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Rotational Film Dosimetry Using a Simple Phantom for Patient-Specific QA of SBRT/SRS Plans

M Grams

M Grams*, A Deisher , L Fong de los Santos , Mayo Clinic, Rochester, MN


SU-E-108-7 (Sunday, July 30, 2017) 1:00 PM - 1:55 PM Room: 108

Purpose: To describe the design and use of a simple rotational phantom for patient-specific QA of SBRT/SRS treatment plans using radiochromic film dosimetry.

Methods: A cylindrically shaped phantom consisting of separable upper and lower halves was machined from solid water. The cylinder rests on plastic bearings within an acrylic base and can rotate 360° about its central axis. The center of the phantom accommodates a half sheet of radiochromic film, and when not rotated, the film resides in the coronal plane. By rotating the cylinder, the film can be placed in any plane between coronal and sagittal. Calculated dose planes coinciding with rotated film measurements are exported by rotating the CT image and dose distribution within the treatment planning system. The process is illustrated using a brain SRS treatment plan involving 4 separate targets, as well as a spine SBRT plan. Additionally, 199 patient-specific QA film measurements of SBRT cases involving the brain, spine, lung, liver, and pelvis were obtained with the rotational phantom and analyzed with a 2%/2 mm gamma criterion.

Results: The average 2%/2 mm gamma passing rate for all 199 SBRT plans was 99.5% and ranged from 93.3% - 100%. Thirty-four of the 199 plans were measured with the plane of the film rotated beyond the coronal or sagittal planes and had an average passing of 99.4% (range: 96.7% - 100%).

Conclusion: The rotational phantom technique allows for accurate film measurements in any plane, including those between sagittal and coronal. By rotating the cylinder containing the film, regions of a dose distribution which might otherwise require multiple sagittal or coronal measurements can be verified with as few as a single measurement. This increases efficiency, and in combination with the high spatial resolution inherent to film dosimetry makes the rotational technique an attractive option for SRS/SBRT patient-specific QA.

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