A system has been developed which uses Monte Carlo computer simulations to aid the design and optimisation of a polarised source for in vivo X-ray fluorescence (XRF) analysis of heavy metals: The system is based on a version of the Monte Carlo code EGS4 which includes polarised photon interactions, running on a personal computer. The code was used to construct a model of a clinical polarised XRF system (based on a 300 kV therapy X-ray source) under development at Swansea for the measurement of Pt-based chemotherapy drugs in head and neck tumours. Several simulations were performed to investigate the variation of XRF measurement sensitivity with the material composition and geometry of the system components. Pt XRF spectra generated by the model were found to be in good agreement with experimental data. The optimum operating voltage for the system, predicted by the simulations and confirmed by experiment, was approx. 200 kV. The accuracy of the results obtained to date indicates that this technique will greatly facilitate future design and optimisation studies of a wide variety of XRF systems.

Antineoplastic Agents↗

PubMedDate not supplied

Benchmarking the MCNP code for Monte Carlo modelling of an in vivo neutron activation analysis system.

The Monte Carlo computer code MCNP (version 4A) has been used to develop a personal computer-based model of the Swansea in vivo neutron activation analysis (IVNAA) system. The model included specification of the neutron source (252Cf), collimators, reflectors and shielding. The MCNP model was 'benchmarked' against fast neutron and thermal neutron fluence data obtained experimentally from the IVNAA system. The Swansea system allows two irradiation geometries using 'short' and 'long' collimators, which provide alternative dose rates for IVNAA. The data presented here relate to the short collimator, although results of similar accuracy were obtained using the long collimator. The fast neutron fluence was measured in air at a series of depths inside the collimator. The measurements agreed with the MCNP simulation within the statistical uncertainty (5-10%) of the calculations. The thermal neutron fluence was measured and calculated inside the cuboidal water phantom. The depth of maximum thermal fluence was 3.2 cm (measured) and 3.0 cm (calculated). The width of the 50% thermal fluence level across the phantom at its mid-depth was found to be the same by both MCNP and experiment. This benchmarking exercise has given us a high degree of confidence in MCNP as a tool for the design of IVNAA systems.

Body Composition↗

PubMedDate not supplied

A linear array, scintillation crystal-photodiode detector for megavoltage imaging.

An imaging device has been developed to acquire images during external photon-beam radiotherapy treatments. It consists of a linear array of 128 zinc tungstate (ZnWO4) scintillation crystals each of which is individually optically coupled to a photodiode and associated electronics. The image is formed by scanning the linear array across the radiation field using a stepping motor under the control of a microcomputer. Image archive, display, and analysis are performed using a microVAX II computer. Results from a general theoretical analysis are presented before a detailed description of the particular detector construction. The mechanical design of the detector is such that the detector is automatically positioned to within a millimeter relative to the treatment source. This simplifies procedures for analyzing setup variations when comparing a treatment image to any other treatment, or planning, images. Image acquisition takes under 4 s with a contrast resolution of better than 1% at a spatial resolution of 2.5 mm in the object plane. The primary dose used to form these images is 0.55 cGy although the dose received by the patient will be closer to 25 cGy due to the linear scanning geometry and 3.8-s scan time that is used.

Humans↗

PubMedDate not supplied

The design of megavoltage projection imaging systems: some theoretical aspects.

This study investigates factors associated with the imaging of a patient using a high-energy radiotherapy treatment beam. Both single-stage (e.g., solid-state detector) and two-stage (e.g., scintillation screen plus TV) systems are considered. First an expression is derived that relates dose at the buildup depth in the object to the structure of the object, the scatter-to-primary signal-variance ratio and the differential-signal-to-noise ratio in the image. Second the number of bits required to digitize the image is derived. Third the effect of scattered radiation is investigated for photon counting, photopeak, and Compton detector types. Fourth the effect of noise in the detection process is considered. Finally, the relationship between x-ray source size, detector aperture, and image magnification is derived. The optimum magnification for given source size and detector aperture is discussed in terms of the system transfer function. The study indicates that at a primary beam energy of 2 MeV, a dose of 10(-3) cGy is required to detect reliably the presence of a bone section of area 10 x 10 mm and thickness 4 mm in 250 mm of soft tissue. For this example, it is also estimated that a digitization accuracy of 10 bits is required. The calculations indicate that for a Compton detector, the scatter-to-primary signal-variance ratio drops from a value of around 30% at the exit surface of the object to 5% at a distance of 80 cm from the object with a consequent small reduction in the dose required to form the image.

Equipment Design↗

PubMedDate not supplied

A dose distribution evaluation utilizing megavoltage CT imaging system.

An isodose distribution (collision kinetic energy distribution) was acquired using a megavoltage CT imaging system. Direct comparison of dose distributions has become important for verification in innovative treatment techniques. Three-dimensional implementation of this system is considered highly feasible.

Humans↗