QUANTITATIVE ASSESSMENT OF BEAM PERTURBATIONS CAUSED BY SILICON DIODES USED FOR IN-VIVO DOSIMETRY

Purpose: To measure the effect of silicon diode detectors used for in vivo dosimetry on beam characteristics and determine whether this effe ct is clinically significant. Methods and Materials: Commercially avai lable photon and electron diodes were placed on the central axis of ph oton and electron beams, The beam characteristics were measured for 6- and 10-MV photon and 6-20-MeV electron energies from a Varian Clinac 1800 medical linear accelerator, Water was used for the medium, and me asurements were made for various clinically common field sizes and dep ths. Results: Beam attenuations along the central axis were 10 and 7.5 % for 6- and 10-MV photons, respectively, Electron beam dose reduction s were between 13 and 25% for 20-6-MeV electrons, Photon beam flatness varied up to 7% at different depths, but the symmetry was not affecte d much, Electron beam flatness and symmetry were significantly changed to as much as 18 and 6%, respectively. Conclusion: Use of diode detec tors on central axis of photon and electron beams for in vivo dosimetr y causes significant attenuation and alteration of the beam characteri stics. The percentage of the volume affected is significant (e.g., 23% of the volume in a 4 x 4 field gets 10% less dose for a 6-MV photon b eam), especially if these diodes are used for in vivo dosimetry on the central axis every day for every treatment, as is done in some clinic s, Other beam parameters such as penumbra and skin dose are also affec ted, It is therefore recommended that the diodes be used only as neede d.