title: Linearity of the dose monitor system at low monitor units.
abstract: OBJECTIVE
  The accuracy of the dose monitor system of a medical electron accelerator is of crucial importance for the precise delivery of the prescribed dose. Special attention is given to the case of low monitor units (MU). The long-term stability of the dose monitor system is analyzed.
  
  
  METHODS
  Photon and electron fields with monitor units between 1 and 500 MU were measured with field sizes of 10 x 10 cm(2) and 20 x 20 cm(2) at source-skin distances (SSDs) of 100 cm and 95 cm, respectively. The relative dose per MU [see text] was determined for an Elekta linear accelerator.
  
  
  RESULTS
  The relative dose per MU was measured to be constant within about 1% down to fields with only 2 MU. The deviation remained below 2% down to 1 MU which is the minimum setting of the accelerator. The long-term stability is better than 0.2% per month.
  
  
  CONCLUSIONS
  No therapeutically relevant deviation of the prescribed dose could be detected for the Elekta linear accelerator. This is a prerequisite for the implementation of intensity-modulated radiotherapy (IMRT) techniques.
authors.affiliation: Strahlentherapie, Diakoniekrankenhaus Schwäbisch Hall, Diakoniestrasse 10, 74523 Schwäbisch Hall, Germany. Peter.Mohr@diaksha.de
authors.forename: Peter
authors.initials: P
authors.lastname: Mohr
authors.forename: Sabrina
authors.initials: S
authors.lastname: Brieger
authors.forename: Jürgen
authors.initials: J
authors.lastname: Stahl
authors.forename: Gerlo
authors.initials: G
authors.lastname: Witucki
mesh_terms: D001822:Body Burden; D004583:Electrons; D004867:Equipment Design; D006801:Humans; D010315:Particle Accelerators / Q000295:instrumentation*; D017785:Photons; D011834:Radiation Monitoring / Q000295:instrumentation*; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation*