Findings at mammographic screening on only one standard projection: outcomes analysis.

PURPOSE: To determine the radiographic and clinical outcomes of findings seen at mammographic screening on only one standard projection.
MATERIALS AND METHODS: To identify prospectively marked benign-appearing and abnormal findings that were seen on only one standard projection, 63,836 consecutive two-view mammographic screening studies were reviewed. Subsequent outcomes analysis included determination of the frequency of occurrence, mammographic features, work-up performed, and final imaging assessment. For imaging findings that prompted tissue sampling, histopathologic diagnosis was recorded. To identify breast cancers among the remaining findings, screening cases were linked with a regional tumor registry.
RESULTS: Of the 61,273 screening studies available for review, 2,023 (3.3%) involved prospectively identified findings seen on only one standard projection. One thousand eighty-six (53.7%) studies with one-view-only findings were judged to represent superimposition of normal breast structures (summation artifact) simply from the standard projections obtained at screening; findings in an additional 587 (29.0%) studies were characterized as representing superimposition of normal structures after recall for further diagnostic imaging. None of these 1,673 cases was subsequently found to be cancer. Cancers were identified in 36 one-view-only studies; six involved ductal carcinoma in situ; 18, invasive ductal carcinoma; and 12, invasive lobular carcinoma (a large percentage [33%], since only 10% of all cancers are invasive lobular carcinoma).
CONCLUSION: Findings seen on only one standard projection are common among lesions identified at mammographic screening. More than 80% can be correctly assessed as representing superimposition of normal structures, either without or with the aid of additional imaging studies. Among those findings that truly are cancer, a disproportionately high percentage are invasive lobular carcinoma.