Impact of spot size on plan quality of spot scanning proton radiosurgery for peripheral XXXX .

@entity1 @entity39 @entity75 @entity5 @entity34
PURPOSE: To determine the plan quality of proton spot scanning (SS) radiosurgery as a function of spot size (in-air sigma) in comparison to x-ray radiosurgery for treating peripheral @entity75 . METHODS: Single-field optimized (SFO) proton SS plans with sigma ranging from 1 to 8 mm, cone-based x-ray radiosurgery (Cone), and x-ray @entity34 ( @entity34 ) plans were generated for 11 @entity1 . Plans were evaluated using secondary @entity5 risk and brain @entity39 normal tissue complication probability (NTCP). RESULTS: For all @entity1 , secondary @entity5 is a negligible risk compared to brain @entity39 NTCP. @entity5 risk was lower in proton SS plans than in photon plans regardless of spot size (p = 0.001). Brain @entity39 NTCP increased monotonically from an average of 2.34/100 (range 0.42/100-4.49/100) to 6.05/100 (range 1.38/100-11.6/100) as sigma increased from 1 to 8 mm, compared to the average of 6.01/100 (range 0.82/100-11.5/100) for Cone and 5.22/100 (range 1.37/100-8.00/100) for @entity34 . An in-air sigma less than 4.3 mm was required for proton SS plans to reduce NTCP over photon techniques for the cohort of @entity1 studied with statistical significance (p = 0.0186). Proton SS plans with in-air sigma larger than 7.1 mm had significantly greater brain @entity39 NTCP than photon techniques (p = 0.0322). CONCLUSIONS: For treating peripheral @entity75 -where proton therapy would be expected to have the greatest depth-dose advantage over photon therapy-the lateral penumbra strongly impacts the SS plan quality relative to photon techniques: proton beamlet sigma at @entity1 surface must be small (<7.1 mm for three-beam single-field optimized SS plans) in order to achieve comparable or smaller brain @entity39 NTCP relative to photon radiosurgery techniques. Achieving such small in-air sigma values at low energy (<70 MeV) is a major technological challenge in commercially available proton therapy systems.
