Radiation Protection in Mammography: Recommended Safety Procedures for the Use of Mammographic X-Ray Equipment - Safety Code 33

1. Introduction

Diagnostic X-ray imaging is an essential part of present day medical practice. In Canada, over 60 percent of the population undergoes radiological procedures each year. One of these radiological procedures is mammography, a valuable tool used in modern health care for the detection of breast cancer. It is the only detection modality proven reliable and effective in detecting preclinical breast cancer. However, mammographic X-ray procedures must be carefully managed, because X-radiation has the potential for damaging healthy cells and tissues, and great care must be taken to obtain optimal image quality for interpretation. The aim of radiation protection in radiology is to obtain the desired clinical information with minimum radiation exposure to patients, medical personnel, and the public.

During a properly carried out mammographic X-ray examination, the radiation dose received by an individual is generally low and relatively few cells are affected by the radiation. The body will replace or repair most, but not all, of the affected cells. Therefore, the effect of exposure to even low levels of ionizing radiation over long periods of time can accumulate and may represent a health risk. The need for radiation protection exists because exposure to ionizing radiation can result in deleterious effects that manifest themselves in the exposed individuals, and other effects that manifest themselves in their descendants. These are called somatic and genetic effects, respectively. Somatic effects are characterized by observable changes occurring in the body organs of the exposed individual. These changes may appear within a time frame of a fewhours to many years, depending on the amount and duration of exposure of the individual. Genetic effects attributable to chromosomal damage of the germ cells giving rise to genetic defects are also a cause for concern at the lower doses used in diagnostic radiology. These genetic defects showthemselves in the progeny of exposed individuals. The radiation doses may be low and appear to cause no observable damage, but genetic effects can be significant when considered for a large population. In mammography, the risk of genetic defects in well-conducted examinations is very small, and for post-menopausal women, there is no genetic risk.

It must be emphasized that it is not possible to measure carcinogenic effects at low doses and that estimates of the incidence of such effects are based on linear extrapolation from relatively high doses. It is generally accepted that there is no safe level of radiation dose and that no matter how low a dosage is used, there exists the probability of an effect. Since the projected effect of a low dose increases the incidence of a deleterious effect only minimally above the naturally occurring level, it is impossible to prove by observation either the validity or falsity of this hypothesis. However, the linear extrapolation hypothesis has been widely adopted in radiological protection and has led to the formulation of the ALARA (As Low As Reasonably Achievable) principle. This requires that a dose which can be reduced without significant loss of critical diagnostic information and without too much expense or inconvenience should be reduced, no matter howlowit is and that a dose which can be avoided altogether, without unfavourable consequences, should be avoided.

With modern equipment and techniques, mammography is the most accurate diagnostic modality used in the detection of breast cancer. The two most common mammographic modalities presently in use are film-screen mammography and Xeromammography. Filmscreen mammography utilizes low-energy X-rays with a compatible film-screen combination, and must be done using special dedicated mammographic X-ray equipment. To obtain consistent optimum image quality, a stringent Quality Control system is required for the entire diagnostic imaging chain, including X-ray equipment with consistent performance, good imaging techniques, appropriate loading factors, adequate image processing, and optimal viewing conditions. Xeromammography is a technique that uses X-ray tube voltages between 40 and 50 kVp and an electrostatic technique to record the image instead of film. In this technique, a selenium-coated positively charged plate is exposed to X-rays, and the X-ray photons cause the selenium to release electrons, neutralizing part of the positive charges. The plate is then processed and an image is obtained. As the image receptor is less efficient than a film-screen combination, glandular doses received by the patient are typically three to six times higher than those of film-screen mammography

There are four main aspects of radiation protection to be considered in mammography. Firstly, patients should not be subjected to unnecessary radiographic procedures. Secondly, when a procedure is required, it is essential that the patient be protected from excessive exposure to radiation during the examination. Thirdly, it is necessary that personnel in radiology facilities be protected from excessive exposure to radiation in the course of their work. Finally, personnel and the general public in the vicinity of such facilities require adequate protection.

In diagnostic mammography, the application of radiation protection begins with the request of the examination followed by the execution of the mammographic X-ray examination including positioning of the patient, irradiation, image processing and interpretation. In screening mammography, the application of radiation protection begins with the proper selection of participants followed by the execution of the mammographic X-ray examination. It is essential that close cooperation between radiologists, radiology technologists, medical physicists, and other support staff be maintained to obtain a consistent and effective level of radiation protection and image quality.

While dose limits have been established for radiation workers and the general public, these limits do not apply to doses received by a person as a patient undergoing diagnostic X-ray procedures. However, this Safety Code recommends limits on the mean glandular dose for a representative mammographic X-ray examination. Other limits may be recommended by Provincial standards where applicable. For patients, the risk involved with exposure to radiation must always be weighed against the clinical benefit of an accurate diagnosis, and there must always be a conscious effort to reduce patient doses to the lowest practical levels consistent with optimal image quality.

New modalities, such as stereotactic breast localisation, digital mammography, and storage phosphor mammography, are being introduced or are in development. While many characteristics and requirements for these modalities are comparable to the ones of film-screen mammography, there may be a need, in some situations, to modify requirements to suit these imaging modalities. Imaging procedures, where X-rays are not utilized, such as ultrasound, thermography, transillumination, and magnetic resonance imaging, are beyond the scope of this document.