摘要:

In this paper we explore the feasibility of using DNA molecules as a biophysical radiation dosimeter. Supercoiled φX174 bacteriophage DNA molecules were irradiated with different gamma radiation doses. The strand breakage produced by ionizing radiation within supercoiled double‐stranded DNA molecules (RFI) yields relaxed circular DNA molecules (RFII) and linear DNA molecules (RFIII) as a result of single‐strand breaks and double‐strand breaks, respectively. The irradiated samples were subjected to electrophoresis on agarose gels to separate the three forms. A proprietary fluorescent dye was used to detect DNA bands within the gel, which was photographed under UV transillumination. The negative was scanned with a computerized imaging densitometric system for DNA band quantitation. The relative fractions of the three molecular forms are dose dependent, and can be modeled mathematically with five parameters. The values of the parameters were determined by optimizing the fit of the model to the data, using a nonlinear regression procedure of a commercial statistical analysis package. Once the parameters of DNA breakage have been determined, absorbed dose can be measured by this technique, which we have termed supercoil relaxation dosimetry. The average accuracy of dose determination for our system over the range of 1–40 Gy was about 5%. Supercoil relaxation dosimetry may be well suited to certain difficult dosimetric problems.

ISSN:0094-2405    

DOI:10.1118/1.597420

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The influence of the shape of a region of interest (ROI) on the uncertainty in the sampled volume of the ROI is investigated for computations with regular Cartesian grids. Both mathematically defined volumes and clinically relevant ROIs were studied. The sampling uncertainty is shown to depend on the compactness of the ROI and on effects of grid matching and translational symmetry. In clinical ROIs without translational symmetry the estimate of the sampling uncertainty is improved up to a factor of 2.3 by taking the compactness of the ROI into account. In a spherical ROI grid‐matching effects were demonstrated by means of Fourier transforms. In this type of ROI, grid‐matching effects decrease as well as increase the sampling uncertainty up to a factor of 1.6. Translational symmetry is shown to cause a decrease in the sampling uncertainty convergence power from23for spherical ROIs, to12for stringlike or13for pancakelike cylinders. For clinical ROIs with translational symmetry, similar decreases were found. With the theory derived and these symmetry effects taken into account the experimental uncertainty of volume computation can be estimated for most clinical ROIs within a factor of 2.5. Special care should be taken in grid sampling of volumes inside isodose surfaces of rectangular field techniques. For the volume of a prostate an uncertainty level of 1% or 5% is obtained with less than 1050 or 80 grid points, respectively, while for such an isodose surface up to 16 000 or 500 grid points are required for the same uncertainty levels.

ISSN:0094-2405    

DOI:10.1118/1.597619

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A generic Monte Carlo model of a photon therapy machine is described. The model, known asMcRad, is based onegs4and has been in use since 1991. Its primary function has been the characterization of the incident photon fluence for use by dose calculation algorithms. The accuracy ofMcRadis examined by comparing the dose distributions in a water phantom generated using only the Monte Carlo data with measured dose distributions for two machines in our clinic; a 6 MV Varian Clinac 600C and the 15 MV beam from a Clinac 2100C. The Monte Carlo generated dose distributions are computed using a dose calculation algorithm based on the use of differential pencil beam kernels. It was found that the match to measured data could be improved if the model is tuned by adjusting the energy of the electron beam incident on the target. The beam profiles were found to be more sensitive indicators of the electron beam energy than the depth dose curves. Beyond the depths reached by contaminant electrons, the computed and measured depth dose curves agree to better than 1%. The comparison of beam profiles indicate that in regions up to within 1 cm of the field edge, the measured and computed doses generally agree to within 2%–3%.

ISSN:0094-2405    

DOI:10.1118/1.597620

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been developed to measure a photon penumbra‐generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution‐adapted ratio‐TAR algorithm in our three‐dimensional treatment planning system. The kernel is assumed to be invariant with respect to off‐axis position, axially symmetric, and is divided into short‐ and long‐range components, with a different measurement technique for each. The data required to obtain the short‐range component are measured by scanning across a split‐field geometry incident on a water phantom. The derivative of the measured profile is proportional to one‐dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long‐range portion of the kernel is obtained by examining tissue–air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square‐field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.

ISSN:0094-2405    

DOI:10.1118/1.597621

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have studied the usefulness of both pre‐ and post‐ADC frame summing for the purpose of reducing the effect of quantum noise and digitization noise in portal imaging systems. The study is based on the fluorescent‐screen video‐camera type of system. The study predicts the not‐surprising result that provided the noise level at the ADC input is sufficiently large, the overall SNR can be increased by a factor of=M1M2, whereM1andM2are the number of frames summed before and after the ADC. The study also predicts, somewhat unexpectedly, that there is an operating region in which increasingM1actually decreases the SNR in the final image. To avoid this regionM1must be less than approximately 6×22B(1+δ̄−1)1/2/(iaccf), whereBis the number of ADC bits, δ̄ is the mean number of optical photons detected by the video camera per detected x‐ray photon,iaccis the open‐field number of detected x‐ray photons per accelerator pulse per pixel, andfis the patient transmission factor. An equivalent statement is that the rms noise at the input to the ADC, σin, must exceed approximately 0.4qwhereqis the quantization interval of the ADC. It is possible that some systems operate in or close to this region. A second feature of this anomalous behavior is that the final image is not necessarily improved by increasing the numberM2of post‐ADC‐summed frames. For example, when σin/q=0.2, there is no improvement in the overall rms error forM2>32. It is also shown that the standard deviation of the final image is not a suitable indicator of output image quality and that its use can give rise to meaningless results.

ISSN:0094-2405    

DOI:10.1118/1.597618

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fermi–Eyges electron‐scattering theory has been incorporated into the primary dose calculation for external x‐ray beam radiotherapy using the convolution method. Incorporating scattering theory into the convolution technique accounts for the densitydistributionbetween the interaction and deposition sites, whereas conventional convolution methods only consider the average density between these two points. As the lateral spread of electrons ejected from an interaction site depends on the density distribution, the energy deposition (and hence dose distribution) is predicted more accurately if scattering is accounted for. This new method gives depth dose curves which show better agreement with Monte Carlo calculations in a (slab inhomogeneity) lung phantom than a conventional convolution method, especially at high energies and small field sizes where lateral electronic disequilibrium exists at the central axis. For a 5×5‐cm218‐MV beam incident on the lung phantom, a reduction in the maximum error between the convolution and Monte Carlo depth dose curves from 5% to 2.5% is obtained when scattering theory is used in the primary dose calculation. Incorporating scattering theory into the convolution calculation increases the computation time of the primary dose by a factor of 3.

ISSN:0094-2405    

DOI:10.1118/1.597623

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Applicators (or cones), used in conjunction with patient specific cutouts in electron‐beam radiotherapy, may interact with the primary electron beam to produce a secondary beam component (applicator scatter). This component affects machine output as well as the shape of resulting dose distributions. A model has been developed to simulate this scatter component for applicators consisting of trimming plates of arbitrary shape. This model involves sampling established kernels of scatter from edge elements of appropriate materials, obtained through Monte Carlo simulations. The result of the model is a phase space (position, direction, energy, charge, weighting) of applicator scattered particles which can be incorporated into a further Monte Carlo simulation, or as input into another advanced treatment planning algorithm. This model is evaluated by comparison of measured profiles and applicator scatter component depth dose curves with Monte Carlo simulations using simulated phase‐space data as input. Results are very consistent and reveal information on the angular and spatial variation characteristics of this beam component. The results obtained verify the developed model as an accurate predictor of the characteristics of applicator scattered particles.

ISSN:0094-2405    

DOI:10.1118/1.597415

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An EGS4 Monte Carlo investigation into applicator scatter in clinical electron beams has been undertaken in order to establish the characteristics of electrons incident on the patient surface which have interacted with collimation systems. The applicator scattered component of an electron beam (including that from irregularly shaped cutouts) should be considered when modeling the electron phase space since it represents a component of the beam incident on the patient with widely varying characteristics to those of the primary beam. Scattering off an edge of applicator material is considered in terms of the types and characteristics of incident primary beam, the resulting interactions in the edge, and the fluence and energy characteristics of the emerging particles. Results indicate that the principal component to consider is scattered electrons due to the electron component of the primary beam, and that the fluence and energy characteristics of this component are dependent upon primary beam energy and the configuration of the applicator apertures.

ISSN:0094-2405    

DOI:10.1118/1.597414

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597565

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.597566

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY