A Monte Carlo study of the interaction of external beam X-radiation with inhomogeneous media

Monte Carlo modelling of the performance of a rotating slit-collimator for improved planar gamma-camera imaging

Planar imaging with a gamma camera is currently limited by the performance of the collimator. Spatial resolution and sensitivity trade off against each other; it is not possible with conventional parallel-hole collimation to have high geometric sensitivity and at the same time excellent spatial resolution unless field-of-view is sacrificed by using fan- or cone-beam collimators. We propose a rotating slit-collimator which collects one-dimensional projections from which the planar image may be reconstructed by the theory of computed tomography. The performance of such a collimator is modelled by Monte Carlo methods and images are reconstructed by a convolution and backprojection technique. The performance is compared with that of a conventional parallel-hole collimator and it is shown that higher spatial resolution with increased sensitivity is possible with the slit-collimator. For a point source a spatial resolution of some 6 mm at a distance of 100 mm from the collimator with a x7 sensitivity compared with a parallel-hole collimator was achieved. Applications to bone scintigraphy are modelled and an improved performance in hot-spot imaging is demonstrated. The expected performance in cold-spot imaging is analytically investigated. The slit-collimator is not expected to improve cold-spot imaging. Practical design considerations are discussed.

Evaluation of lung dose correction methods for photon irradiations of thorax phantoms

Radiation absorbed dose in lung is measured and calculated using several algorithms available on commercial treatment planning systems. Phantoms resembling the human thorax are used and irradiated with small and large photon beams of 60Co, 4, 6, and 10 MV X ray energies. The applicability and usefulness of the different calculation methods in clinical situations is discussed.

Radiation transport calculations in treatment planning

This article reviews the role of radiation transport calculations in radiotherapy dose computation. The physical and mathematical principles underlying transport theory, as applied to electrons and photons, are discussed. Practical methods of solving the transport equation, with emphasis on Monte Carlo techniques, are reviewed. The contribution of analytic and Monte Carlo transport calculations to electron beam, photon beam, and brachytherapy treatment planning dosimetry is assessed. Currently, the most important roles of transport theory include using approximate solutions of the transport equation as theoretical foundations of dose computation algorithms, and using Monte Carlo simulation to calculate basic treatment planning data, characteristic of the treatment modality, which can serve as input to a dose computation algorithm.

Theoretical and experimental analysis of scatter from inhomogeneous slabs in a 60Co beam: the differential tissue-air ratio method (DTAR)

A theoretical and experimental analysis of scatter from inhomogeneous slabs in a 60Co beam has been developed. This analysis has been used to examine the differential scatter-air ratio method (DSAR). It has been found that a simplification of this method gives more accurate results than the original one. Thus, a method which takes advantage of this simplification has been developed, called the 'differential tissue-air ratio method (DTAR)'. It is compared with experimental results. The accuracy is satisfactory for a wide range of applications (usually approaching the limits of the precision of the measurements, about 2%) and leads to calculation times not significantly longer than any conventional method (e.g. Batho).

Comparison of measured and calculated absorbed doses from tangential irradiation of the breast

Calculated absorbed dose distributions from tangential irradiation of the breast have been compared with TLD measurements in a anthropomorfic body-shaped phantom which has gone through all phases of the radiation treatment planning cycle, including mapping of electron densities with a CT scanner, simulation of beam set-up and several treatments with an accelerator. The absorbed doses, measured in the breast are 2-6% lower than those calculated with the clinically used treatment planning system. The deviation is slightly higher when wedges are used. The main source for this deviation is shown to be the limitations in the loss-of-scatter correction in the dose calculation algorithm used. It has also been noted that the variation of the absorbed dose in the target volume is in general smaller than predicted by the calculations.

The effect of photon scatter and consequent electron build-up in air on the calculation of dose equivalent quantities in the ICRU sphere for photon energies from 0.662 to 10 MeV

A Monte Carlo computer program, DEIPHOS has been modified and employed to calculate depth-dose distributions within the 30 cm diameter ICRU tissue-equivalent sphere for initially parallel beams of photons (0.662 to 10 MeV) when the sphere is in an air medium. Dose equivalent values are computed as the sum of two contributions, the dose equivalent when the sphere is in vacuo and the dose equivalent from scattered photons and the corresponding electrons and positrons produced in an air column in front of the sphere. The dimensions of the air cylinder required to attain full electronic equilibrium were derived for each photon energy. The calculation of absorbed dose to air in air is described. Dose equivalents at 300 and 1000 mg cm-2 depths, the restricted dose equivalent indices, the average dose equivalent in the sphere as well as depth-dose equivalent and angle-dose equivalent distributions are presented, normalised both to fluence and absorbed dose to air.

Verification by Monte Carlo methods of a power law tissue-air ratio algorithm for inhomogeneity corrections in photon beam dose calculations

A Monte Carlo computer program has been used to calculate axial and off-axis depth dose distributions arising from the interaction of an external beam of 60Co radiation with a medium containing inhomogeneities. An approximation for applying the Monte Carlo data to the configuration where the lateral extent of the inhomogeneity is less than the beam area, is also presented. These new Monte Carlo techniques rely on integration over the dose distributions from constituent sub-beams of small area and the accuracy of the method is thus independent of beam size. The power law correction equation (Batho equation) describing the dose distribution in the presence of tissue inhomogeneities is derived in its most general form. By comparison with Monte Carlo reference data, the equation is validated for routine patient dosimetry. It is explained why the Monte Carlo data may be regarded as a fundamental reference point in performing these tests of the extension to the Batho equation. Other analytic correction techniques, e.g. the equivalent radiological path method, are shown to be less accurate. The application of the generalised power law equation in conjunction with CT scanner data is discussed. For ease of presentation, the details of the Monte Carlo techniques and the analytic formula have been separated into appendices.

The absorbed dose in the vicinity of an interface between two media irradiated by a 60Co source

Monte Carlo techniques are used to generate the distribution of absorbed dose in the vicinity of an interface between two media. Computer-generated distributions for absorbers comprising the media carbon-copper, water-aluminium and aluminium-water are presented. The features of the distributions in regions of electron equilibrium and non-equilibrium, in particular finite discontinuities at the boundaries and subsequent build up or build down, are discussed both qualitatively and quantitatively. The difficulties inherent in interpreting ionization measurements in the neighbourhood of interfaces are outlined.

The direct use of CT numbers in radiotherapy dosage calculations for inhomogeneous media

Techniques by which the quantitative anatomical data inherent in a CT scan can be directly used in treatment planning are described. The correction algorithms used in the RAD-8 system, based on an effective path length, have been extended to a pixel-by-pixel approach. By calibrating the X-ray transmission CT scanner in terms of electron densities (electron cm-3) inhomogeneity corrections may be made automatically.

The dose in water surrounding point isotropic gamma-ray emitters

A Monte Carlo method has been used to calculate the dose distribution around point gamma-ray emitters in water. Five gamma-ray emitting radionuclide, 60Co, 137Cs, 192Ir, 198Au and 226Ra, which are commonly used in radiotherapy interstitial implants have been considered. The results are in substantial agreement with those derived using a combination of experimental and theoretical data.