title: Measurements of lateral penumbra for uniform scanning proton beams under various beam delivery conditions and comparison to the XiO treatment planning system.
abstract: OBJECTIVE
  The main purposes of this study were to (1) investigate the dependency of lateral penumbra (80%-20% distance) of uniform scanning proton beams on various factors such as air gap, proton range, modulation width, compensator thickness, and depth, and (2) compare the lateral penumbra calculated by a treatment planning system (TPS) with measurements.
  
  
  METHODS
  First, lateral penumbra was measured using solid-water phantom and radiographic films for (a) air gap, ranged from 0 to 35 cm, (b) proton range, ranged from 8 to 30 cm, (c) modulation, ranged from 2 to 10 cm, (d) compensator thickness, ranged from 0 to 20 cm, and (e) depth, ranged from 7 to 15 cm. Second, dose calculations were computed in a virtual water phantom using the XiO TPS with pencil beam algorithm for identical beam conditions and geometrical configurations that were used for the measurements. The calculated lateral penumbra was then compared with the measured one for both the horizontal and vertical scanning magnets of our uniform scanning proton beam delivery system.
  
  
  RESULTS
  The results in the current study showed that the lateral penumbra of horizontal scanning magnet was larger (up to 1.4 mm for measurement and up to 1.0 mm for TPS) compared to that of vertical scanning magnet. Both the TPS and measurements showed an almost linear increase in lateral penumbra with increasing air gap as it produced the greatest effect on lateral penumbra. Lateral penumbra was dependent on the depth and proton range. Specifically, the width of lateral penumbra was found to be always lower at shallower depth than at deeper depth within the spread out Bragg peak (SOBP) region. The lateral penumbra results were less sensitive to the variation in the thickness of compensator, whereas lateral penumbra was independent of modulation. Overall, the comparison between the results of TPS with that of measurements indicates a good agreement for lateral penumbra, with TPS predicting higher values compared to measurements.
  
  
  CONCLUSIONS
  Lateral penumbra of uniform scanning proton beams depends on air gap, proton range, compensator thickness, and depth, whereas lateral penumbra is not dependent on modulation. The XiO TPS typically overpredicted lateral penumbra compared to measurements, within 1 mm for most cases, but the difference could be up to 2.5 mm at a deep depth and large air gap.
authors.affiliation: Department of Medical Physics, ProCure Proton Therapy Center, Oklahoma City, Oklahoma 73142, USA. suresh.rana@okc.procure.com
authors.forename: Suresh
authors.initials: S
authors.lastname: Rana
authors.forename: Omar
authors.initials: O
authors.lastname: Zeidan
authors.forename: Eric
authors.initials: E
authors.lastname: Ramirez
authors.forename: Michael
authors.initials: M
authors.lastname: Rains
authors.forename: Junfang
authors.initials: J
authors.lastname: Gao
authors.forename: Yuanshui
authors.initials: Y
authors.lastname: Zheng
mesh_terms: D000388:Air; D019047:Phantoms, Imaging; D061766:Proton Therapy / Q000379:methods*; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D011522:Protons
chemical_list: D011522:Protons