Absorbed dose conversion factors for therapeutic kilovoltage x-ray beams calculated by the Monte Carlo method.
This thesis describes techniques for performing calculations of absorbed dose conversion
factors for therapeutic kilovoltage and megavoltage x-ray beams, by application of the Monte
Carlo method. These factors are low-energy x-ray backscatter factors, B, low- and medium-energy
x-ray mass energy absorption coefficient ratios, water-to-air, We,IP)w.a,r ' and megavoltage x-ray
stopping power ratios, water-to-air, s. The EGS4 and ITS Monte Carlo systems were used to
model the relevant particle transport.
Consistent theoretical expressions for absorbed dose in kilovoltage x-ray beams have been
proposed. For low-energy x-rays, the expression for absorbed dose to water, D, requires values
of defined as a water kerma ratio at the surface of a water phantom, and also of [(Jie,,/P)w.,r]p,
evaluated over the primary spectrum (free-in-air). For medium-energy O x-rays, values of
(Pen(Z,f)I'P)w,air are necessary, which are dependent on depth, z, and field-size,f. Bragg-Gray cavity
theory entails values of SWthI(z) to convert readings from recommended ionisation chambers,
calibrated in terms of air-kerma or exposure, into D.
Bremsstrahlung spectra have been calculated by detailed Monte Carlo simulations of the
NPL standard accelerator, a Philips SL series linac and a Siemens Stabilipan x-ray unit. Values of
(i1 ,,/P)w air and B' have been calculated for therapeutic polyenergetic kilovoltage beams, over a
comprehensive range of field sizes and beam qualities, following the Monte Carlo calculation of
photon fluence in water and by a kerma-weighted averaging technique utilising primary fluence
spectra and pre-calculated values of monoenergetic Values of Spencer-Attix s,, have
similarly been calculated for the NPL standard and therapeutic linac beams following the Monte
Carlo calculation of electron fluence in water and by a dose-weighted averaging technique utilising
primary fluence spectra and pre-calculated monoenergetic dose distributions. Accurate TPR
values, which account for linac spectrum quality shift off-axis, have been acquired by convolving
water terma distributions with point-energy-deposition kernels. The beam quality dependence of
N, the NFL absorbed dose-to-water calibration factor, has subsequently been investigated.
Data appropriate for insertion into recommended expressions for D is presented and
compared to data provided in the literature.