2087 - Commissioning and First Clinical Treatment of Mini-Lattice Radiation Therapy

Purpose/Objective(s): Spatially fractionated radiation therapy (SFRT) intentionally delivers heterogenous dose distributions consisting of alternating regions of high dose “peaks” and low dose “valleys”. The first clinical implementation of submillimeter-wide photon minibeams was recently demonstrated. However, this treatment uses low (keV) energy x-rays from an orthovoltage machine, which limits treatment depths. Here, we address this limitation and describe the commissioning and first patient treatment using mini-lattice radiation therapy (MLRT). MLRT uses a linac and decreases the size and spacing of standard lattice SFRT by using the width of individual multileaf collimators (MLCs) to deliver 5mm wide high dose regions and low dose valleys to targets.

Materials/Methods: MLRT plans were created in the treatment planning system equipped with Millennium 120 MLCs. MLRT uses 6MV flattening filter free high dose rate and the width of individual MLCs to define 5mm by 5mm openings separated by closed MLCs to deliver alternating opened and blocked regions. Dynamic conformal arcs were used to conform MLCs to 4mm spherical mini-lattice structures in the gross tumor volume (GTV) throughout the arcs. An in-house ESAPI script was employed to maintain 5mm MLC openings at all control points, move closed MLCs under a jaw, and shift jaw positions to 2mm beyond the most lateral MLC position. A MLRT-specific beam model was commissioned to accurately model the small MLRT fields. Film measurements were performed to validate MLRT plans. Plans for 7 treatment sites in different parts of the body were retrospectively created to assess treatment quality. Ultimately, a pediatric patient with multiply recurrent metastatic Ewing’s sarcoma in the left lung was treated with two fractions of MLRT.

Results: The MLRT-specific beam model resulted in gamma passing rates (1%/0.5mm criteria) of 90-99% for all film measurements. MLRT plans resulted in two to three times more lattices within the GTV compared to standard VMAT lattice. No significant difference between VMAT lattice and MLRT plan max dose outside the GTV was observed. MLRT plans resulted in a mean dose of 396cGy, a mean equivalent uniform dose of 295cGy, and D10%/D90% of 10. MLRT plans using 3 and 5 mini-lattices in the first and second fraction, respectively, were delivered to a 16cm³ target for the patient treatment. Both plans delivered a D90% to mini-lattice structures greater than 1800cGy. The patient reported pain relief in the left arm following the first fraction, had stable disease, and no acute toxicities following both fractions.

Conclusion: MLRT is a feasible approach to target small and irregularly shaped tumors with SFRT. It can be delivered to deep-seated tumors anywhere in the body using widely available linacs and existing radiation oncology infrastructure. Clinical trials evaluating this technique are in development.