Room: Exhibit Hall
Purpose: Feasibility and related plan parameters for adapting VMAT technique for spatially fractionated radiation therapy (VMAT-GRID).
Methods: 23 grid patients with tumor volumes of 71.6-4683.9cc were selected. The prescription was 20Gy to maximum dose of GTV in a single fraction. VMAT-GRID plan was generated using Eclipse TPS: Mean dose (Dmean) with range of 8.5 to 9.5 Gy, equivalent uniform dose (EUD) with range of 6 to 9 Gy were used for GTV dose constraints. A virtual grid block (VGB) was optimized by satisfying the dose constraints and minimizing the valley to peak ratio (V/P). 6MV flattening-filter-free (FFF) beams, two to six arcs with collimator of 90 and 270 degrees were used except one elongated tumor which used 10 arcs and two iso-centers. Dosimetric parameters included: GTV Dmean, EUD and V/P, normal tissue mean dose (ring1: 5mm from GTV surface and extend outward by 5mm; ring2: an extension outward of 1cm from the out-surface of ring1), delivery MU.
Results: The optimized VGB had a median diameter of opening area (dopen) of 9mm, range (8-14mm), a median center-to-center distance (dc-c) of 27mm, range (20-46mm). For GTV, the median and ranges of Dmean was 9.19Gy(8.77 to 9.38Gy), EUD was 6.29Gy (3.60 to 8.78Gy) and V/P was 0.08(0.028 to 0.26). For normal tissue, the median mean dose for ring1 was 4.64Gy(3.61 to 6.45Gy) and was 3.09Gy(2.10 to 5.19Gy) for ring2. The median number of MUâ€™s was 4833MU (2724 to 17367MU).
Conclusion: All plans satisfied the GTV dose constraints. A 2.5mm truebeam HD MLC-leaf width allows a smaller dopen and dc-c, similar to our cerrobend alloy grid block (dopen=14mm, dc-c=21mm at isocenter plane). VGB has the capability for customizing the treatment field to spare of OARs of individual patients. In addition, VMAT-GRID plan uses FFF beam which significantly reduce treatment times without compromising dosimetric quality.
Not Applicable / None Entered.