title: Cerenkov emission induced by external beam radiation stimulates molecular fluorescence.
abstract: OBJECTIVE
  Cerenkov emission is induced when a charged particle moves faster than the speed of light in a given medium. Both x-ray photons and electrons produce optical Cerenkov photons in everyday radiation therapy of tissue; yet, this phenomenon has never been fully documented. This study quantifies the emissions and also demonstrates that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.
  
  
  METHODS
  In this study, Cerenkov emission induced by radiation from a clinical linear accelerator is investigated. Biological mimicking phantoms were irradiated with x-ray photons, with energies of 6 or 18 MV, or electrons at energies 6, 9, 12, 15, or 18 MeV. The Cerenkov emission and the induced molecular fluorescence were detected by a camera or a spectrometer equipped with a fiber optic cable.
  
  
  RESULTS
  It is shown that both x-ray photons and electrons, at MeV energies, produce optical Cerenkov photons in tissue mimicking media. Furthermore, we demonstrate that the Cerenkov emission can excite a fluorophore, protoporphyrin IX (PpIX), embedded in biological phantoms.
  
  
  CONCLUSIONS
  The results here indicate that molecular fluorescence monitoring during external beam radiotherapy is possible.
authors.affiliation: Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755, USA. johan.axelsson@dartmouth.edu
authors.forename: Johan
authors.initials: J
authors.lastname: Axelsson
authors.forename: Scott C
authors.initials: SC
authors.lastname: Davis
authors.forename: David J
authors.initials: DJ
authors.lastname: Gladstone
authors.forename: Brian W
authors.initials: BW
authors.lastname: Pogue
mesh_terms: D005453:Fluorescence; D008163:Luminescent Measurements / Q000379:methods*; D015336:Molecular Probe Techniques; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*