title: Medical reference dosimetry using EPR measurements of alanine: development of an improved method for clinical dose levels.
abstract: BACKGROUND
  Electron spin resonance (EPR) is used to determine the absorbed dose of alanine dosimeters exposed to clinical photon beams in a solid-water phantom. Alanine is potentially suitable for medical reference dosimetry, because of its near water equivalence over a wide energy spectrum, low signal fading, non-destructive measurement and small dosimeter size.
  
  
  METHODS
  A Bruker EMX-micro EPR spectrometer with a rectangular cavity and a measurement time of two minutes per dosimeter was used for reading of irradiated alanine dosimeters. Under these conditions a new algorithm based on scaling of known spectra was developed to extract the alanine signal.
  
  
  RESULTS
  The dose accuracy, including calibration uncertainty, is less than 2% (k=1) above 4 Gy (n=4). The measurement uncertainty is fairly constant in absolute terms (approximately 30 mGy) and the relative uncertainty therefore rises for dose measurements below 4 Gy. Typical reproducibility is <1% (k=1) above 10 Gy and <2% between 4 and 10 Gy. Below 4 Gy the uncertainty is higher. A depth dose curve measurement was performed in a solid-water phantom irradiated to a dose of 20 Gy at the maximum dose point (d(max)) in 6 and 18 MV photon beams. The typical difference between the dose measured with alanine in solid water and the dose measured with an ion chamber in a water tank was about 1%. A difference of 2% between 6 and 18 MV was found, possibly due to non-water equivalence of the applied phantom.
  
  
  CONCLUSIONS
  Compared to previously published methods the proposed algorithm can be applied without normalisation of phase shifts caused by changes in the g-value of the cavity. The study shows that alanine dosimetry is a suitable candidate for medical reference dosimetry especially for quality control applications.
authors.affiliation: Risø-DTU, Risø High Dose Reference Laboratory, Technical University of Denmark, Roskilde, Denmark. jakob.helt-hansen@risoe.dk
authors.forename: Jakob
authors.initials: J
authors.lastname: Helt-Hansen
authors.forename: Flemming
authors.initials: F
authors.lastname: Rosendal
authors.forename: Inger Matilde
authors.initials: IM
authors.lastname: Kofoed
authors.forename: Claus Erik
authors.initials: CE
authors.lastname: Andersen