title: Seeing the invisible: direct visualization of therapeutic radiation beams using air scintillation.
abstract: OBJECTIVE
  To assess whether air scintillation produced during standard radiation treatments can be visualized and used to monitor a beam in a nonperturbing manner.
  
  
  METHODS
  Air scintillation is caused by the excitation of nitrogen gas by ionizing radiation. This weak emission occurs predominantly in the 300-430 nm range. An electron-multiplication charge-coupled device camera, outfitted with an f/0.95 lens, was used to capture air scintillation produced by kilovoltage photon beams and megavoltage electron beams used in radiation therapy. The treatment rooms were prepared to block background light and a short-pass filter was utilized to block light above 440 nm.
  
  
  RESULTS
  Air scintillation from an orthovoltage unit (50 kVp, 30 mA) was visualized with a relatively short exposure time (10 s) and showed an inverse falloff (r(2) = 0.89). Electron beams were also imaged. For a fixed exposure time (100 s), air scintillation was proportional to dose rate (r(2) = 0.9998). As energy increased, the divergence of the electron beam decreased and the penumbra improved. By irradiating a transparent phantom, the authors also showed that Cherenkov luminescence did not interfere with the detection of air scintillation. In a final illustration of the capabilities of this new technique, the authors visualized air scintillation produced during a total skin irradiation treatment.
  
  
  CONCLUSIONS
  Air scintillation can be measured to monitor a radiation beam in an inexpensive and nonperturbing manner. This physical phenomenon could be useful for dosimetry of therapeutic radiation beams or for online detection of gross errors during fractionated treatments.
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Fahimian
authors.affiliation: Department of Electrical Engineering, Stanford University, Stanford, California 94305.
authors.forename: Andrew
authors.initials: A
authors.lastname: Ceballos
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Silvan
authors.initials: S
authors.lastname: Türkcan
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Kapp
authors.affiliation: Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305.
authors.forename: Guillem
authors.initials: G
authors.lastname: Pratx
mesh_terms: D000388:Air; D009010:Monte Carlo Method; D019047:Phantoms, Imaging; D011878:Radiotherapy; D012588:Scintillation Counting / Q000295:instrumentation / Q000379:methods*