Clinical Practice Guidelines
QUALITY CONTROL OF MAMMOGRAPHY FOR BREAST CANCER SCREENING
Robert Clark, MD H. Lee Moffitt Cancer Center & Research
Institute
Introduction
Major randomized, controlled trials of breast cancer screening have shown that
screening with mammography reduces breast cancer mortality by approximately 30%.1,2
The most problematic characteristic of screening mammography is the lack of
standardization in its interpretation by radiologists.3 Interpretation of the
mammographic image is difficult, as much an art as a science, and variation exists in the
quality of interpretation. Guidelines have been developed to improve interpretation by
radiologists and to monitor the quality of interpretation with outcome audits.4-6
Outcome audits define the characteristics of the women examined, the interpretations
given, the recommendations made, and the follow-up of outcomes to determine the accuracy
of both the interpretations and the size of cancers detected. Intermediate or short-term
benchmark measures that correlate with mortality reduction have been derived from the
controlled screening trials.
Guidelines for quality assurance have evolved from these measures by which a screening
program can document its effectiveness. By performing regular, periodic outcome audits of
screening mammography practice and then comparing outcomes to the benchmarks of screening
trials, screening providers can ensure that their own practices conform to the standards
of the defined population trials.
Mammography Interpretation
The mammographic signs of early breast cancer are (1) tumor mass, usually irregularly
marginated or spiculated, (2) small, grouped (clustered) calcifications with or without a
mass, (3) poorly defined, asymmetric breast density, especially if this developed since a
prior examination, and (4) distortion of the breast parenchymal architecture by scirrhous
tumor.
The distinction between benign and malignant masses is made by analyses of the margins,
shape, density, and size of the detected lesions. Generally, a benign mass such as a cyst,
lymph node, and fibroadenoma has a sharply circumscribed margin, oval or round shape, and
density equal to or less than the surrounding parenchyma. Fat-containing masses that are
well circumscribed are always benign. Malignancies have a "benign" appearance in
less than 1% to 2% of cases. Papillary, medullary, and colloid carcinomas are more likely
to be well circumscribed than the more common ductal and lobular carcinomas.
Benign calcifications in the breast are usually more evenly scattered than clustered.
Benign secretory calcifications are typically thick, linear, smooth, or ring-like.
Calcifications in fibroadenomas are characteristic and coarse. Other benign types of
calcifications in-clude the ring calcifications of the skin, typical parallel linear
vascular calcifications, and the layered appearance of "milk of calcium" in
small cysts. Alternatives to biopsies may be considered for lesions with benign
characteristics.7-10 Mammographic surveillance at six-month intervals of a
nonpalpable lesion with a very low probability of malignancy can confirm benignity and
lack of growth, thus avoiding the cost and morbidity of biopsy. Stability of the size and
appearance of the lesion for two years is reasonable evidence of its benignity. Several
types of "probably benign" lesions are candidates for short-term surveillance:
(1) punctate calcifications that are clustered and round or oval, (2) a solid,
nonpalpable, noncalcified mass with a round or oval shape and sharply circumscribed
margins, (3) focal asymmetric tissue density with concave margins and/or interspersed with
fat, (4) a single dilated duct without mass, calcifications, or nipple discharge, (5) an
architectural distortion in an area of known biopsy, and (6) multiple low-risk lesions
that are similar in both breasts.
The most important and specific feature of malignant masses is a spiculated margin,
which is due to the infiltrative nature of the breast cancer. Irregularity and
indistinctness of margins are lesser manifestations of this phenomenon. Malignant
calcifications are clustered, are usually greater than five per cubic centimeters of
breast tissue, and may occur with or without a mass lesion. They typically have
pleomorphic sizes and shapes, often with irregular margins and branching configurations.
The presence of microcalcifications accounts for approximately half of all the biopsies
recommended for nonpalpable breast lesions, and their biopsy accounts for ap-proximately
half of all the nonpalpable cancers detected by screening. Of the microcalcifications that
are associated with cancer, approximately 50% are ductal carcinoma in situ (DCIS). Of all
DCIS cases, 75% to 90% demonstrate microcalcifications, alone or with a mass or density.
Thus, microcalcifications are an important sign of early breast cancer.
Diagnostic mammography, with specialized views tailored to the clinical
problem, is appropriate for a woman with a palpable mass. Breast ultrasound may be a
better primary imaging modality to detect suspected breast cysts; alternatively,
fine-needle aspiration of palpable abnormalities may be more useful than imaging. In any
case, for women of any age, a palpable ab-normality with a normal mammographic
interpretation should not be ignored. The main purpose of mammography is to detect breast
cancer prior to its clinical detection, when it is nonpalpable. For the evaluation of
palpable abnormalities, positive mammographic findings may confirm a diagnosis, but normal
mammographic findings neither confirm benignity nor exclude cancer.
(Please see hard copy for algorithm #1).
References
- Fletcher SW, Black W, Harris R, et al. Report of the International Workshop on Screening
for Breast Cancer. J Natl Cancer Inst. 1993;85:1644-1656.
- Kerlikowske K, Grady D, Rubin SM, et al. Efficacy of screening mammography: a
meta-analysis. JAMA. 1995;273:149-154.
- Elmore JG, Wells CK, Lee CH, et al. Variability in radiologistsí interpretations of
mammograms. N Engl J Med. 1994;331:1493- 1499.
- Spring DB, Kimbrell-Wilmot K. Evaluating the success of mammography at the local level:
how to conduct an audit of your practice. Radiol Clin North Am. 1987;25: 983-992.
- Murphy WA Jr, Destouet JM, Monsees BS. Professional quality assurance for mammography
screening programs. Radiology. 1990;175:319-320.
- Sickles EA. Quality assurance: how to audit your own mammography practice. Radiol
Clin North Am. 1992;30:265-275.
- Brenner RJ, Sickles EA. Acceptability of periodic follow-up as an alternative to biopsy
for mammographically detected lesions interpreted as probably benign. Radiology.
1989;171:645-646.
- Helvie MA, Pennes DR, Rebner M, et al. Mammographic follow-up of low-suspicion lesions:
compliance rate and diagnostic yield. Radiology. 1991;178:155-158.
- Sickles EA. Periodic mammographic follow-up of probably benign lesions: results in 3,184
consecutive cases. Radiology. 1991;179:463-468.
- Varas X, Leborgne F, Leborgne JH. Nonpalpable, probably benign lesions: role of
follow-up mammography. Radiology 1992;184:409-414.
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