Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting "dose spread arrays" were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three-dimensional dose distributions. The method gives good agreement for the 15-MV x-ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low-density heterogeneities.

The EGS Monte Carlo code was used to generate photon energy deposition kernels which describe the energy deposited by charged particles set in motion by primary, first scattered, second scattered, multiple scattered and bremsstrahlung plus annihilation photons. These were calculated for a water medium irradiated with monoenergetic photons with energies in the range 0.1-50 MeV. In addition to the primary energy deposition kernels, primary charged particle transport was further characterised by computing the effective centre of the voxels, and the effective penetration depth, effective radius and effective lateral distance travelled by these particles. The dose per unit collision kerma for parallel monoenergetic primary photons beta' was calculated. Additional applications of the energy deposition kernels are discussed.

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100c's, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25's, (iii) two AECL Therac 6's, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.

In recent years, magnetic-resonance imaging of gelatin doped with the Fricke solution has been applied to the direct measurement of three-dimensional (3D) radiation dose distributions. However, the 3D dose distribution can also be imaged more economically and efficiently using the method of optical absorption computed tomography. This is accomplished by first preparing a gelatin matrix containing a radiochromic dye and mapping the radiation-induced local change in the optical absorption coefficient. Ferrous-Benzoic-Xylenol (FBX) was the dye of choice for this investigation. The complex formed by Fe3+ and xylenol orange exhibits a linear change in optical attenuation (cm-1) with radiation dose in the range between 0 and 1000 cGy, and the local concentration of this complex can be probed using a green laser light (lambda = 543.5 nm). An optical computed tomography (CT) scanner was constructed analogous to a first-generation x-ray CT scanner, using a He-Ne laser, photodiodes, and rotation-translation stages controlled by a personal computer. The optical CT scanner itself can reconstruct attenuation coefficients to a baseline accuracy of < 2% while yielding dose images accurate to within 5% when other uncertainties are taken into account. Optical tomography is complicated by the reflection and refraction of light rays in the phantom materials, producing a blind spot in the transmission profiles which, results in a significant dose artifact in the reconstructed images. In this report we develop corrections used to reduce this artifact and yield accurate dosimetric maps. We also report the chemical reaction kinetics, the dose sensitivity and spatial resolution (< 1 mm3) obtained by optical absorption computed tomography. The article concludes with sample dose distributions produced by "cross-field" 6 MV x-ray beams, including a radiosurgery example.