices and separation distances to assess the impact of varying these parameters on treatment dose metrics. MLRT was used for the first time to treat a patient with two fractions of MLRT. RESULTS: The AcurosXB calculation algorithm with modified x and y spot sizes, dosimetric leaf gap, transmission factor, and output factor table was used to generate a beam model for accurate MLRT calculations. The MLRT-specific beam model resulted in gamma passing rates (1%/0.5 mm criteria) of 90%-99% for retrospective patient MLRT film measurements. Dose volume histogram statistics, equivalent uniform dose, and mean dose showed a higher number of mini-lattices with smaller separation increased the dose to the GTV and surrounding tissue. Separation distances between mini-lattices did not impact plan heterogeneity as measured by D10%/D90%. The first patient treated with the MLRT technique reported pain relief, had stable disease, and no acute toxicities following both fractions. CONCLUSIONS: MLRT is feasible using clinical linear accelerators and existing radiation oncology infrastructure. It enables targeting smaller tumors with SFRT, allows for a greater number of high dose regions within the target compared to standard VMAT lattice techniques, and may be a useful technique for challenging treatment scenarios.