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Patient Advocacy - Breast Cancer Diagnosis/Technical Aspects of Mammography and The Role of the Medical Physicist

Introduction

Mammography is a very technically difficult radiographic exam, however, it is presently the best diagnostic imaging test available for detecting early breast cancer.  The exam should only be performed by qualified personnel with specialized training in mammography.  Also, only dedicated equipment for breast imaging should be used.  The goal of a mammography exam is to detect very small abnormalities in the breast tissue before they develop into breast cancer.  This is accomplished by producing high quality images at an adequate, but not unnecessary radiation dose.  Mammography performed in this manner will lead to a substantial benefit for the patient, at low risk.

The medical physicist plays a significant role in establishing and maintaining good standards of practice for mammography facilities.  The physicist works with other personnel and is consulted on various aspects such as equipment performance, imaging protocols, and radiation dose/risk assessments for staff and patients.

Mammography - Technical Aspects

  1. What does the equipment do?

    Conventional mammography equipment consists of two major components: an x-ray tube and an image receptor. The x-ray beam originates at the x-ray tube, and is transmitted through the breast.  A film contained in the image receptor records the x-ray distribution pattern that has passed through the breast tissue.  This black and white 'photograph' or image of tissue structures is known as the mammogram.

    1. Mammography Unit

      Mammography Unit

    2. Film image of breast (mammogram)

      (ACR website) ref. (www.radiologyinfo.org)

      Mammogram

  2. Why must the breast be compressed with this equipment?

    Firm breast compression decreases the thickness of breast tissue that the x-ray beam must penetrate and immobilizes the breast.  Sufficient compression of the breast during mammography, improves image quality and reduces radiation dose.

  3. What is the radiation dose from a mammogram?  What does it mean (radiation risk)?  How does it compare with other exams? Is the dose a concern?  What about if the patient is pregnant? Even though it may be a higher dose than other exams, why is it beneficial?

    The standard mammography exam usually includes four exposures; two to each breast.  With a well calibrated mammography unit, the breast tissue is the only tissue exposed to x-rays during a mammography exam.  The radiation dose from mammography is regulated by the FDA, and is evaluated routinely by a qualified medical physicist.  The dose to the breast tissue from mammography is approximately equivalent to a whole-body dose from annual background radiation.

    Even though the dose from mammography might seem high when compared to other x-ray exams, the patient's overall risk from a single mammography exam is quite low.

    Patients should also remember that a mammographic film produced with a radiation dose that is too low can be potentially harmful.  The image of the breast created with a very low dose will be inferior in image quality when compared to a film produced with an adequate radiation dose.  Adequate radiation doses are established by a medical physicist evaluation and a good quality assurance program.

    Thus, the great benefit of mammography in its ability to detect cancer early, far outweighs the radiation risk from the exam.

    It is safe to have a mammogram while pregnant.  The radiation dose received by the fetus is estimated to be extremely small - 60 times less than the dose received by the patient from one month of background radiation.

    Medical Physicist Evaluation of Radiation Dose

    The radiation dose from a mammogram is a direct consequence of the amount of x-rays that are absorbed in the breast tissue.  Medical physicists routinely evaluate radiation dose from mammography by exposing a mammography phantom (object that simulates breast tissue) to the x-ray beam under conditions identical to those encountered during patient exams.  This evaluation is performed to ensure that the dose is sufficient for patient exams while not exceeding regulatory limits.  Several factors affect the radiation dose, such as the type of film used, the amount of compression, the energy of the x-ray beam, and the thickness and composition of the breast.

    In summary, the diagnostic ability of mammography to detect breast cancer early significantly outweighs the radiation risk.  Radiation doses must continually be monitored by medical physicists to ensure they are appropriate, however, patients should not be discouraged from having mammograms due to the associated radiation risk.

  4. What is mammography quality assurance all about?

    Mammography quality assurance is a program established by the health care facility to ensure that mammography is being performed in compliance with minimum standards established by federal regulation.  These standards include interacting with patients, testing of imaging equipment, evaluation of clinical films, and qualifications of personnel.  The goal of the program is to maintain optimum image quality standards using an appropriate but not excess radiation dose.

    The three primary persons responsible for this program are the radiologist, mammography x-ray technologist, and medical physicist.

  5. Role of Medical Physicist regarding quality assurance responsibilities

    The medical physicist is responsible for evaluating equipment performance as it pertains to clinical exams.  The physicist's evaluation includes specific tests of the mammography unit, image quality assessment, consistency of exposures, and radiation dose measurement.  These tests are performed at least annually.  The physicist is also responsible for reviewing all equipment tests, and overall improvement in the quality assurance program.

  6. What are the other diagnostic exams (MRI, Ultrasound)?

    MRI and ultrasound are specialized diagnostic exams used to examine the breast tissue when a problem has been discovered with mammography.

  7. What is digital mammography?

    Digital mammography is a relatively new technology that makes use of digital detectors to capture the x-ray distribution pattern that has transmitted the breast.  The mammogram is performed the same way as with conventional units, i.e., x-rays expose the breast tissue.  However, a digital receptor is used instead of film to record the x-ray pattern.  This is similar to the way digital cameras create photos without the use of film.  With the use of specialized electronics, the x-ray pattern is then displayed as a black and white image on a computer screen.  Computer controls allow the radiologist to change the appearance of the image by adjusting the brightness, contrast, and magnification.  This image manipulation, not possible with film, may provide more information available for improved detection.

  8. What is an automated mammogram reader?

    Automated mammogram readers are computer controlled systems that perform the same task as the radiologist - search and identify abnormalities in the mammogram image.  It is thought that these devices, which act as a 'second set of eyes', may provide physicians another tool for improvement in diagnosing breast cancer.