title: Diamond detector measurements near simulated air channels for narrow photon beams.
abstract: OBJECTIVE
  To evaluate the combined effect of increased photon transmission, reduced photon scatter, increased secondary electron range and loss of electronic equilibrium for narrow 6-MV beams in and around a simulated air channel.
  
  
  METHODS
  A measuring method was developed in-house for relative dose measurements near simulated air-like/soft-tissue interfaces in an automated water phantom. A Styrofoam cylinder (density 0.03 g/cm3) of 2-cm diameter was submersed in the water phantom and irradiated with small rectangular radiation fields. The field length was fixed at 10 cm and the field widths ranged from 1 to 4 cm. The axis of the foam cylinder and the long side of the field were parallel. A water layer of 2 cm was realised upstream of the cylinder. Relative depth dose and profiles behind the foam cavity were assessed using a diamond detector with a sensitive crystal thickness of 0.21 mm located at 1 mm from the top of the encapsulation.
  
  
  RESULTS
  The dose at central axis 1.1 mm behind the cavity was found to be 92 and 74% for a field size of 10 x 2 and 10 x 1 cm2, respectively. The highly convex dose profile of the 10 x 1-cm2 field, characterising the homogeneous case, is flattened.
  
  
  CONCLUSIONS
  The diamond detector is an excellent choice as a detector in small photon fields with high-dose gradients as they occur near air channels, provided the orientation of the detector is appropriate. Doses near air channels are subject to significant local variations as a function of small changes of field width, and local underdosing may occur in particular cases.
authors.affiliation: Division of Radiotherapy, University Hospital Gent, Belgium.
authors.forename: K
authors.initials: K
authors.lastname: De Vlamynck
authors.forename: C
authors.initials: C
authors.lastname: De Wagter
authors.forename: W
authors.initials: W
authors.lastname: De Neve
mesh_terms: D008961:Models, Structural; D017785:Photons; D011882:Radiotherapy, High-Energy; D013679:Technology, Radiologic