Radioactive materials in biosolids: dose modeling.

The Interagency Steering Committee on Radiation Standards (ISCORS) has recently completed a study of the occurrence within the United States of radioactive materials in sewage sludge and sewage incineration ash. One component of that effort was an examination of the possible transport of radioactivity from sludge into the local environment and the subsequent exposure of humans. A stochastic environmental pathway model was applied separately to seven hypothetical, generic sludge-release scenarios, leading to the creation of seven tables of Dose-to-Source Ratios (DSR), which can be used in translating from specific activity in sludge into dose to an individual. These DSR values were then combined with the results of an ISCORS survey of sludge and ash at more than 300 publicly owned treatment works, to explore the potential for radiation exposure of sludge workers and members of the public. This paper provides a brief overview of the pathway modeling methodology employed in the exposure and dose assessments and discusses technical aspects of the results obtained.

Radioactive materials in biosolids: national survey, dose modeling, and publicly owned treatment works (POTW) guidance.

The Nuclear Regulatory Commission (NRC) announced the availability of three new documents concerning radioactive materials in sewage sludge and ash from publicly owned treatment works (POTW). One of the documents is a report presenting the results of a volunteer survey of sewage sludge and ash samples provided by 313 POTWs. The second document is a dose modeling document, using multiple exposure pathway modeling focused on a series of generic scenarios, to track possible exposure of POTW workers and members of the general public to radioactivity from the sewage sludge or ash. The third document is a guidance report providing recommendations on the management of radioactivity in sewage sludge and ash for POTW owners and operators. This paper explains how radioactive materials enter POTWs, provides criteria for evaluating levels of radioactive material in sludge and ash, and gives a summary of the results of the survey and dose modeling efforts.

An overview of EPA regulation of the safe disposal of transuranic waste at the Waste Isolation Pilot Plant.

In March of 1999, the Waste Isolation Pilot Plant (WIPP) in southeast New Mexico, the world's first deep geological repository for radioactive materials, began receiving defense-related transuranic waste. The WIPP was designed and constructed by the U.S. Department of Energy, but critical to its opening was certification by the U.S. Environmental Protection Agency that the repository complies with the radioactive waste disposal regulations set forth as environmental radiation protection standards (40 CFR Part 191) and compliance criteria (40 CFR Part 194). This paper provides a summary of the regulatory process, including the Environmental Protection Agency's waste containment, groundwater protection, and individual dose regulations for the WIPP; the Department of Energy's performance assessment and the other parts of its compliance certification application; and the Environmental Protection Agency's review and analysis of the compliance certification application and related documentation.

Looking within: Part 3. More ... photons, protons, and all that....

To summarize: Most radiographic and all fluoroscopic and CT studies make use of bremsstrahlung X-ray photons, with beam energies typically between 60 and 120 kVp. Breast imaging uses the nearly monochromatic (of only one wavelength and energy) characteristic X-rays of molybdenum, with energies just under 20 keV.

Sites in the United States contaminated with radioactivity.

Over the century that radioactive materials have been mined, processed, produced, and utilized, many sites across the United States have become contaminated. Such sites include bases and installations of the Department of Defense, weapons production and research facilities of the Department of Energy, properties under the authority of other Federal agencies, privately-owned and governmental facilities that are licensed by the Nuclear Regulatory Commission and its Agreement States, and sites licensed by or the responsibility of states. This review reports on aspects of work by the Environmental Protection Agency, the Department of Defense, the Department of Energy, the Nuclear Regulatory Commission, and others to identify sites contaminated with radioactive materials. It also describes the principal programs that have been instituted to deal with them.

Model for estimating population impacts averted through the remediation of contaminated soil.

This is the second in a series of papers that discuss methodologies being developed and employed by the U.S. Environmental Protection Agency in support of its decisions on cleanup levels for radioactively contaminated sites that are to be remediated and released for public use. It describes a model, CU-POP, designed by the U.S. Environmental Protection Agency to obtain estimates of the potential collective radiological health impacts over specific periods of time (100, 1,000 and 10,000 y following cleanup), both on and off site, due to residual radioactive materials in on-site soil. Collective doses and risks are linear in population density for the direct exposure, dust and indoor radon inhalation, and soil ingestion pathways; it is assumed that specific fractions of all food grown and all groundwater pumped at a site are consumed by on- and off-site populations. The model was developed for application to a set of hypothetical "reference" sites; its testing on a simple generic site is discussed briefly here.

Technical basis for EPA's proposed regulation on the cleanup of sites contaminated with radioactivity.

The U.S. Environmental Protection Agency is proposing a regulation for the protection of the public from radioactive contamination at sites that are to be cleaned up and released for public use. The rule will apply to sites under the control of Federal agencies, and will impose limits on radiation doses to individuals living or working on a site following cleanup; it will thereby provide site owners and managers with uniform, consistent cleanup criteria for planning and carrying out remediation. This paper presents an overview of EPA's approach to assessing some of the beneficial and adverse effects associated with various possible values for the annual dose limit. In particular, it discusses the method developed to determine how the choice of cleanup criterion affects (1) the time-integrated potential numbers of non-fatal and fatal radiogenic cancers averted among future populations, (2) the occurrence of radiogenic cancers among remediation workers and the public caused by the cleanup process itself, and (3) the volumes of contaminated soil that may require remediation. The analytic methods described here were used to provide input data and assumptions for the Regulatory Impact Analysis (RIA) that supports the proposed regulation; the RIA also considered non-radiological benefits and costs (i.e., public health, economic, and ecological) of the standards.

Management of radiation oncology patients with implanted cardiac pacemakers: report of AAPM Task Group No. 34. American Association of Physicists in Medicine.

Contemporary cardiac pacemakers can fail from radiation damage at doses as low as 10 gray and can exhibit functional changes at doses as low as 2 gray. A review and discussion of this potential problem is presented and a protocol is offered that suggests that radiation therapy patients with implanted pacemakers be planned so as to limit accumulated dose to the pacemaker to 2 gray. Although certain levels and types of electromagnetic interference can cause pacemaker malfunction, there is evidence that this is not a serious problem around most contemporary radiation therapy equipment.

Optimization of radiation therapy, IV: A dose-volume histogram reduction algorithm.

A general formalism for estimating the complication probability for non-uniform irradiation of a normal tissue structure has been developed and used by the NCI-sponsored committee on "Evaluation of Treatment Planning for Particle Beam Radiotherapy." The approach involves reducing an N-step dose-volume histogram for the tissue (obtained from the 3D treatment plan) to an equivalent (N-1)-step histogram; this procedure is repeated until there remains a single-step histogram, the complication probability of which can readily be determined. This note provides technical details concerning the histogram-reduction algorithm. Results obtained using it are compared with those for two alternative histogram-reduction algorithms.

Optimization of radiation therapy, III: A method of assessing complication probabilities from dose-volume histograms.

To predict the likelihood of success of a therapeutic strategy, one must be able to assess the effects of the treatment upon both diseased and healthy tissues. This paper proposes a method for determining the probability that a healthy organ that receives a non-uniform distribution of X-irradiation, heat, chemotherapy, or other agent will escape complications. Starting with any given dose distribution, a dose-cumulative-volume histogram for the organ is generated. This is then reduced by an interpolation scheme (involving the volume-weighting of complication probabilities) to a slightly different histogram that corresponds to the same overall likelihood of complications, but which contains one less step. The procedure is repeated, one step at a time, until there remains a final, single-step histogram, for which the complication probability can be determined. The formalism makes use of a complication response function C(D, V) which, for the given treatment schedule, represents the probability of complications arising when the fraction V of the organ receives dose D and the rest of the organ gets none. Although the data required to generate this function are sparse at present, it should be possible to obtain the necessary information from in vivo and clinical studies. Volume effects are taken explicitly into account in two ways: the precise shape of the patient's histogram is employed in the calculation, and the complication response function is a function of the volume.

Optimization of radiation therapy II: the critical-voxel model.

The Complication Factor (CF) is an objective function recently introduced for use in the optimization of radiation therapy X treatment planning. Unlike earlier objective functions based upon physical/geometrical criteria, such as tumor dose uniformity, minimal integral-dose, etc., the CF stems from a simple biological/probabilistic model of radiation damage in living organisms. The CF defines the integral-response of an organ as that fraction of it rendered non-functional by irradiation; this parameter is significant if the net amount of damage to the organ is of importance but details of its spatial distribution are not as, for example, might be nearly the case with liver. This approach does not work, however, if complications in any one individual volume-element are critical, as with spinal cord or tumor recurrence. Several authors have addressed the latter problem, and we find that the probabilistic argument common to their methods fits comfortably within the CF framework. Drawing attention to the distinct differences between the integral-response and critical-voxel cases hopefully will be of value in the further development of biological modelling, for application in radiotherapy and elsewhere.

Measurements of dose from secondary radiation outside a treatment field.

Radiation dose to organs outside the radiotherapy treatment field can be significant and therefore is of clinical interest. We have made measurements of dose at distances up to 70 cm from the central axes of 5 X 5, 15 X 15 and 25 X 25 cm radiation fields of 300 kVp, 4 MV and 8 MV X rays, and 60Co gamma rays, at the surface and at depths in water of 5 and 10 cm. Contributions to the total secondary radiation dose from water scatter, machine (collimator) scatter and leakage radiation have been separated. We have found that the component of dose from water scatter can be described by a simple exponential function of distance from the central axis of the radiation field for all energies and field sizes. Machine scatter contributes 20 to 40% of the total secondary dose depending on machine, field size and distance from the field. Leakage radiation contributes very little dose, but becomes the dominant component at distances beyond 60 cm from the central axis. Estimates of the risk of second tumors in long term survivors indicate a small incremental increase above the natural incidence rate based on information from the 1980 BEIR Committee report.

Optimization of radiation therapy: integral-response of a model biological system.

Several radiotherapy treatment planning criteria have been proposed for dose distribution optimization. Here we present a simple mathematical model of an idealized biological system. From it we have derived an objective function designed to achieve an extremum for that particular plan which minimizes the probabilities of occurrence of unacceptable complications in healthy tissue and of recurrence or spread of disease. The model assumes that an organism is separable into physiologically discrete compartments or organs, each consisting of a set of microscopic functional units with their own dose-response characteristics. In analogy to the integral-dose, we define an integral-response parameter v as a measure of radiation-induced damage; the value of this v may be calculated for any given spatial distribution of dose in a compartment or organ. A Probability of Serious Complications function, PSC(v), then provides an estimate of the likelihood of occurrence of unacceptable complications. Special problems arising with paired organs (kidneys), "series" organs (spinal cord), and the recurrence and spread of disease are addressed. The PSC for the various organs and neoplasia can be combined to form a compound Complication Factor (CF) objective function; the lower the value of the CF, the better the overall plan. Prospects for making the model explicitly time/fractionation dependent, and for incorporating utility theoretic ideas, are discussed.

A FORTRAN program for the optimization of radiotherapy treatment planning using the complication probability factor (CPF).

The complication probability factor (CPF) is an objective function, based directly on radiobiological principles and clinical data, for the optimization of radiotherapy treatment planning; it measures the likelihood that a given radiation dose distribution will lead to serious complications in the patient as a result of damage to healthy tissue. A computerized search can be made for that treatment plan which delivers an acceptable tumoricidal dose, yet minimizes the CPF as averaged over the total volume of healthy tissue irradiated. The CPF FORTRAN program, run on a PDP 11/55 in conjunction with a commercially available radiotherapy treatment planning package, is described in detail.

Of pacemakers and statistics: the actuarial method extended.

Pacemakers cease functioning because of either natural battery exhaustion (nbe) or component failure (cf). A study of four series of pacemakers shows that a simple extension of the actuarial method, so as to incorporate Normal statistics, makes possible a quantitative differentiation between the two modes of failure. This involves the separation of the overall failure probability density function PDF(t) into constituent parts pdfnbe(t) and pdfcf(t). The approach should allow a meaningful comparison of the characteristics of different pacemaker types.