title: Impact of spot size on plan quality of spot scanning proton radiosurgery for peripheral brain lesions.
abstract: OBJECTIVE
  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 brain lesions.
  
  
  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 volumetric modulated arc therapy (VMAT) plans were generated for 11 patients. Plans were evaluated using secondary cancer risk and brain necrosis normal tissue complication probability (NTCP).
  
  
  RESULTS
  For all patients, secondary cancer is a negligible risk compared to brain necrosis NTCP. Secondary cancer risk was lower in proton SS plans than in photon plans regardless of spot size (p = 0.001). Brain necrosis 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 VMAT. 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 patients studied with statistical significance (p = 0.0186). Proton SS plans with in-air sigma larger than 7.1 mm had significantly greater brain necrosis NTCP than photon techniques (p = 0.0322).
  
  
  CONCLUSIONS
  For treating peripheral brain lesions--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 patient surface must be small (<7.1 mm for three-beam single-field optimized SS plans) in order to achieve comparable or smaller brain necrosis 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.
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Dongxu
authors.initials: D
authors.lastname: Wang
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Blake
authors.initials: B
authors.lastname: Dirksen
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Hyer
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: John M
authors.initials: JM
authors.lastname: Buatti
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Arshin
authors.initials: A
authors.lastname: Sheybani
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Eric
authors.initials: E
authors.lastname: Dinges
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Nicole
authors.initials: N
authors.lastname: Felderman
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Mindi
authors.initials: M
authors.lastname: TenNapel
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: John E
authors.initials: JE
authors.lastname: Bayouth
authors.affiliation: Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52242.
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Flynn
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001921:Brain / Q000528:radiation effects / Q000601:surgery; D001932:Brain Neoplasms / Q000556:secondary / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009336:Necrosis / Q000209:etiology; D061766:Proton Therapy / Q000009:adverse effects / Q000379:methods*; D016634:Radiosurgery / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000009:adverse effects / Q000379:methods; D012306:Risk; D014964:X-Ray Therapy / Q000009:adverse effects / Q000379:methods; D055815:Young Adult