Initial versus subsequent screening mammography: comparison of findings and their prognostic significance.

OBJECTIVE: The goal of this study was to compare findings on initial and subsequent screening mammograms to determine the prognostic significance of screening-detected abnormalities. MATERIALS AND METHODS: All 3386 abnormal examinations from a 9-year mammographic screening program were studied. An initial examination was defined as one for which there were no prior films available for comparison (even if one or more prior examinations had been performed); the remainder were called subsequent examinations. The principal mammographic feature of each abnormality was recorded, as well as whether a biopsy was performed. For all screening-detected cancers, we also determined several surrogate markers of prognosis (tumor size, presence of axillary lymph node metastasis, and tumor stage). These various parameters were analyzed as a function of initial versus subsequent screening. RESULTS: The frequency of abnormal examinations was more than 2 times greater for initial examinations (7%) than for subsequent examinations (3%). Only minor differences were noted between initial and subsequent screenings when comparing the principal mammographic features of the abnormalities. However, the number of cancers found per number of biopsies performed was significantly greater (p = .02) for subsequent screenings (41%) than for initial screenings (32%). Among the 333 cancers detected, tumor size was significantly smaller for subsequent screenings (p = .0076). Node-negative status and early tumor stage (stage 0 or 1) also were found more frequently for subsequently screened cancers, but these differences were not statistically significant. CONCLUSION: Substantially fewer abnormal screening interpretations are made when mammography has been performed previously and when the prior films are available for comparison. This results in cost savings and reduced morbidity at subsequent screening (no further work-up, less patient anxiety, fewer benign biopsies). Surrogate markers of prognosis also appear to be more favorable for cancers detected at subsequent screening.