Biomedical subjects

T R Mackie

Publications and source records attributed to T R Mackie.

80 records · Page 5Linked to original sources

PubMedDate not supplied

Lung dose corrections for 6- and 15-MV x rays.

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Animals↗

PubMedDate not supplied

A convolution method of calculating dose for 15-MV x rays.

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.

Humans↗

PubMedDate not supplied

EGS4 Monte Carlo determination of the beta dose kernel in water.

The EGS4 Monte Carlo code has been used to simulate the emission and energy deposition in H2O about point sources of monoenergetic electrons and radionuclides of potential use in radioimmunotherapy. The radiations studied were 0.05, 0.1, 0.5, 1.0, 2.0, and 3.0 MeV monoenergetic electrons and 32P, 67Cu, 90Y, 105Rh, 131I, 153Sm, 186Re, and 188Re beta particles and conversion and Auger electrons. Ten batches of 10,000 electrons (or 10,000 radionuclide decays) each were started isotropically at a point in an infinite homogeneous H2O phantom. The parameter-reduced electron-step transport algorithm (PRESTA) version of the EGS4 Monte Carlo code was used to follow these and their progeny, scoring the energy deposited in thin spherical shells. The scaled dose kernels are calculated and compared to kernels available in the literature. These previously published kernels either completely ignore secondary electrons or are based on a Monte Carlo code which improperly sampled the Landau energy straggling distribution.

Beta Particles↗

PubMedDate not supplied

The spatial and energy dependence of bremsstrahlung production about beta point sources in H2O.

A Monte Carlo simulation was performed to characterize the spatial and energy distribution of bremsstrahlung radiation from beta point sources important to radioimmunotherapy (RIT). Using the EGS4 Monte Carlo code, the isotropic emission and transport of monoenergetic 0.1-, 0.5-, 1.0-, 2.0-, and 3.0-MeV electrons and 32P and 90Y beta particles was simulated in an infinite, homogeneous H2O phantom. The probability of bremsstrahlung production for each Monte Carlo-simulated electron step was accumulated in energy intervals not exceeding 5 keV and stored as a function of radial position. To validate this scheme, the EGS4 code was tested in the continuous slowing down approximation (csda) mode, with resulting radiation yields seen to agree with values in ICRU Report No. 37 (ICRU, Bethesda, MD, 1984) to better than 1.6%. The radiation yield calculated with the simulation of secondary particles is seen to be 3%-5% greater than the csda yield. The photon energy distributions are characterized by a typically broad bremsstrahlung spectrum with the probability of photon generation decreasing with radial distance. In the energy range 0.05-0.511 MeV, the probability for bremsstrahlung production from 90Y (2.76 x 10(-2) decay-1) is twice that from 32P (1.35 x 10(-2) decay-1). When passed through 10 cm of H2O and put upon a standard NaI scintillation camera, count rates of 2.3 x 10(-6) and 1.2 x 10(-6) counts s-1 Bq-1 are estimated from point sources of 90Y and 32P. These results predict the inherent spatial resolution limitation and provide the initial data required for modeling and analyzing the scatter, attenuation, and image formation processes in quantitative imaging of bremsstrahlung for RIT dosimetry.

Humans↗

PubMedDate not supplied

Tomotherapy: a new concept for the delivery of dynamic conformal radiotherapy.

Tomotherapy, literally "slice therapy," is a proposal for the delivery of radiation therapy with intensity-modulated strips of radiation. The proposed method employs a linear accelerator, or another radiation-emitting device, which would be mounted on a ring gantry like a CT scanner. The patient would move through the bore of the gantry simultaneously with gantry rotation. The intensity modulation would be performed by temporally modulated multiple independent leaves that open and close across the slit opening. At any given time, any leaf would be (1) closed, covering a portion of the slit, (2) open, allowing radiation through, or (3) changing between these states. This method would result in the delivery of highly conformal radiation. Overall treatment times should be comparable with contemporary treatment delivery times. The ring gantry would make it convenient to mount a narrow multisegmented megavoltage detector system for beam verification and a CT scanner on the treatment unit. Such a treatment unit could become a powerful tool for treatment planning, conformal treatment, and verification using tomographic images. The physical properties of this treatment delivery are evaluated and the fundamental design specifications are justified.

Humans↗

PubMedDate not supplied

On the cause of the variation in tissue-maximum ratio values with source-to-detector distance.

While tissue-maximum ratios (TMR) for 60cobalt treatment units have been shown to be independent of source-to-axis distance (SAD), high-energy photon beams demonstrate variations in their TMR as a function of SAD. Some authors have asserted that the distance dependence of the TMR stems from electron contamination in the beams, while others have suggested low-energy, scattered photons as the cause. Using a magnet to sweep contaminant electrons out of the photon treatment beam eliminates any variation in TMR with distance. Thus, electron contamination accounts for all of the distance dependence, and any low-energy, scattered photons behave indistinguishably like the high-energy photons.

Cobalt Radioisotopes↗

PubMedDate not supplied

Investigation of the convolution method for polyenergetic spectra.

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two convolution approaches that properly account for beam hardening effects. Dose deposition kernels calculated previously using the EGS4 Monte Carlo code are convolved with the primary terma to give the dose for monoenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fluence spectrum to yield the polyenergetic dose distribution. Alternatively, a single convolution for the polyenergetic beam is considered, where a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels for a diverging beam geometry were also examined. The depth dose data produced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.

Biophysical Phenomena↗

PubMedDate not supplied

3D rendering of SAR distributions from Thermotron RF-8 using a ray casting technique.

A comprehensive 3D visualization package developed for CT-based 3D radiation treatment planning has been modified to volume-render SAR data. The program accepts data from sequential thermographic thermometry measurements as well as calculated data from thermal models. In this presentation sample data obtained from a capacitive heating system 'Thermotron-RF8' is presented. This capability allows the generation of accurate standardized volumetric images of SAR and provides a valuable tool to better preplan hyperthermia treatments.

Evaluation Studies as Topic↗