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Very Small MLC-Defined Arcing Fields: Accuracy of Planning and Delivery

I Brezovich

I Brezovich*, X Wu , R Popple , S Shen , E Covington , University of Alabama at Birmingham , Birmingham, AL


SU-I-GPD-T-625 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To experimentally verify accuracy of a standard treatment planning system (TPS) to compute dose distributions of small MLC-defined arcing fields for use in SRS, and to test spatial accuracy of dose delivery.

Methods: A block phantom was constructed, allowing GafChromic film placement in the coronal plane. Single 360° arcs with 1.6x5, 2.0x5, 2.4x5, 5x5, 10x10, 15x15, 20x20, 25x25 and 30x30 mm fields were planned on an Eclipse workstation for delivery with 10X flattening-filter-free beams on a Varian Edge accelerator. Films were evaluated using institution-developed software. Computed and measured isodose lines and beam profiles were displayed.

Results: Averaged over all 10 measurements/field and all field sizes, delivery error was 2.4 ± 2.0% of planned dose. Average geographic vector error of all measurements (vector error = square root of sum of squared x-and y- errors), was 0.18 ± 0.07 mm, maximum 0.20 mm. Planned and measured beam profiles from 20% to 80% of maximum agreed within better than 0.2 mm. The number of monitor units required for 8 Gy at isocenter increased from 1138 MU for a 30x30 field to 2701 for 1.6x5mm. Accuracy of treatment planning and delivery with multiple arcs was tested by a 9-arc arrangement at 6 different couch angles (0°, 10°, 45°, 90°, 350°, 315°), using 2.4x5 mm fields arcing through 160°. Average dose error was 0.3±2.0%, 3.1% maximum. Vector errors averaged 0.20±0.07 mm, maximum 0.43 mm. The larger spatial error in these experiments was attributed to slight mismatch between couch axis and MV isocenter.

Conclusion: Even for field sizes as small as 1.6x5 mm the TPS provides accurate doses. These can be used to find arrangements of static arcs as needed for individual treatment sites. The 9-arc arrangement was tested because it closely replicated the dose distribution of a GammaKnife with 4 mm collimators.

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