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A Method for Improving Dose Gradient for Robotic Radiosurgery

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T Li

T Li*, S Burton , J Flickinger , D E Heron , M Saiful Huq , University of Pittsburgh Medical Center, Pittsburgh, PA


SU-E-T-40 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:For targets with substantial volume, collimators of relatively large sizes are usually selected to minimize the treatment time in robotic radiosurgery. Their large penumbrae may adversely affect the dose gradient around the target. In this study, we implement and evaluate an inner-shell planning method to increase the dose gradient.

Methods:Ten patients previously treated with CyberKnife M6 system were randomly selected with the only criteria that PTV be larger than 2cm³. A new plan was generated for each patient, in which the PTV was split into two regions: an inner shell and a core, where the core was created by shrinking the PTV by 5mm using 3D erosion, and the shell was obtained by subtracting the core from the PTV; then a 7.5mm Iris collimator was exclusively applied to the shell, with other appropriate collimators applied to the core depending on its size. The optimization objective functions and constraints were kept the same as the corresponding clinical plans. The results were analyzed for V12Gy, V9Gy, V5Gy, and gradient index (GI).

Results:Volume reduction is found for the inner-shell method at all studied dose levels as compared to the clinical plans. The absolute dose volume reduction ranged from 0.05cm³ to 18.5cm³ with a mean of 5.6cm³ for 12Gy, from 0.2cm³ to 38.1cm³ with a mean of 9.8cm³ for 9Gy, and from 1.5cm³ to 115.7cm³ with a mean of 24.8cm³ for 5Gy, respectively. The relative GI reduction ranged from 3.2% to 23.6%, with a mean of 12.6%. Paired t-test for GI has a p-value of 0.0014. The range for treatment time increase is from -3 min to 20 min, with a mean of 7.0 min.

Conclusion:The inner-shell planning method can significantly increase the dose gradient outside the PTV, while maintaining good coverage, conformity, and reasonable treatment time.

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