Program of the 1999 Annual Meeting, Radiation Protection in Medicine: Contemporary Issues
April 7-8, 1999
General Issues
Henry D. Royal, Session Chair
The Linear-Nonthreshold Dose-Response Model: A Critical Reappraisal
Arthur C. Upton, Rutgers University
It is customary, for radiation protection purposes, to assume that the risks of genetic and carcinogenic effects of ionizing radiation increase as linear-nonthreshold (LNT) functions of the dose; however, the existing data do not exclude the possibility that thresholds for such effects may exist in the low-dose domain. It is generally acknowledged, therefore, that the LNT dose-response model needs to be evaluated periodically in the light of advancing knowledge.
Such a re-evaluation has recently been conducted by NCRP Scientific Committee 1-6, guided by the charge specifically to review the weight of scientific evidence for and against the LNT model, without reference to any of the associated policy implications.
From its review of the relevant data, the Committee has concluded that the weight of evidence suggest that lesions which are precursors to cancer (i.e., mutations and chromosome aberrations), and certain types of cancer as well, increase linearly with the dose in the low-dose domain. Hence, although other dose-response relationships cannot be excluded, no alternative dose-response model appears more plausible than the LNT model on the basis of present scientific knowledge.
Update of Medical Radiation Dosimetry
Keith F. Eckerman, Oak Ridge National Laboratory
Recent advances in both hardware and software have had substantial impacts in medical dosimetry. The revolution in diagnostic imaging during the past decade has enabled visualization of the anatomy in tomographic planes making it possible to calculate three dimensional distributions of the absorbed dose during treatment planning or following administration of radiopharmaceuticals. Existing computing power provides the dosimetrist with various dose statistics to characterize the irradiation. Most of the statistics and the criteria used in quantitative evaluations of the irradiation are based on the physical dose, however there is increasing interest in developing and using biological models of tissue response to radiation. These advances, most evident in radiotherapy, are finding applications to other areas of medical dosimetry.
Developmental and Reproductive Risks of Radiological Procedures During Pregnancy
Robert L. Brent, Alfred I. duPont Hospital for Children
The term “radiation” evokes emotional responses and fears from lay persons and professionals, especially when exposures occur during pregnancy. Yet, in the field of ionizing radiation, we have a better understanding of the biologic effects and the reproductive risks of ionizing radiation than for any other environmental hazard. The human and animal data support the conclusion that there is a threshold dose for the production of congenital malformations, fetal growth retardation, severe mental retardation, and abortion (deterministic effects) which exceeds the exposures occurring in the vast majority of diagnostic radiological procedures.
Whether there exists a linear or quadralinear dose-response relationship for the risk of stochastic phenomena; namely, genetic diseases and carcinogenesis, has not been determined. But it is obvious that the risks of radiation induced genetic disease and carcinogenesis from diagnostic radiological exposures to the developing embryo are far below the spontaneous occurrence of genetic and carcinogenic diseases.
Medically indicated diagnostic radiological procedures are appropriate for pregnant women, and there is no justification for terminating a pregnancy in women exposed to 0.05 Sv or less because of the radiation exposure. On the other hand, diagnostic radiological studies that can be replaced by ultrasound or other procedures should be avoided simply because the unnecessary use of radiological procedures is not good medical practice. Embryonic and fetal exposures to isotopes present variable risks, depending on the isotope and the exposure, and have to be individually assessed. Counseling women of reproductive age should be based on sound information about the risks of radiation exposure.
Diagnostic Radiology
Andrew K. Poznanski, Session Chair
Radiation Protection Issues in Imaging Infants and Children
Andrew K. Poznanski, Children's Memorial Hospital
Although there is evidence that the risk of radiation in children is greater than in adults, the risk for most examinations may not be, since a lower dose is usually given to produce an image in a child than in an adult in most radiographic studies. In some radiologic exams, particularly in computed tomography (CT), there is a tendency to use similar doses in children and adults. In most cases a lower dose can be used in the child without significantly affecting image quality.
Many methods can be used to decrease radiation exposure in children. Collimation to the region examined is very important in children as this is usually less than the size of the film. Immobilization devices decrease the chance of motion and decrease the need to repeat a study. A technologist who is trained in pediatric techniques and in dealing with children will minimize the need for repeat examinations. Last image hold and pulsed fluoro at low rates will help reduce the radiation dose. Substitution of exams without ionizing radiation is sometimes practical. Ultrasound can, in some situations, substitute for use of ionizing radiation. MRI, however, in children less than six years usually requires sedation and this may be a greater risk than that of radiation from CT.
Mammography
Lawrence N. Rothenberg, Memorial Sloan-Kettering Cancer Center
Mammography is the most effective method for the early detection of breast cancer. The quality and consistency of mammography examinations have improved significantly within the past decade. Much of this improvement can be attributed to the implementation of quality assurance (QA) programs developed in response to recommendations from a variety of sources including NCRP Scientific Committee 72 (NCRP SC-72), the American College of Radiology through its Mammography Accreditation Program, and the U. S. government through the programs created to implement the Mammography Quality Standards Act of 1992. These quality assurance programs require the participation of the entire mammography staff including radiologists, technologists and medical physicists.
The essential aspects of an effective mammography QA program will be reviewed along with a synopsis of the radiation dose evaluation procedures employed and a review of representative patient dose obtained from several nationwide surveys.
Radiation dose is an important indicator of patient risk from mammography procedures. Randomized controlled trials of mammographic screening for asymptomatic women have provided information on the benefits of mammography. Benefit versus risk of mammography will be presented in the format of NCRP SC-72.
New Developments in Computed Tomography
Elliott K. Fishman, Johns Hopkins Hospital
Computed tomography (CT) has become one of the mainstays of radiologic imaging. CT provides unique diagnostic capabilities by allowing the imaging of multiple organ systems in a single examination with a high degree of sensitivity and specificity. The technology behind CT has continued to evolve. Whether it be real-time data reconstruction in the late 80’s, the introduction of clinical spiral CT in the early 90’s, or multidetector scanners being introduced at RSNA 1998, CT continues to evolve. This lecture will discuss these changes and their effect on our diagnostic capabilities. The development of newer applications including CT angiography and 3D vascular imaging will also be addressed.
Patient Dose and Quality Control in Computed Radiography
J. Anthony Seibert, University of California, Davis
Computed radiography (CR) is the generic name describing the digital acquisition of x-ray projection images with photostimulable storage phosphor (PSP) detectors, and the subsequent image readout using a laser beam stimulation source, signal capture assembly, and digital electronics. In most examinations, patient exposures are typically higher by a factor of two for CR than with conventional 400 speed screen-film detectors because of lower x-ray absorption efficiency. In other exams, radiation exposures can be lower, because of the wide exposure latitude of the PSP detector and the ability to amplify and scale the digital information prior to display. A departure from the contrast-limited analog screen-film paradigm of the past 100 y is changing to a noise-limited digital detector paradigm, which has implications regarding the optimal use of CR (and other future digital detectors) to obtain high-quality images at the lowest possible radiation dose. Quality control procedures must evolve to ensure the proper function of the equipment, to monitor radiation exposure levels for specific exams, and to provide feedback regarding image quality and radiation dose.
Doses in Diagnostic Radiology—How and Why They Vary
Orhan Suleiman, Food and Drug Administration
Doses in diagnostic radiology vary because of a wide range of reasons. The Nationwide Evaluation of X-Ray Trends surveys, and the Mammography Quality Standards Act of 1992 inspections use standard, clinically representative phantoms for the determination of dose. Examinations for which phantoms exist and surveys have been conducted include chest and abdominal radiography, fluoroscopy, computed tomography, mammography, dental radiography, and most recently pediatric radiography. Despite these standard protocols, doses in diagnostic radiology still vary widely. Variation in dose due to technical factors such as beam quality, the use of grids, processing quality, type of image receptor, and the relationship of these factors to image quality will be presented.
Nuclear Medicine
Henry D. Royal, Session Chair
Therapy with Monoclonal Antibodies
Richard L. Wahl, University of Michigan Medical Center
The use of radiolabeled monoclonal antibodies for the treatment of cancer has moved from the research laboratory to a technique that is expected to soon be an important part of routine clinical practice of nuclear medicine. A variety of radiolabeled monoclonal antibodies are in clinical trials, however the largest experience to date is in the treatment of non-Hodgkin's lymphomas.
Several monoclonal antibodies have shown promising clinical results, but the most advanced in clinical trials is an anti CD20 monoclonal antibody labeled with 131I. This agent has recently been shown active in Phase II and Phase III multicenter clinical trials. The therapeutic administration of this radioconjugate is performed following patient-specific dosimetry calculations based on tracer imaging studies with a small dose of the radioconjugate. This is used to calculate a specific mCi dose for a given patient to deliver a prescribed whole-body radiation dose (generally 75 cGy). Tumor doses are typically 10 to 15 times higher than the total body dose, with renal radiation dose the highest normal tissue, in most cases.
In patients who have failed chemotherapy, responses with 131I anti CD20 are common, and in the Phase III study (including heavily chemotherapy-treated patients), a response rate of 65 percent was reported. Response rates are higher in less heavily pre-treated patients. Recently, such treatments have been instituted on an outpatient basis, in selected cooperative patients after appropriate education.
In summary, radiolabeled antibody therapy is emerging as a new and effective form of cancer treatment. This lecture will briefly review the field and emphasize the clinical experience in non-Hodgkin's lymphoma.
Outpatient Radionuclide Therapy
Jeffry A. Siegel, Nuclear Physics Enterprises
The Nuclear Regulatory Commission (NRC) regulations (10 CFR 35.75) for the release of patients administered radioactive material have been revised to include dose-based, in addition to the conventional activity-based, criteria. A licensee may now release patients if the total effective dose equivalent to another individual from exposure to a released patient is not likely to exceed 0.5 rem. Compliance with this dose limit is demonstrated by licensees by either using a default table for activity or dose rate provided in Regulatory Guide 8.39 or performing a patient-specific dose calculation.
The purpose of this manuscript is to review the revised regulations and the formulas that can be used to determine when an individual patient administered radioactivity for therapy is releasable. The release of patients administered NaI-131, for treatment of thyroid cancer and hyperthyroidism, and I-131-labeled antibodies, for cancer treatment, is discussed. The implications of the revised NRC regulations on patient release following radionuclide therapy are also discussed. It is concluded that the new regulations will permit many radionuclide therapy procedures to be conducted on an outpatient basis.
Radiation Effects on the Thyroid: Emphasis on Iodine-131
Elaine Ron, National Cancer Institute
External radiation is the major known cause of thyroid cancer. Exposure to x-ray or gamma radiation at doses as low as 0.1 Gy is associated with an increased risk of thyroid cancer. Among atomic-bomb survivors, thyroid cancer exhibits one of the highest excess relative risk coefficients of any organ, and a strong trend for risk to decrease with increasing age at exposure has been demonstrated. Benign thyroid tumors and other benign thyroid diseases also have been linked to external radiation.
In contrast, the role of 131I in inducing long-term health effects are much less well understood. Most epidemiologic studies in which individual 131I organ dose estimates are available are of adult patients who have received either relatively low diagnostic or extremely high therapeutic exposure to 131I. From these studies, there is little evidence of a dose response. However, over the last few years, a sharp increase in the incidence of thyroid cancer has been reported among children exposed to radionuclides from the Chernobyl accident. Recent studies of Chernobyl exposed populations provide some evidence of a dose-response relation between childhood thyroid cancer and estimated 131I dose to the thyroid. Since damage to the thyroid gland from 131I is caused primarily by beta irradiation, differences in the radiation quality and dose rate of beta particles compared with x-ray and gamma radiation need to be considered.
An evaluation of data from medical and environmental exposure to external radiation and 131I will be presented, with particular attention to recent information.
Radiation Oncology/Biology
Sarah S. Donaldson, Session Chair
Three-Dimensional Conformal Radiation Therapy
Steven A. Leibel, Memorial Sloan-Kettering Cancer Center
Failure to control localized cancer results from resistance of tumor clonogens to the maximal dose levels feasible with conventional radiotherapy techniques. Because of difficulties in precisely identifying the location of tumors using traditional radiation therapy planning methods, large safety margins were used in the past to ensure that the tumor was encompassed in its entirety. These safety margins encroached into normal tissue structures. To avoid severe side effects, it was necessary to restrict the tumor dose to the tolerance level of the normal tissues.
Three-dimensional conformal radiation therapy (3D-CRT) permits an increase in tumor dose that is expected to improve local tumor control. As a proof-in-principle, studies in patients with localized prostate cancer have demonstrated that dose escalation is feasible, and a relationship of dose to tumor response and long-term therapeutic outcome has been established. Further, despite the use of high-dose levels, the overall rate of clinically relevant radiation-induced toxicities was reduced when compared to conventional radiotherapeutic approaches.
To further improve the dose distribution, intensity modulated radiation therapy (IMRT) has been developed. This advanced 3D-CRT approach permits the high-dose radiation volume to be exquisitely shaped to the cancerous area and sculpted around adjacent normal organs, allowing higher, more effective, doses to be delivered to tumors without increasing the side effects of treatment. IMRT is being implemented for treatment of prostate, head and neck, and breast cancers and for brain tumors.
New methods of biological imaging are being integrated into the planning of radiation treatment. Using resonance spectroscopy and positron emission tomography, regions of rapid proliferation, hypoxia and high clonogen burden can be identified within the tumor. IMRT may permit “dose painting” to selectively increase the dose to more aggressive or less radiosensitive regions within the tumor. This approach represents a new frontier in 3D-CRT.
Second Malignancies and Genetic Susceptibility
Eric J. Hall, Columbia University
A sizeable proportion of cancer cases presenting themselves for treatment are now second malignancies. The exact number is not known with any certainty, but estimates vary from 7 to 13 percent. They can be attributed to three sources. (1) Continued lifestyle: For example, an individual with a cancer of the head and neck region due to tobacco and alcohol use is very likely to develop additional cancers in adjacent tissues. (2) Genetic susceptibility: Some defective genes result in a condition characterized by multiple malignancies in different sites. (3) Treatment induced second malignancies: A combination of earlier diagnosis and more effective therapy means that more patients are living long enough after treatment for malignancies to become evident that are induced by radiation and/or chemotherapy agents. Risk estimates for second cancers in patients receiving radiotherapy will be discussed.
Radiation Accidents and Accident Prevention in Radiation Oncology
Fred A. Mettler, Jr., University of New Mexico Medical School
The definition of an “accident” in the setting of radiotherapy has been problematic. Various definitions are reviewed. The main difficulty is the narrow range that the radiation oncologist must work within in order to cure a tumor and at the same time, spare normal tissues. The risk of accidents in radiotherapy will be covered with information available from a number of countries. Any calculated risk is likely to be low since many overexposures are not reported to a central registry. There will be an overview of the causes and consequences of several accidents including the 1996 cobalt accident in Costa Rica. Lessons learned from these examples and other accidents include the need for a “safety culture” which includes a good quality assurance program and policies and procedures. Prevention of accidents will be covered both in terms of organization, education and specific issues related to external beam and brachytherapy. The progress of Task Group of ICRP Committee 3 in this subject area will be reviewed.
Twenty-Third Lauriston S. Taylor Lecture on Radiation Protection and Measurements
Introduction of the Lecturer, Isabel Fisenne
Back to Background, Naomi H. Harley
Interventional Procedures
Jerrold T. Bushberg, Session Chair
Radiation Dosage During Interventional Neuroradiological Procedures
Richard E. Latchaw, University of Miami Jackson Memorial Hospital
Interventional neuroradiology is a subspecialty in which complex diagnostic and therapeutic procedures are performed on the head, neck and spine, including cerebral angiography, intravascular occlusive or reconstructive procedures, spinal procedures, and nerve block and injections. These procedures require a high degree of spatial resolution, and are usually performed with prolonged fluoroscopy.
Many of the angiographic studies require repeated radiographic acquisition sequences in the same distribution. Repeat and follow-up procedures over days to weeks are common. Thus, a significant dose of radiation may be administered to a given patient. Depending upon the type of procedure, there is the potential for temporary and long term erythema and hair loss; relatively high doses to the lens of the eye or to the thyroid; or relatively high doses to the pelvic organs, especially the ovaries and testicles.
While these procedures may result in relatively high doses of radiation, they may replace more invasive and morbid surgical procedures, or make surgery more effective. It is important that the dosage of radiation for a given procedure be monitored, and that both the interventionalist and the patient weigh this information and other factors that define the risk/benefit ratio of the procedure. Recommendations of techniques to decrease the dosage or to minimize its effect will be made.
Vascular and Interventional Radiology—Spectrum of Procedures and Radiation Risks
Helen C. Redman, Southwestern Medical Center at Dallas
Body intervention is generally referred to as Vascular and Interventional Radiology (VIR). It includes all arterial and venous studies excluding the heart and head. The arteries in the neck are evaluated by both neuroradiology and VIR. In addition, there are a large number of nonvascular interventional procedures, such as drainage of a blocked kidney or ablation of a liver cancer, that are generally included in VIR, though procedures using computed tomography or ultrasound guidance may be performed by body imagers.
The potential radiation dose varies for each procedure because of many uncontrollable factors. Body habitus affects not only the kilovolt and milliampere-seconds but also the difficulty of a procedure. A very obese patient is often technically more challenging than the same procedure on a marathon runner. Age is also a factor. Both the very young in whom everything is small and more prone to spasm and the elderly with atherosclerotic arteries are more difficult than studying a 20 y old. An agitated or uncooperative patient often requires more fluoroscopy than a calm one. The manual dexterity, experience and observational skills of the radiologist all affect the length of fluoroscopy.
Controllable factors include the precise equipment used and how the equipment is used. Newer equipment generally leads to lesser radiation than equipment that is 5 or 10 y old. The other controllable factor is the number of films obtained on a given patient. These can be digital or overhead film. Though this is a controllable factor, there is a wide variation among radiologists in film usage.
The presentation will describe selected procedures and provide an estimated average fluoroscopy time and an estimated number of overhead films. It is important to remember that fluoroscopy times over 15 min are uncommon and over 30 min are very rare. Therefore, problems seen in cardiac and neurologic intervention are exceedingly rare.
Interventional Cardiology
Bruce D. Lindsay, Washington University School of Medicine
The procedures performed in cardiac catheterization laboratories have evolved over the past decade to encompass increasingly complex curative interventions associated with higher radiation exposure for patients and health care workers. Percutaneous transluminal coronary angioplasty, often requiring the deployment of stents to maintain vessel patency, is routinely performed for patients with complex multivessel coronary artery disease. Electrophysiology studies employ catheter guided applications of radiofrequency energy to selectively destroy small amounts of tissue that cause abnormal heart rhythms. These curative or palliative procedures require the extensive use of fluoroscopy with or without cine angiography to visualize structural lesions and the position of catheters. This presentation will provide a general overview of these procedures and the associated radiation exposure to patients and health care workers.
Perspectives on Risks of Radiation Skin Dose in Fluoroscopy
Louis K. Wagner, University of Texas, Houston Medical School
Radiation doses to the skin from some fluoroscopically-guided interventional procedures are among the highest delivered in medicine. Many reports of burns, some of which have resulted in dermal necroses, have been reported to the U.S. Food and Drug Administration and/or have been reported in the literature. The factors behind these effects include long fluoroscopy times, large patients or thick body parts, close proximity to the x-ray source, multiple procedures, equipment malfunction, and possibly pre-existing skin disease. This presentation will focus on the relationships among the above factors, the severities of effects that have been observed, the types of procedures that appear to involve the greatest risks, and the prevalence of the problem.
Management of Patient Dose During Fluoroscopy
Benjamin R. Archer, Baylor College of Medicine
Modifications to the work habits of some practitioners and to the fluoroscopic room environment are necessary to maintain an acceptable level of benefit/risk for patients. These modifications include improved patient management, physician training, dose monitoring, and equipment maintenance. Dose optimization can be achieved by: (1) managing cumulated dose from multiple procedures, (2) controlling dose rate at skin entrance, (3) performing real-time dose monitoring and establishing action levels, and (4) using quality control measures to avoid excessive radiation output by equipment. This presentation will focus on the implementation of each of these measures.
High-Dose Rate Brachytherapy in Coronary Artery Restenois
Ron Waksman, Cardiology Research Foundation
Vascular brachytherapy for prevention of restenosis is an evolving field. In recent years, numerous animal experiments and feasibility clinical pilot trials have demonstrated that low doses of radiation applied following intracoronary intervention reduces neointimal proliferation, prevents vessel contraction, and decreases the restenosis rate. However implementing this technology in the cardiac catheterization laboratory is associated with safety, logistic and regulatory issues. The training requirements for the personnel involved in the procedure have not yet been well-defined.
Several systems for delivering the radiation dose are available including radioactive implants, catheter based system using solid sources and non-solid sources such as liquid or gas. The delivery systems include hand loading or automatic remote afterloaders. The radionuclide that is used and thus the shielding requirements also vary among systems. Given the encouraging results from clinical trials, it is likely that this technology will be approved for routine clinical use in the near future and will be used commonly in the next millennium.
Policy Issues
Barbara McNeil, Session Chair
Training of Physicians and Support Staff—The Time Has Come for Credentialing and Privileging for the Use of X Rays
Joel E. Gray
Real estate agents, beauticians, and restaurants have one thing in common—they are licensed in most states. Physicians, surgeons, and x-ray technologists have one thing in common—they do NOT need to be licensed or trained before they expose patients, the public, and themselves to ionizing radiation!
In most states a physician, surgeon, dentist, veterinarian, chiropractor, podiatrist, and some others, may use x-ray equipment, whether or not they have been trained in its effective use or risks. Likewise, in about one-half of the states, individuals under the direction of these professionals may use x-ray equipment without training. These may be laboratory technicians, nurses, secretaries or others employed in the medical practice. In addition, radiation exposures for medical purposes are higher than necessary as evidenced by national surveys carried out by the Food and Drug Administration. Many of the individuals using radiation do not know the doses to their patients.
What can and should the NCRP do to assist the medical community in addressing these issues? This presentation will focus on this question, as well as address the potential for radiation exposure reduction and improved healthcare through the credentialing, privileging and licensing of individuals using ionizing radiation in the medical community.
Irradiation of Human Subjects in Research
Ruth R. Faden, Johns Hopkins University School of Hygiene and Public Health
Two concepts central to research ethics, informed consent and acceptable risk, are reviewed, with particular emphasis on research involving radiation. The moral meaning of informed consent is contrasted with regulatory requirements to illustrate the differences between informed consent as a process and informed consent as a legal document. Each of the elements of informed consent—disclosure, absence of controlling influences, adequate understanding and competence—are discussed. This discussion includes consideration of what it means for potential subjects to have an adequate understanding of risks associated with a specific radiation exposure. The concept of acceptable risk, often translated in research ethics to mean an acceptable risk to benefit ratio, is examined. Special attention is paid to the criterion of “minimal risk” as a threshold for the involvement of human subjects in “nontherapeutic” research. The implications of the concept of “minimal risk” for nontherapeutic research involving the exposure of pregnant women and children to radiation is examined.
The Program Committee
Henry D. Royal, Chairman
Jerrold T. Bushberg
Sarah S. Donaldson
Bruce D. Lindsay
Barbara J. McNeil
Fred A. Mettler, Jr.
Andrew K. Poznanski
Telephone: 1-800-229-2652, Ext. 25
Email: NCRPpubs@NCRPonline.org
Fax: 301-907-8768
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