title: Backscatter correction factor for megavoltage photon beam.
abstract: OBJECTIVE
  For routine clinical dosimetry of photon beams, it is often necessary to know the minimum thickness of backscatter phantom material to ensure that full backscatter condition exists.
  
  
  METHODS
  In case of insufficient backscatter thickness, one can determine the backscatter correction factor, BCF(s,d,t), defined as the ratio of absorbed dose measured on the central-axis of a phantom with backscatter thickness of t to that with full backscatter for square field size s and forward depth d. Measurements were performed in SAD geometry for 6 and 15 MV photon beams using a 0.125 cc thimble chamber for field sizes between 10 × 10 and 30 × 30 cm at depths between d(max) (1.5 cm for 6 MV and 3 cm for 15 MV) and 20 cm.
  
  
  RESULTS
  A convolution method was used to calculate BCF using Monte-Carlo simulated point-spread kernels generated for clinical photon beams for energies between Co-60 and 24 MV. The convolution calculation agrees with the experimental measurements to within 0.8% with the same physical trend. The value of BCF deviates more from 1 for lower energies and larger field sizes. According to our convolution calculation, the minimum BCF occurs at forward depth d(max) and 40 × 40 cm field size, 0.970 for 6 MV and 0.983 for 15 MV.
  
  
  CONCLUSIONS
  The authors concluded that backscatter thickness is 6.0 cm for 6 MV and 4.0 cm for 15 MV for field size up to 10 × 10 cm when BCF = 0.998. If 4 cm backscatter thickness is used, BCF is 0.997 and 0.983 for field size of 10 × 10 and 40 × 40 cm for 6 MV, and is 0.998 and 0.990 for 10 × 10 and 40 × 40 cm for 15 MV, respectively.
authors: Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, USA.
authors: Yida
authors: Y
authors: Hu
authors: Timothy C
authors: TC
authors: Zhu
mesh_terms: D000465:Algorithms; D003037:Cobalt Radioisotopes / Q000032:analysis; D003198:Computer Simulation; D004867:Equipment Design; D006801:Humans; D015233:Models, Statistical; D008962:Models, Theoretical; D017785:Photons; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D015203:Reproducibility of Results; D012542:Scattering, Radiation
chemical_list: D003037:Cobalt Radioisotopes