Mass-energy absorption coefficient and backscatter factor ratios for kilovoltage x-ray beams
- 1 January 1999
- journal article
- Published by IOP Publishing in Physics in Medicine & Biology
- Vol. 44 (1), 131-143
- https://doi.org/10.1088/0031-9155/44/1/011
Abstract
For low-energy (up to 150 kV) x-rays, the ratio of mass-energy absorption coefficients for water to air, , and the backscatter factor B are used in the conversion of air kerma, measured free-in-air, to water kerma on the surface of a water phantom. For clinical radiotherapy, similar conversion factors are needed for the determination of the absorbed dose to biological tissues on (or near) the surface of a human body. We have computed the ratios and B factor ratios for different biological tissues including muscle, soft tissue, lung, skin and bone relative to water. The ratios were obtained by integrating the respective mass-energy absorption coefficients over the in-air primary photon spectra. We have also calculated the ratios at different depths in a water phantom in order to convert the measured in-phantom water kerma to the absorbed dose to various biological tissues. The EGS4/DOSIMETER Monte Carlo code system has been used for the simulation of the energy fluence at different depths in a water phantom irradiated by a kilovoltage x-ray beam of variable beam quality (HVL: 0.1 mm Al-5 mm Cu), field size and source-surface distance (SSD). The same code was also used in the calculation of the B factor ratios, soft tissue to water and bone to water. The results show that the B factor for bone differs from the B factor for water by up to 20% for a 100 kV beam (HVL: 2.65 mm Al) with a 100 field. On the other hand, the difference in the B factor between water and soft tissue is insignificant (well within 1% generally). This means that the B factors for water may be directly used to convert the `in-air' water kerma to surface kerma for human soft tissues.Keywords
This publication has 11 references indexed in Scilit:
- Lithium borate TLD for determining the backscatter factors for low‐energy x rays: Comparison with chamber‐based and Monte Carlo derived valuesMedical Physics, 1998
- Correction factors for water-proofing sleeves in kilovoltage x-ray beamsMedical Physics, 1997
- The IPEMB code of practice for the determination of absorbed dose for x-rays below 300 kV generating potential (0.035 mm Al - 4 mm Cu HVL; 10 - 300 kV generating potential)Physics in Medicine & Biology, 1996
- Calculation of absorbed dose ratios using correlated Monte Carlo samplingMedical Physics, 1993
- Effect of size and composition of the central electrode on the response of cylindrical ionization chambers in high-energy photon and electron beamsPhysics in Medicine & Biology, 1993
- Report on the IPSM working party on low- and medium-energy x-ray dosimetryPhysics in Medicine & Biology, 1991
- Dependence of the photon backscatter factor for water on source-to-phantom distance and irradiation field sizePhysics in Medicine & Biology, 1990
- Experimentally determined backscatter factors for X-rays generated at voltages between 16 and 140 kVPhysics in Medicine & Biology, 1989
- Photon mass attenuation and energy-absorption coefficientsThe International Journal of Applied Radiation and Isotopes, 1982
- Computation of bremsstrahlung X-ray spectra and comparison with spectra measured with a Ge(Li) detectorPhysics in Medicine & Biology, 1979