Biomedical subjects

B A Faddegon

Publications and source records attributed to B A Faddegon.

9 recordsLinked to original sources

PubMed1999

The flatness of Siemens linear accelerator x-ray fields.

The primary definer for Siemens MXE and MDX linear accelerators projects a circular opening with a radius of 25 cm at 100 cm from the target. Our measurements of photon beam profiles, however, indicate that the photon fluence drops to 95% of the central axis value at a radius of 18 cm. The flattening filter for these machines projects a flattened field size that is much smaller than the primary definer would allow. The clinical implications of this mismatch for large rectangular fields and for fields defined by asymmetric jaws are discussed and solutions are considered. A large field flattener was designed for our Siemens MXE 6 MV beam using Monte Carlo simulation of the treatment head and water phantom. The accuracy required of source and geometry details for dose distributions calculation is presented. The key parameters are the mean energy and focal spot size of the electron beam incident on the exit window, the material composition, and thickness profile of the exit window, target, flattener, and primary collimator, and the position of the primary collimator relative to the target. Profiles were more sensitive than central axis depth doses to simulation details. The beam energy and primary collimator position were selected to achieve good agreement between measured and calculated dose distributions. The flattener we designed with Monte Carlo was machined from brass and mounted on our MXE treatment unit. Measurements demonstrate that the large field flattener extends the useful radius of the field out to 22 cm, right into the penumbra cast by the primary collimator.

Computer Simulation↗

PubMed1998

Megavoltage imaging with low Z targets: implementation and characterization of an investigational system.

The poor quality of stereotactic radiotherapy portal images is a limiting factor in precise image registration. To alleviate this problem, a low atomic number (Z) target was implemented on our Siemens MXE linear accelerator. This investigational system was used to assess the performance of various target materials by filming an aluminum contrast object. Beryllium, carbon and conventional target materials were studied. The bremsstrahlung spectra of these materials were simulated using Monte Carlo techniques. These spectra were used to calculate the dependence of narrow beam contrast on phantom thickness for verification of the data measured from film. A Monte Carlo simulation of the beryllium spectrum in a wide beam geometry was used to evaluate the effect of phantom-to-film distance on contrast. Although the same degree of contrast improvement with distance was not realized in practice, the improvement in image quality rivaled that achieved using a scatter reduction grid. A comparison of conventional localization images of the head and neck of an anthropomorphic phantom with images produced with a beryllium or carbon target and a mammography film and screen system supports earlier suggestions that the technique is clinically useful.

Biophysical Phenomena↗

PubMed1997

Accurate characterization of Monte Carlo calculated electron beams for radiotherapy.

Monte Carlo studies of dose distributions in patients treated with radiotherapy electron beams would benefit from generalized models of clinical beams if such models introduce little error into the dose calculations. Methodology is presented for the design of beam models, including their evaluation in terms of how well they preserve the character of the clinical beam, and the effect of the beam models on the accuracy of dose distributions calculated with Monte Carlo. This methodology has been used to design beam models for electron beams from two linear accelerators, with either a scanned beam or a scattered beam. Monte Carlo simulations of the accelerator heads are done in which a record is kept of the particle phase-space, including the charge, energy, direction, and position of every particle that emerges from the treatment head, along with a tag regarding the details of the particle history. The character of the simulated beams are studied in detail and used to design various beam models from a simple point source to a sophisticated multiple-source model which treats particles from different parts of a linear accelerator as from different sub-sources. Dose distributions calculated using both the phase-space data and the multiple-source model agree within 2%, demonstrating that the model is adequate for the purpose of Monte Carlo treatment planning for the beams studied. Benefits of the beam models over phase-space data for dose calculation are shown to include shorter computation time in the treatment head simulation and a smaller disk space requirement, both of which impact on the clinical utility of Monte Carlo treatment planning.

Biophysical Phenomena↗

PubMed1997

Monte Carlo based phase-space evolution for electron dose calculation.

A system of computer codes based on phase-space evolution is developed and applied to low energy therapeutic electron beams. Monte Carlo (EGS4) is used to pre-calculate the electron transport and dose deposition in a 0.5 cm width cubic voxel. Dose calculations at larger scales are computed from the pre-calculated data using phase-space evolution. This approach has the theoretical accuracy of Monte Carlo with potentially significant speed gains resulting from the pre-calculation. This study demonstrates the accuracy of this technique while providing a preliminary assessment of the calculation time. For a 4.3 MeV electron beam in water with a 0.5 cm thick slab of either water (homogeneous), air, or aluminum at 1 cm depth, we observe differences relative to Monte Carlo of less than 3% along the central axis for a pencil-beam. For a 3.5 cm x 3.5 cm field we observe a maximum difference on the central axis of 4% in the build-up region and less than 0.1 cm in the fall-off region for all three phantoms. Calculation times are disappointing; however, there is high potential for their reduction to values comparable to or better than condensed history Monte Carlo while retaining clinically acceptable accuracy.

Algorithms↗

PubMed1995

Calculation of x-ray spectra for radiosurgical beams.

As conformal radiosurgery using micromultileaf collimators gains feasibility, dose calculation algorithms based on Monte Carlo or convolution techniques may become necessary. These require radiosurgical x-ray spectra. The most accurate method currently available to estimate clinical radiosurgery spectra is the Monte Carlo method. In this study the EGS4 Monte Carlo system was used to simulate the thick target of a 6 MV linear accelerator used for radiosurgery in our center. The calculated spectrum was attenuated through any significant mass thickness of material downstream from the target. The attenuated thick-target spectral distributions calculated both with and without the flattening filter were compared to the attenuated, thin target spectrum based on the small angle Schiff analytical spectrum calculated for the same target and attenuator material, as well as with a published spectrum from a full Monte Carlo simulation of a treatment head with a flattener in place. The Schiff spectrum neglects contributions from lower-energy scattered electrons that significantly degrade the quality of the beam. The flattener is removed from our accelerator during radiosurgery to increase the dose rate to approximately 750 cGy/min for a 10 x 10 cm2 field at the depth of dose maximum. This leaves a substantial fluence of photons below 1 MeV that are not observed in published spectra calculated for accelerators with flattening filters. Removal of the flattening filter has a measurable effect on the central axis depth dose, reducing the percentage dose at 10 cm depth from 59.2% to 54.3% for a 10 mm diam field. Radiosurgical off-axis ratios and percentage depth dose distributions calculated from these spectra with the EGS4 Monte Carlo code were compared to measured data. Measured and calculated dose distributions both with and without flattener were in good agreement. The dose distributions were found to be insensitive to the differences in the various calculated spectral distributions. Thus, although the attenuated Schiff spectrum is significantly harder than the clinical beam, it is adequate for dose calculations of radiosurgical beams.

Computer Simulation↗

PubMed1995

BEAM: a Monte Carlo code to simulate radiotherapy treatment units.

This paper describes BEAM, a general purpose Monte Carlo code to simulate the radiation beams from radiotherapy units including high-energy electron and photon beams, 60Co beams and orthovoltage units. The code handles a variety of elementary geometric entities which the user puts together as needed (jaws, applicators, stacked cones, mirrors, etc.), thus allowing simulation of a wide variety of accelerators. The code is not restricted to cylindrical symmetry. It incorporates a variety of powerful variance reduction techniques such as range rejection, bremsstrahlung splitting and forcing photon interactions. The code allows direct calculation of charge in the monitor ion chamber. It has the capability of keeping track of each particle's history and using this information to score separate dose components (e.g., to determine the dose from electrons scattering off the applicator). The paper presents a variety of calculated results to demonstrate the code's capabilities. The calculated dose distributions in a water phantom irradiated by electron beams from the NRC 35 MeV research accelerator, a Varian Clinac 2100C, a Philips SL75-20, an AECL Therac 20 and a Scanditronix MM50 are all shown to be in good agreement with measurements at the 2 to 3% level. Eighteen electron spectra from four different commercial accelerators are presented and various aspects of the electron beams from a Clinac 2100C are discussed. Timing requirements and selection of parameters for the Monte Carlo calculations are discussed.

Electrons↗

PubMedDate not supplied

Computer aided design and verification of megavoltage tissue compensators for oblique beams.

A computer based radiotherapy dose compensator system is presented. A plane of uniform dose (pud) is chosen which passes through the central axis at a reference depth in the patient. The pud is oriented at an arbitrary angle. Tissues thicknesses are the ray line distances from the patient surface to the pud. Relative dose estimates on the pud are derived from tissue-air ratio, tissue-maximum ratio, or tissue-phantom ratio tables interpolated for field size and tissue thickness. An inverse square law correction is also applied. Compensator thicknesses are calculated from measured effective attenuation coefficients to attenuate the beam by a factor proportional to the estimated dose on the pud. Water phantom measurements for 60Co for both oblique incidence and wedge geometries demonstrate dose uniformity to better than +/- 4%. Dose measurement with radiographic films sandwiched between the two halves of a wax phantom verify adequate compensation for neck irradiation by 60Co opposed lateral beams and 6-MV x-ray inferiorly angled lateral beams. Compensator factors used for absolute dose determination are accurate to better than +/- 3%. The dose modeling routine, QCBEAM, is found to be sufficiently accurate for routine compensator verification for these geometries.

Biophysical Phenomena↗

PubMedDate not supplied

Forward-directed bremsstrahlung of 10- to 30-MeV electrons incident on thick targets of Al and Pb.

Bremsstrahlung spectra from thick targets of Al and Pb have been measured absolutely (photons per incident electron) along the beam axis for electrons of 10-, 15-, 20-, 25-, and 30-MeV incident energy. The spectra have a 220-keV low-energy cutoff. The targets were cylinders with nominal thicknesses of 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm diameter by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and the collimator effect. Spectra were calculated using the EGS4 Monte Carlo system for simulating the radiation transport. The simulation model included the small amount of material upstream of the target. This material contributed about 40% of the spectrum, but its presence or absence had little effect on the calculated bremsstrahlung yield. The shapes of the measured and calculated spectra were in excellent agreement. The ratio of the total number of photons in each measured spectrum to those in the corresponding calculated spectrum varied from 0.97 +/- 0.06 to 1.12 +/- 0.06, depending largely on the atomic number of the target. Absolute spectral measurements in the literature agreed with our calculations of spectral shape but showed a range of +/- 30% in the number of photons per incident electron relative to the calculated values, which is contrary to our result.

Aluminum↗

PubMedDate not supplied

Angular distribution of bremsstrahlung from 15-MeV electrons incident on thick targets of Be, Al, and Pb.

Bremsstrahlung spectra from thick cylindrical targets of Be, Al, and Pb have been measured at angles of 0 degrees, 1 degree, 2 degrees, 4 degrees, 10 degrees, 30 degrees, 60 degrees, and 90 degrees relative to the beam axis for electrons of 15-MeV incident energy. The spectra are absolute (photons per incident electron) and have a 145-keV lower-energy cutoff. The target thickness were nominally 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm-diam by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and collimator effects. Spectra were also calculated using the EGS4 Monte Carlo system for simulating the radiation transport. There was excellent agreement between the measured and calculated spectral shapes. The measured yield of photons per incident electron was 9% and 7% greater than the calculated yield for Be and Al, respectively, and 2% less for Pb, all with an uncertainty of +/- 5%. There was no significant angular variation in the ratio of the measured and calculated yields. The angular distributions of bremsstrahlung calculated using available analytical theories dropped off more quickly with angle than the measured distributions. The predictions of the theories would be improved by including target-scattered photons.

Aluminum↗