title: Calibration of helical tomotherapy machine using EPR/alanine dosimetry.
abstract: OBJECTIVE
  Current codes of practice for clinical reference dosimetry of high-energy photon beams in conventional radiotherapy recommend using a 10 x 10 cm2 square field, with the detector at a reference depth of 10 cm in water and 100 cm source to surface distance (SSD) (AAPM TG-51) or 100 cm source-to-axis distance (SAD) (IAEA TRS-398). However, the maximum field size of a helical tomotherapy (HT) machine is 40 x 5 cm2 defined at 85 cm SAD. These nonstandard conditions prevent a direct implementation of these protocols. The purpose of this study is twofold: To check the absorbed dose in water and dose rate calibration of a tomotherapy unit as well as the accuracy of the tomotherapy treatment planning system (TPS) calculations for a specific test case.
  
  
  METHODS
  Both topics are based on the use of electron paramagnetic resonance (EPR) using alanine as transfer dosimeter between the Laboratoire National Henri Becquerel (LNHB) 60Co-gamma-ray reference beam and the Institut Curie's HT beam. Irradiations performed in the LNHB reference 60Co-gamma-ray beam allowed setting up the calibration method, which was then implemented and tested at the LNHB 6 MV linac x-ray beam, resulting in a deviation of 1.6% (at a 1% standard uncertainty) relative to the reference value determined with the standard IAEA TRS-398 protocol.
  
  
  RESULTS
  HT beam dose rate estimation shows a difference of 2% with the value stated by the manufacturer at a 2% standard uncertainty. A 4% deviation between measured dose and the calculation from the tomotherapy TPS was found. The latter was originated by an inadequate representation of the phantom CT-scan values and, consequently, mass densities within the phantom. This difference has been explained by the mass density values given by the CT-scan and used by the TPS which were not the true ones. Once corrected using Monte Carlo N-Particle simulations to validate the accuracy of this process, the difference between corrected TPS calculations and alanine measured dose values was then found to be around 2% (with 2% standard uncertainty on TPS doses and 1.5% standard uncertainty on EPR measurements).
  
  
  CONCLUSIONS
  Beam dose rate estimation results were found to be in good agreement with the reference value given by the manufacturer at 2% standard uncertainty. Moreover, the dose determination method was set up with a deviation around 2% (at a 2% standard uncertainty).
authors.affiliation: CEA, LIST Laboratoire National Henri Becquerel, 91191 Gif-sur-Yvette cedex, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Perichon
authors.forename: Tristan
authors.initials: T
authors.lastname: Garcia
authors.forename: Pascal
authors.initials: P
authors.lastname: François
authors.forename: Valérie
authors.initials: V
authors.lastname: Lourenço
authors.forename: Caroline
authors.initials: C
authors.lastname: Lesven
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Bordy
mesh_terms: D000409:Alanine; D002138:Calibration; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D015203:Reproducibility of Results; D036542:Tomography, Spiral Computed / Q000295:instrumentation*
chemical_list: D000409:Alanine