Measurement of the restricted linear energy transfer of stray radiation close to the treatment volume of 12 and 18 MeV clinical photon beams

The restricted dose mean linear energy transfer (LET) (L500,D) of the stray radiation field a few centimeters outside the treatment volume has been measured for 12 and 18 MV photons produced by a clinical Therac-20 (AECL) accelerator. The measurements were performed as a function of field size and distance from the edge of the treatment volume, using the method of the high-pressure ionization chamber. Contrary to what was found in a previous investigation for a clinical Co-60 unit and despite the presence of photoneutrons (in the case of 18 MV photons), the L500,D outside the beam does not increase significantly relative to the L500,D of the primary beam.