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Explore the Dosimetric Impact of the Prescription Isodose Line (IDL) On the Quality of Robotic SRS Plans

Q Xu

Q Xu1*, J Fan2 , S Asbell1 , J Park1 , G Kubicek1 , (1) MD Anderson Cancer Center at Cooper, Mt Laurel, NJ, (2) Fox Chase Cancer Center, Philadelphia, PA


SU-I-GPD-T-575 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Currently, there is no consensus on the optimal prescription isodose line (IDL) for CyberKnife SRS. We investigate the dosimetric impact on the quality of CyberKnife plans with low (50%) and high IDL (90%).

Methods: Ten patients with 10 brain tumors treated with CyberKnife were included in the study. The mean volume and size of the tumors were 7±6 cc and 25.5±8 mm. For all plans we used a prescription of 25 Gy in 5 fractions. The allowed maximum dose was 50 Gy for low IDL and 27 Gy for high IDL. Four shells were generated around the target with distances from 3 mm to 60 mm, and the constraint dose to the first shell (D1) was the key to reach optimal plans. The optimal low IDL plan was reached after gradually reducing D1 from 25 Gy to near 21.5 Gy when coverage started to degrade. For high IDL plans, a similar technique was used and the optimal D1 reached around 24 Gy.

Results: The V5Gy, V10Gy, V15Gy, V20Gy, V25Gy of low IDL (50%±2%) plans were 26.3%, 28.6%, 38.2%, 36.6% and 8.2% lower than those from high IDL (89%±2%) plans. The Conformality Index of low IDL plans outperformed those from high IDL plans (mean 1.14 vs. 1.26) except for a lesion under 0.5 cc. The low IDL plans also had fewer beams (131 vs. 147) and fewer MUs (15542 vs. 16803).

Conclusion: Low IDL plans outperform high IDL plans for all metrics in tumors > 0.5 cc. Although there is a higher maximum dose, the lower dose to normal CNS tissue and better CI could potentially allow for less radiation necrosis.

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