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A Dosimetric Study of Spatially Fractionated Radiation (GRID) Using Volumetric Modulated Arc Therapy Technique(VMAT-GRID)

X Zhang1*, S Morrill1 , H Zhang2 , G Narayanasamy1 , E Galhardo1 , J Fang1 , T Kim1 , S Maraboyina1 , J Penagaricano1 , (1) University of Arkansas for Medical Sciences, Little Rock, AR, (2) Northwestern Memorial Hospital, Chicago, IL.


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

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 MU’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.


TH- External beam- photons: Development (new technology and techniques)

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