A Study on the Monte Carlo Calculation of Kilovoltage X-ray Beam Backscatter Factors

Backscatter factors in water (B_w) are important dosimetric quantities used in reference and relative dosimetry of kilovoltage (kV) radiotherapy x-ray beams. B_w is defined as the ratio of water kerma on the beam central axis at a point at the surface of a water phantom to the water kerma at the same point free in air. It is not straightforward to measure B_w and data are typically determined from Monte Carlo (MC) calculations; these are found in dosimetry protocols for kilovoltage x-ray beams [5]. Published values may, however, be for x-ray beams quite different from the beams found in the clinic or laboratory. In addition, the publication of ICRU Report 90 providing updated physics data, which may impact the values of B_w, has been one rationale for a new database of B_w values calculated by Andreo for an extensive number of x-ray beam spectra, field sizes and source-to-surface distances. In this work, independent calculations of B_w for a broad range of x-ray beam qualities and field sizes using Monte Carlo techniques has been made for clinical beams generated by Pantak SXT150 and DXT300 generators. The results have been compared with those from the new database of Andreo, based on the Penelope MC system.