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W D D'Souza

Publications and source records attributed to W D D'Souza.

5 recordsLinked to original sources
PubMed2004

Magnetic collimation and metal foil filtering for electron range and fluence modulation.

We investigated the use of magnetically collimated electron beams together with metal filters for electron fluence and range modulation. A longitudinal magnetic field collimation method was developed to reduce skin dose and to improve the electron beam penumbra. Thin metal foils were used to adjust the energies of magnetically collimated electrons. The effects for different types of foils such as Al, Be, Cu, Pb, and Ti were studied using Monte Carlo calculations. An empirical pencil beam dose calculation model was developed to calculate electron dose distributions under magnetic collimation and foil modulation. An optimization method was developed to produce conformal dose distributions for simulated targets such as a horseshoe-shaped target. Our results show that it is possible to produce an electron depth dose enhancement peak using similar techniques of producing a spread-out Bragg peak. In conclusion, our study demonstrates new aspects of using magnetic collimation and foil filtration for producing fluence and range modulated electron dose distributions.

Algorithms↗
PubMed2001

Dose homogeneity as a function of source activity in optimized I-125 prostate implant treatment plans.

PURPOSE: In conventional treatment planning for permanent I-125 prostate implants, it has been suggested that lower seed activities result in more homogeneous dose distributions and also less overdose of the critical structures. We sought to determine if this hypothesis holds by analyzing treatment plans constructed using an automated optimized approach. METHODS AND MATERIALS: We studied treatment plans for 10 patients using mixed-integer programming and the branch-and-bound method. Two mixed-integer models (that yielded somewhat different treatment plans) were developed: a "basic" model and a "dose homogeneity" model. For each resulting treatment plan, we examined dose homogeneity (by evaluating the dose non-uniformity ratio [DNR] and the full-width half-maximum [FWHM] of the differential dose-volume histogram [DVH]) as a function of three different source activities (0.35 mCi, 0.44 mCi, and 0.66 mCi). In addition, target coverage and critical structure dose distributions were evaluated. Plans using multiple source activities were also evaluated for resulting dose inhomogeneities. RESULTS: The homogeneity model results in a more homogeneous dose distribution than the basic model. DNR is lowered by an average of 42% (standard deviation [SD] = 19%), 39% (SD = 21%), and 33% (SD = 21%) for the 0.35 mCi, 0.44 mCi, and 0.66 mCi seeds, respectively, when the homogeneity model is employed over the basic model. Corresponding average decreases in the FWHM of the DVH for 0.35 mCi, 0.44 mCi, and 0.66 mCi, respectively, are 29 Gy (SD = 28 Gy), 24 Gy (SD = 22 Gy), and 27 Gy (SD = 13 Gy). Seeds of 0.35 mCi and 0.44 mCi result in the lowest DNR and narrower FWHM of the DVH relative to 0.66 mCi seeds. In general, the 0.44 mCi seeds produce greater target coverage and require fewer seeds and needles than the 0.35 mCi seeds. Although 0.66 mCi seeds result in the greatest target coverage, they yield highest critical structure doses. They also yield solutions requiring the least number of seeds and needles. However, the dose distributions from 0.66 mCi seeds are highly inhomogeneous. Multiple source activities in the same treatment plan produce dose distributions that are comparable in homogeneity to 0.44 mCi seed implants. CONCLUSIONS: Even when an optimization model that seeks to minimize dose inhomogeneity is employed, all factors involved in seed implants make 0.44 mCi the best activity choice in comparison with 0.35 mCi and 0.66 mCi.

Algorithms↗
PubMed2001

An iterative sequential mixed-integer approach to automated prostate brachytherapy treatment plan optimization.

Conventional treatment planning for interstitial prostate brachytherapy is generally a 'trial and error' process in which improved treatment plans are generated by iteratively changing, via expert judgement, the configuration of sources within the target volume in order to achieve a satisfactory dose distribution. We have utilized linear mixed-integer programming (MIP) and the branch-and-bound method, a deterministic search algorithm, to generate treatment plans. The rapidity of dose falloff from an interstitial radioactive source requires fine sampling of the space in which dose is calculated. This leads to a large and complex model that is difficult to solve as a single 3D problem. We have therefore implemented an iterative sequential approach that optimizes pseudo-independent 2D slices to achieve a fine-grid 3D solution. Using our approach, treatment plans can be generated in 20-45 min on a 200 MHz processor. A comparison of our approach with the manual 'trial and error' approach shows that the optimized plans are generally superior. The dose to the urethra and rectum is usually maintained below harmful levels without sacrificing target coverage. In the event that the dose to the urethra is undesirably high, we present a refined optimization approach that lowers urethra dose without significant loss in target coverage. An analysis of the sensitivity of the optimized plans to seed misplacement during the implantation process is also presented that indicates remarkable stability of the dose distribution in comparison with manual treatment plans.

Algorithms↗
PubMed2001

Tissue mimicking materials for a multi-imaging modality prostate phantom.

Materials that simultaneously mimic soft tissue in vivo for magnetic resonance imaging (MRI), ultrasound (US), and computed tomography (CT) for use in a prostate phantom have been developed. Prostate and muscle mimicking materials contain water, agarose, lipid particles, protein, Cu++, EDTA, glass beads, and thimerosal (preservative). Fat was mimicked with safflower oil suffusing a random mesh (network) of polyurethane. Phantom material properties were measured at 22 degrees C. (22 degrees C is a typical room temperature at which phantoms are used.) The values of material properties should match, as well as possible, the values for tissues at body temperature, 37 degrees C. For MRI, the primary properties of interest are T1 and T2 relaxations times, for US they are the attenuation coefficient, propagation speed, and backscatter, and for CT, the x-ray attenuation. Considering the large number of parameters to be mimicked, rather good agreement was found with actual tissue values obtained from the literature. Using published values for prostate parenchyma, T1 and T2 at 37 degrees C and 40 MHz are estimated to be about 1,100 and 98 ms, respectively. The CT number for in vivo prostate is estimated to be 45 HU (Hounsfield units). The prostate mimicking material has a T1 of 937 ms and a T2 of 88 ms at 22 degrees C and 40 MHz; the propagation speed and attenuation coefficient slope are 1,540 m/s and 0.36 dB/cm/MHz, respectively, and the CT number of tissue mimicking prostate is 43 HU. Tissue mimicking (TM) muscle differs from TM prostate in the amount of dry weight agarose, Cu++, EDTA, and the quality and quantity of glass beads. The 18 microm glass beads used in TM muscle increase US backscatter and US attenuation; the presence of the beads also has some effect on T1 but no effect on T2. The composition of tissue-mimicking materials developed is such that different versions can be placed in direct contact with one another in a phantom with no long term change in US, MRI, or CT properties. Thus, anthropomorphic phantoms can be constructed.

Adipose Tissue↗