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Biomedical subjects

B A Fraass

Publications and source records attributed to B A Fraass.

69 records · Page 4Linked to original sources
PubMed1983

Whole-skin electron treatment: patient skin dose distribution.

Low-energy electron irradiation of the whole skin is used to treat skin diseases such as mycosis fungoides. The literature on the related dosimetry concentrates almost exclusively on idealized conditions, such as the "in-air" distribution of radiation and the dose received by body-shaped phantoms. The results of a detailed study of dose to five patients, using measurements from thermoluminescent dosimeters, are reported. The dose to different points on the trunk was fairly uniform, while there were significant deviations from uniformity for the arms, legs, and head. The data show that in-air measurements are of limited value as a measure of the uniformity of the dose received by the patient.

Electrons↗
PubMed1982

Late effects of radiation therapy in the treatment of Hodgkin's disease.

In the last two decades, a considerable amount of clinical and experimental animal data has been accumulated regarding the tolerance of the major normal tissues and organs traversed by the radiation beams in treating Hodgkin's disease. Typically, the tolerance of several tissues and organs must be considered in designing the very large radiation portals such as the mantle. Today, sophisticated machinery such as computerized tomography scanners, simulators, computers, and linear accelerators are available to improve the radiation planning and treatment of Hodgkin's disease. In this paper, we discuss the late effects of radiation therapy to the lung, heart, thyroid, kidney, gastrointestinal tract, and the gonads of adults with Hodgkin's disease. Emphasis is placed on our recommendations for modifications of radiation technique to improve the complication-free cure rate.

Adult↗
PubMedDate not supplied

Peripheral dose from megavolt beams.

The peripheral dose (PD), defined as the dose outside of therapeutic radiation beams, has been investigated for 60Co, 4-, 6-, and 10-MV x-ray machines. The measurements have been carried out down to dose levels of about 0.1% of the peak dose in the beam, since that dose level may be of clinical importance in some situations. The PD measurements for the various machines are qualitatively similar, which allows the identification of a simple basic data set which can characterize the PD for any particular machine. The PD has been separated into two components: in-phantom scatter dose and transmission (leakage) dose. Knowledge of the two components is important clinically when shielding is considered.

Cobalt Radioisotopes↗
PubMedDate not supplied

The net fractional depth dose: a basis for a unified analytical description of FDD, TAR, TMR, and TPR.

The net fractional depth dose (NFD) is defined as the fractional depth dose (FDD) corrected for inverse square law. Analysis of its behavior as a function of depth, field size, and source-surface distance has led to an analytical description with only seven model parameters related to straightforward physical properties. The determination of the characteristic parameter values requires only seven experimentally determined FDDs. The validity of the description has been tested for beam qualities ranging from 60Co gamma rays to 18-MV x rays, using published data from several different sources as well as locally measured data sets. The small number of model parameters is attractive for computer or hand-held calculator applications. The small amount of required measured data is important in view of practical data acquisition for implementation of a computer-based dose calculation system. The generating function allows easy and accurate generation of FDD, tissue-air ratio, tissue-maximum ratio, and tissue-phantom ratio tables.

Cobalt Radioisotopes↗
PubMedDate not supplied

Practical methods of electron depth-dose measurement compared to use of the NACP design chamber in water.

Central axis relative dose versus depth measurements were performed using two different small volume thimble ionization chambers and a p-type silicon diode in a water phantom and with two parallel-plate ionization chambers, thermoluminescent dosimeters, and radiographic film in a popular clear polystyrene phantom. Values obtained were compared to the results of similar measurements in a water phantom performed with a plane-parallel ionization chamber designed and optimized for use in electron beams by the Nordic Association of Clinical Physicists (NACP). The NACP chamber is expected to minimally perturb the electron fluence and be least prone to point of measurement uncertainties. Its use in a water phantom closely approximates the spirit of recent international protocols. Data were obtained for the foil scattered electron beams generated by two different accelerators for field sizes from 6 cm X 6 cm to 25 cm X 25 cm and energies between 6 and 20 MeV. Easily identifiable effective points of measurements were defined for each measurement device and standard corrections were applied to the raw data to obtain depth-dose curves. The degree of agreement between the various techniques and the NACP-water standard was quantitatively analyzed through comparison of the resulting depths of 50% dose and practical range. All methods were found to yield reasonable results when carefully implemented, with average differences of less than 1 mm being easily achievable. Measurements with p-type silicon diode detectors were found to be particularly useful, as they are pointlike and appear from all practical considerations to directly represent relative dose, thus requiring little or no correction to raw readings.

Air↗
PubMedDate not supplied

Relative electron beam measurements: scaling depths in clear polystyrene to equivalent depths in water.

For purposes of mean incident energy determination and the accrual of consistent treatment planning data, measurements of relative ionization or dose made in clear polystyrene must be scaled in depth to produce depth-ionization or depth-dose curves equivalent to what would have been measured in water. Recommendations from various protocols for clear polystyrene to water depth scaling factors differ by as much as 5%. Here, central axis measurements of relative ionization as a function of depth have been made with parallel-plate chambers both in a popular clear polystyrene phantom of density 1.045 g/cm3 and in water. Perturbation and displacement corrections were thus minimized. Both depth-ionization and depth-dose curves were formed for electron beams with nominal incident energies between 6 and 20 MeV and field sizes from 6 cm X 6 cm to 15 cm X 15 cm. Comparisons of the depths for 50% relative reading and practical range between the two phantoms yield average empirical scaling factors of 0.990 and 1.002, respectively.

Electrons↗
PubMedDate not supplied

Determination of electron beam mean incident energy from d50 (ionization) values.

Depth-ionization measurements have been obtained with an air-filled Nordic Association of Clinical Physicists (NACP) design parallel-plate ionization chamber in a water phantom for ten foil-scattered electron beams from two different machines with nominal energies between 6 and 20 MeV and field sizes from 6 X 6 to 25 X 25 cm2. Depths of 50% ionization and practical range have been determined from least-squares fits to both the raw data and values corrected to parallel-beam geometry using measured virtual source distances. Depths of 50% dose have also been obtained from fits to divergence-corrected depth-dose measurements performed under identical conditions using, a p-type silicon diode detector. Utilizing accepted conversion factors between mean incident energy (E0) and depth of 50% dose for parallel incident beams, and taking advantage of the fact that p-type silicon diode detector readings are nearly directly indicative of relative dose, conversion factors between E0 and depth of 50% ionization for divergence-corrected and raw, uncorrected finite source-surface distance depth-ionization data are empirically determined. Those values, obtained using the results of both ETRAN and EGS4 dose calculations as base lines, are compared to values currently recommended for use in clinical dosimetry.

Electrons↗
PubMedDate not supplied

Gypsum mixtures for compensator construction.

The characteristics and properties of a new material used for the fabrication of compensators are presented. This material is a special, refined gypsum. It requires a factor of 3 less water to prepare than ordinary gypsums and as a result the attenuation properties are stable over time. The material may be used by itself or mixed with fine metal particles to increase the attenuation per unit thickness. Gypsum, gypsum + steel, and gypsum + iron were investigated. The results of attenuation measurements in narrow- and broad-beam geometries appropriate to design of clinical dose modifying compensators are presented. Practical and technical details associated with the use of these materials are given. These compounds are found to be easy to use, versatile, reliable, environmentally safe, and inexpensive. In addition, an example of their use for dose compensation is given.

Calcium Sulfate↗
PubMedDate not supplied

Improving precision and safety in the use of beam modifying devices in radiation therapy.

Reliable and safe implementation of beam modifying devices such as wedges and block trays requires careful design and construction. Inappropriate design may pose problems ranging from user-hostile operation to hard-to-track, but significant variations in actual position in a beam. This may cause variation in actual wedge output factors, or variation in the position of a block tray. In case of simple mechanical failure or personnel mistake, design related mechanical conditions may result in injury to either a patient or a staff member. This paper is based on experience with linear accelerators from one manufacturer, but similar conditions are likely to exist with other radiation machines. A simple technical modification is offered which improves both accuracy and reproducibility in the placement of wedge-type filters. For our machines the solution also provides improved safety in the use of both wedge trays and block trays.

Equipment Design↗
PubMedDate not supplied

Verification data for electron beam dose algorithms.

The Collaborative Working Group (CWG) of the National Cancer Institute (NCI) electron beam treatment planning contract has performed a set of 14 experiments that measured dose distributions for 28 unique beam-phantom configurations that simulated various patient anatomic structures and beam geometries. Multiple dose distributions were measured with film or diode detectors for each configuration, resulting in 78, 2-D planar dose distributions and one, 1-D depth-dose distribution. Measurements were made for 9- and 20-MeV electron beams, using primarily 6 x 6- and 15 x 15-cm applicators at several SSDs. Dose distributions were measured for shaped fields, irregular surfaces, and inhomogeneities (1-D, 2-D, and 3-D), which were designed to simulate many clinical electron treatments. The data were corrected for asymmetries, and normalized in an absolute manner. This set of measured data can be used for verification of electron beam dose algorithms and is available to others for that purpose.

Algorithms↗
PubMedDate not supplied

Generation and use of measurement-based 3-D dose distributions for 3-D dose calculation verification.

A 3-D radiation therapy treatment planning system calculates dose to an entire volume of points and therefore requires a 3-D distribution of measured dose values for quality assurance and dose calculation verification. To measure such a volumetric distribution with a scanning ion chamber is prohibitively time consuming. A method is presented for the generation of a 3-D grid of dose values based on beam's-eye-view (BEV) film dosimetry. For each field configuration of interest, a set of BEV films at different depths is obtained and digitized, and the optical densities are converted to dose. To reduce inaccuracies associated with film measurement of megavoltage photon depth doses, doses on the different planes are normalized using an ion-chamber measurement of the depth dose. A 3-D grid of dose values is created by interpolation between BEV planes along divergent beam rays. This matrix of measurement-based dose values can then be compared to calculations over the entire volume of interest. This method is demonstrated for three different field configurations. Accuracy of the film-measured dose values is determined by 1-D and 2-D comparisons with ion chamber measurements. Film and ion chamber measurements agree within 2% in the central field regions and within 2.0 mm in the penumbral regions.

Humans↗
PubMedDate not supplied

Medical accelerator safety considerations: report of AAPM Radiation Therapy Committee Task Group No. 35.

Ensuring safe operation for a medical accelerator is a difficult task. Users must assume more responsibility in using contemporary equipment. Additionally, users must work closely with manufacturers in promoting the safe and effective use of such complex equipment. Complex treatment techniques and treatment modality changeover procedures merit detailed, unambiguous written procedural instruction at the control console. A thorough "hands on" training period after receiving instructions, and before assuming treatment responsibilities, is essential for all technologists. Unambiguous written instructions must also be provided to guide technologists in safe response when equipment malfunctions or exhibits unexpected behavior or after any component has been changed or readjusted. Technologists should be given a written list of the appropriate individuals to consult when unexpected machine behavior occurs. They should be assisted in identifying aberrant behavior of equipment. Many centers already provide this instruction, but others may not. Practiced response and discussion with technologists should be a part of an ongoing quality assurance program. An important aspect of a safety program is the need for continuous vigilance. Table III gives a summary of a comprehensive safety program for medical accelerators. Table IV gives a list of summary recommendations as an example of how one might mitigate the consequences of an equipment failure and improve procedures and operator response in the context of the environment described. Most of these recommendations can be implemented almost immediately at any individual treatment center.

Biophysical Phenomena↗
PubMedDate not supplied

Use of an octree-like geometry for 3-D dose calculations.

A representation used for 3-D graphical objects, the "octree," has been applied to the geometrical calculations needed to perform photon beam does calculations for radiotherapy treatment planning. This representation allows the algorithm to attempt to minimize the number of distinct geometrical calculations that are needed to perform dose calculations to a particular resolution. In this way, the calculational time can be minimized, since the geometrical part of the dose calculations is often the most time-intensive part of the calculation process. The octree-like system used here has sped up the photon dose calculations described here by up to a factor of 10.

Algorithms↗
PubMedDate not supplied

Investigating the potential of three-dimensional treatment planning.

3-D treatment planning has received a great deal of attention in the radiation therapy community over the last several years. This new technology makes use of the continuous improvements in computer hardware and graphics capabilities, along with major improvements in treatment planning software, to provide a fully three dimensional simulation of the patient, radiation beams, and dose distributions which are used for radiation therapy of various cancers. With these capabilities, the physician and treatment planner may now optimize the radiation beams used to treat the patient much more effectively than in the past, when only a limited description of the patient, beams, and doses was available. This paper describes several of the new capabilities of these 3-D planning systems, some research studies which are currently being performed to evaluate the usefulness of the new technology, and finally some of the costs associated with its implementation.

Computer Simulation↗