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Radiosurgery of Peripheral Brain Lesions by Spot Scanning Proton Therapy

B Dirksen

B Dirksen*, D Wang, D Hyer, J Buatti, A Sheybani, M TenNapel, J Bayouth, R Flynn, University of Iowa Hospitals and Clinics, Iowa City, IA

SU-E-T-690 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To determine how small the lateral width in air of proton spots must be in order for proton spot scanning (SS) plan quality to be superior to x-ray radiosurgery for peripheral brain lesions.

Methods: Single-beam proton SS plans with sigma (lateral spot intensity standard deviation) values ranging from 1 to 14 mm, x-ray volumetric modulated arc therapy (VMAT) utilizing a 0.5 cm leaf thickness, and cone-based x-ray radiosurgery plans were generated for eleven patients with clinical target volumes ranging from 0.5 cm3 to 25 cm3. Plans were evaluated using brain necrosis normal tissue complication probability (NTCP), high dose conformity index, and integral dose metrics.

Results: For proton SS plans, as sigma increased from 1 to 14 mm, integral dose relative to the cone plan increased from an average of 0.10 (range 0.05 to 0.17) to 0.63 (range 0.26 to 0.85) compared an average of 1.49 (range 0.90 to 3.00) for VMAT. As SS sigma increased from 1 mm to 14 mm, NTCP for healthy brain necrosis increased from an average of 0.02 (range 0.01 to 0.07) to 0.08 (range 0.01 to 0.18) compared to the averages of 0.06 (range 0.01 to 0.15) for cone and 0.05 (range 0.01 to 0.11) for VMAT plans. An in-air sigma of less than 6 mm was required for the SS plans to have a statistically significant (p < 0.05) NTCP reduction relative to the x-ray techniques.

Conclusion: When treating peripheral brain lesions, which are ideal cases for SS since only a single proton beam direction is needed, SS only provides a superior NTCP to x-ray radiosurgery techniques if the proton therapy system can produce an in-air sigma of 6 mm or less.

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