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HDR-Like Prostate SBRT Plan Considering Physical Characteristics of CyberKnife with MLC

J Suzuki1*, Y Yoshioka2 , H Takahashi1 , T Matsunaga1 , K Hamajima1 , T Nakamura1 , T Okuda1 , (1) TOYOTA Memorial Hospital, Toyota, Aichi, (2) The Cancer Institute Hospital of Japanese Foundation for Cancer Research, Koto, Tokyo


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

Room: Exhibit Hall

Purpose: The CyberKnife system enables delivery of large number of non-isocentric and non-coplanar flattening filter free beams. However, in the case of using MLC, it delivers isocentric beams. Therefore, the beam intensity is optimized with four characteristic beam segment shapes because the target central axis intensity is increased and the peripheral intensity is reduced. In this study, considering the physical characteristics of the MLC beam, we propose that CyberKnife with MLC is advantageous for HDR-like prostate SBRT plan.

Methods: In each of ten patients, the four characteristic beam segment shapes and dose distribution were compared between HDR-like prostate SBRT plan (36.25 Gy/5 fr) and hypofractionated prostate IMRT plan (70 Gy/28 fr). Furthermore, the prostatic urethra of HDR-like heterogeneous prostate SBRT plan was evaluated with dose–volume histogram.

Results: The ratios in which the perimeter segment shapes the narrow fields around the perimeter of the target and the random segment shapes the circular fields around the perimeter were 47.0% and 49.6% in all segment shapes for the HDR-like prostate SBRT plan, and 43.5% and 22.7% for the hypofractionated prostate IMRT plan, respectively. By effectively using the marginal segment, i.e., the perimeter segment and random segment shapes, the prostatic urethra dose could be reduced to D10% = 37.0 Gy, D30% = 36.3 Gy and Dmax = 102.5% in average. The dose distribution for the HDR-like prostate SBRT plan was high dose to low dose gradient toward the center from the margin of the target compared to uniform dose distribution for the hypofractionated prostate IMRT plan.

Conclusion: Use of CyberKnife with MLC was effective and easy to reduce the prostatic urethra dose and ideal for HDR-like prostate SBRT plan.


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