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Dosimetric Effect of Uncorrected Rotations in Lung SBRT with Stereotactic Imaging Guidance

T Zhuang

T Zhuang1*, A Markovich2, P Xia3, (1) Cleveland Clinic Foundation, Cleveland, OH, (2) Cleveland State University, Cleveland, OH, (3) The Cleveland Clinic Foundation, Cleveland, OH

SU-E-T-437 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The stereoscopic imaging guidance can detect three translational and three rotational corrections for the treatment setup in lung SBRT. The rotational corrections may not be performed if a 6-degree treatment table is not available. We aim to investigate the dosimetric effect of uncorrected rotations for Lung SBRT.

Methods: Twelve patients with NSCLC who received SBRT were selected for this study. To simulate uncorrected rotations, the planning CT and contours were rotated around the iso-center by 1°, 2°, 3°, 4°, 5° in all three axes. For each patient, the initial plan was then applied to the rtated CT and contours, and the dose to the rotated CT was recalculated using a Monde Carlo dose calculation method. The ratio of D1, D95 and D99 of the PTV between plans on the original and rotated CT were computed.

Results: The ratios of D95 for different rotations were: 100.0±0.6% (1°), 99.6±0.7% (2°), 99.4±1.0% (3°), 98.7±2.4% (4°), 98.6±1.0% (5°). The ratios of D1 were: 99.8±0.7% (1°), 99.6±0.8% (2°), 99.6±0.7% (3°), 99.5±1.0% (4°), 99.4±1.0% (5°). The ratios of D99 were: 99.0±1.1% (1°), 98.7±1.5% (2°), 98.1±2.2% (3°), 96.2±4.8% (4°), 96.0±4.4% (5°). Compared to the dose distribution in the original plans, less than 1% reduction in D95 of the PTV was observed if the uncorrected rotations were less than 3°. D1 of the PTV was less than 1% for uncorrected rotations up to 5°. D99 of the PTV exceeded 4% for uncorrected rotation greater than 4°.

Conclusion: The rotation of less than 3° in all directions has negligible dose effects in Lung SBRT under stereoscopic image guidance.

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