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1
Department of Radiology, Box 1667, University of California Medical Center,
San Francisco, CA 94143-1667.
2
Present address: Department of Radiology, Box 357115, University of Washington
Medical Center, 1959 N.E. Pacific St., Seattle, WA 98195.
Received July 3, 2000;
accepted after revision September 12, 2000.
Presented in part at the annual meeting of the American Roentgen Ray
Society, Washington, DC, May, 2000.
Abstract
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MATERIALS AND METHODS. We analyzed 46,857 consecutive mammography examinations (10,007 diagnostic, 36,850 screening) from 1997 to 2000, including data on demographics, image interpretation, and biopsy (including size, nodal status, and cancer stage).
RESULTS. The mean age at diagnostic mammography was 55.8 years (mean age at screening mammogram, 59.1 years; p < 0.0001). Among patients who underwent diagnostic examinations, 14.7% had a strong or very strong family history of breast cancer (screening, 11.6%; p < 0.0001). Examination findings were interpreted as abnormal in 14.4% (screening, 5.2%; p < 0.0001). Biopsy was performed in 11.9% (screening, 1.4%; p < 0.0001). Forty-six percent of the biopsies were positive for malignancy (screening, 38%; p < 0.0001). The cancer detection rate was 55 per 1000 (screening, 5/1000; p < 0.0001). Of cancers found, 74.4% were stage 0 or I (screening, 89.3%; p < 0.0001), average size was 18.0 mm (screening, 12.9 mm; p < 0.0001), and axillary nodes were positive for malignancy in 19.9% of invasive cancers (screening, 6.3; p < 0.0001). Differences between diagnostic and screening outcomes were attributable predominantly to the subgroup of diagnostic examinations performed for evaluation of palpable masses.
CONCLUSION. Medical auditing of diagnostic mammography examinations yields substantially different results compared with those of screening examinations, including different patient demographics; higher number of positive biopsies; higher cancer detection rates; and larger, more advanced-stage cancers. Diagnostic and screening data should be segregated during auditing, or if this is not possible, analysis of combined results should be based on known differences between diagnostic and screening outcomes.
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The typical mammography practice includes a mix of screening and diagnostic examinations, all of which are subject to auditing requirements [1]. Diagnostic mammography is performed for a variety of problem-solving indications, including workup of screening examinations with abnormal findings, evaluation of abnormalities found on clinical examination, and short-term follow-up examinations for probably benign lesions (Breast Imaging Reporting and Data System [BI-RADS] category 3) and for patients with cancer who have been recently treated with breast preservation. Other special breast problems, such as the presence of implants or the evaluation of extent of disease of a known malignancy also may represent indications for diagnostic mammography. Because diagnostic examinations involve different patient populations from that of screening mammography, one might expect the clinical outcomes as measured by a medical audit to be different for diagnostic versus screening examinations. However, to our knowledge, only one audit has included data on diagnostic mammography, and that series involved only about 1000 examinations [8]. Thus, there is a paucity of diagnostic mammography outcomes data to use as a benchmark either for assessing the performance of a given practice or for deriving more general performance goals.
In this study, we present detailed audit data involving slightly more than 10,000 consecutive examinations from our diagnostic mammography practice, in comparison with concurrently obtained screening data. We also separately analyze our diagnostic mammography outcomes by major clinical indication to provide insight into the mechanisms by which diagnostic outcomes differ from those encountered at screening.
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Data for all mammography examinations were stored in a computerized database designed at our institution and patterned on principles described previously [17]. The radiologist recorded the indication for each examination during image interpretation, according to the classification scheme listed in the Appendix.
The radiologist also recorded standard BI-RADS assessment categories separately for each breast [16]. Screening examination findings were considered to be abnormal if either breast was assessed as BI-RADS category 0 (incomplete: need additional imaging), category 4 (suspicious), or category 5 (highly suggestive of malignancy). Diagnostic examination findings were considered to be abnormal if either breast was assessed as BI-RADS category 4 or category 5.
For all examination findings interpreted as abnormal, we searched the pathology database at our institution to determine whether biopsy (fine-needle aspiration, core, or surgical biopsy) was performed, and for those cases not identified in this manner, obtained the information directly from the referring clinician [17]. For biopsies resulting in the diagnosis of malignancy (defined herein as either ductal carcinoma in situ or any invasive carcinoma), we also recorded the size, grade, nodal status, and stage, which was based on the American Joint Committee on Cancer staging system [18].
Data tabulations and statistical computations were accomplished using the S-plus programming software (Mathsoft, Seattle, WA). The Student's t test was performed on normal distributions and the chi-square test was performed for comparison of proportional data. A p value of less than 0.05 was considered statistically significant.
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Mammography Interpretation
Results of mammography interpretation are shown in
Table 3. Diagnostic examination
findings were interpreted as abnormal (biopsy recommended) in 14.4% of cases.
This rate is almost three times that observed for screening interpretations
(5.2%) (p < 0.0001), despite the fact that screening examinations
were considered to be abnormal not only if biopsy was recommended, but also if
additional (diagnostic) imaging was requested. It is noteworthy that the
abnormal interpretation rate among diagnostic cases varied greatly according
to the indication for examination. For example, shortterm interval follow-up
examination findings were interpreted as abnormal in only 94 cases (3.6%),
compared with 286 cases (20.6%) interpreted as abnormal among patients with
palpable masses, and 689 cases (34.0%) interpreted as abnormal among patients
having additional workup of abnormal findings on screening examinations.
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Biopsy
As shown in Table 3, the
rate at which biopsy was performed was almost nine times greater for
diagnostic mammography (11.9%) than for screening mammography (1.4%)
(p < 0.0001). The pathology results of all biopsies are summarized
in Table 4. Malignancy was
found in 46% of biopsies for diagnostic mammography, more than the 38%
positive biopsies for screening mammography (p = 0.0027). As also
shown in Table 3, this
translates to a cancer detection rate of 55 per 1000 cases for diagnostic
mammography versus only five per 1000 cases for screening mammography
(p < 0.0001).
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Table 4 shows that the positive biopsy rate for diagnostic mammography varied greatly according to the indication for examination. The positive biopsy rates for patients with cancer having 6-month follow-up after breast preservation surgery (74%) and for patients with palpable masses (63%) were much greater than those for examinations performed for workup of abnormal findings on screening examinations (38%) (p < 0.0001). However, it is not surprising that the 38% positive biopsy rate for patients having workup of abnormal findings on screening examinations was almost identical to the rate observed for our screening population.
Characteristics of Breast Cancers
Tables 5 and
6 summarize the data on tumor
size. The mean size of cancers identified on diagnostic mammography was 18.0
mm, considerably larger than the 12.9-mm mean size of cancers detected at
screening (p < 0.0001). This difference was found to be
statistically significant for invasive tumors (p = 0.0003) and for
intraductal tumors (p = 0.046). Cancers found in patients with
palpable masses were even larger (mean, 26.4 mm), approximately twice the size
of cancers found on screening (p < 0.0001). However, it is
important to note that the size of cancers identified on diagnostic
mammography performed for short-interval follow-up and for workup of abnormal
findings on screening examinations (Table
6) was very similar to that observed for screening-detected
cancers (Table 5).
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There were no cases in our database in which ductal carcinoma in situ was associated with axillary nodal metastasis. Table 7 summarizes the data on nodal status for the invasive cancers. Axillary nodes were positive for metastasis in 20% of invasive cancers found on diagnostic mammography, a rate more than three times greater than the 6% rate observed for screening-detected cancers (p < 0.0001). The rate of nodal metastasis was even greater (33%) for patients with palpable invasive cancers (p < 0.0001). However, the nodal metastasis rate for invasive cancers identified on diagnostic mammography performed for short-interval follow-up and for workup of abnormal findings on screening examinations was very similar to that observed for screening-detected cancers.
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We observed no significant difference between the grade of tumors found at screening versus that of diagnostic mammography, either for ductal carcinoma in situ or for invasive cancers. On diagnostic mammography, 46.9% of ductal carcinoma in situ cases were found to be high grade (52.1% at screening) and 24.1% of invasive tumors were classified as high grade (19.0% at screening).
Table 8 summarizes the data on cancer stage. Of cancers found on diagnostic mammography, 74.4% were stage 0 (ductal carcinoma in situ) or stage I, a considerably smaller percentage than the 89.3% observed for screening-detected cancers (p < 0.0001). Only 46.2% of cancers in patients with palpable masses were stage 0 or stage I (p < 0.0001). However, paralleling previously described observations for cancer size and axillary node status, the stage distribution for cancers identified on diagnostic mammography performed for short-interval follow-up and for workup of abnormal findings on screening examinations was very similar to that observed for screening-detected cancers (i.e., approximately 90% of cancers were stage 0 or stage I in these groups of patients).
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The demographics of patients undergoing diagnostic mammography appear to be somewhat different from those of screened women. Diagnostic patients are, on average, several years younger. This is due, at least in part, to the more widespread use of diagnostic mammography among patients in their 20s and 30s, many of whom have palpable masses. Diagnostic mammography patients also are slightly more likely to have a strong or very strong family history of breast cancer.
The prevalence of cancer among diagnostic mammography patients is much greater than that in screened women. In our series, malignancy was found in 55 per 1000 diagnostic mammography examinations, 11 times more than the five per 1000 cancer detection rate observed in our screening population. We also found that the cancers identified at diagnostic mammography are larger (both for ductal carcinoma in situ and invasive carcinoma), more likely to be associated with axillary nodal metastasis, and more advanced in stage than those detected at screening.
There are a variety of clinical indications for performing diagnostic mammography. One common reason is to work up screening-detected abnormalities. This subset of diagnostic patients differs from the overall screening population only in that mammographic abnormalities are present in all cases, thereby increasing the likelihood of biopsy and cancer diagnosis. However, the size, axillary node status, and cancer stage in this group of diagnostic patients, as expected, are observed to be virtually identical to those found in screening mammography populations.
Another indication for diagnostic mammography examination is short-interval follow-up, either for mammographic lesions previously assessed as probably benign (BI-RADS category 3) or for more-frequent-than-annual surveillance of patients with cancer treated with breast preservation surgery. Our results reinforce the findings of previous reports that cancer is identified very infrequently in the follow-up of probably benign lesions, and that those few cancers in this group of patients are just as favorable in prognosis as those detected at screening [19,20,21,22]. To our knowledge, little has been reported on the detailed clinical outcomes of mammography among patients with cancer treated with breast preservation surgery [23, 24]. It is reassuring that cancers identified by mammographic surveillance in this group of patients have as favorable a prognosis as screening-detected cancers, just as it is readily understandable that cancer is found more frequently in this high-risk group of individuals (18 per 1000 cases in our series) than in screened women, and that the positive biopsy rate is almost twice as great (74% in our series) as that observed in screened women.
The other important indication for diagnostic mammography is the presence of clinical symptoms or signs that suggest the possibility of breast cancer. Our results reveal striking differences in all clinical outcomes between the subgroup of patients who have palpable breast masses versus asymptomatic women undergoing screening mammography. The positive biopsy rate for the palpable-mass subgroup (63%) is much greater, the frequency with which cancer is found (184/1385, 133 per 1000 cases) is much greater, tumor size is much larger (mean size, 26.4 mm), the nodepositivity rate for invasive cancers is much greater (33%), and the stage is more advanced (53.8% stage 2 or higher cancers) than those of patients with screening-detected abnormalities. Similar results have been observed in smaller series of patients undergoing mammography for palpable masses [25,26,27].
There are many other indications for which diagnostic mammography is performed, the spectrum ranging from indications similar to screening (patients with breast implants or breast pain), to patients in whom carcinoma is a certainty (patients with a known nonpalpable breast malignancy being evaluated for the extent of disease or response to neoadjuvant chemotherapy). These cases are grouped together in our database as "other" breast problems because we did not query for these specific indications prospectively. Because of the varied mix of indications in our "other" breast problems group, it is not surprising that our observed outcomes for this group are intermediate between those for screening examinations and those for diagnostic examinations performed to evaluate a palpable mass.
Because the observed clinical outcomes for diagnostic mammography are different from those found for screening mammography, we believe that it is important to analyze the audit data from diagnostic mammography examinations separately from screening data. For example, the combined audit of a mammography practice involving 90% screening and 10% diagnostic examinations can be expected to show a cancer detection rate twice as great as that of a screening-only patient population, thus confounding the interpretation of audit results. Indeed, to derive the most meaningful assessment of results, it would be helpful to analyze diagnostic mammography audit data separately according to indication for examination.
However, for logistic reasons, many mammography practices do not have the ability to perform audits in the segregated manner that we recommend. Rather, they are able to perform only a combined audit of all mammography examinations. If such a practice can derive a reliable estimate of the relative mix of its diagnostic versus screening examinations (or at a minimum, the percentage of patients with palpable masses at the time of examination), they should find the clinical outcomes reported in our study instructive in understanding (by extrapolation) the combined outcomes that they are able to measure. We plan to produce, in a separate report, a primer on how to make such extrapolations, using a wide range of diagnostic versus screening mammography mixes.
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