Interval carcinomas in the Malmö Mammographic Screening Trial: radiographic appearance and prognostic considerations

Interval carcinoma is the term used to describe malignant breast tumors that are detected in the intervals between mammographic screenings. These tumors are important because they contribute significantly to breast cancer mortality in the screened population.

Two radiologists retrospectively reviewed the mammograms of the 96 interval carcinomas (17% of all malignant neoplasms in the screened group) that were detected during the 10-year Malmö Mammographic Screening Trial in Malmö, Sweden (average time between screenings, 21 months), including one sarcoma, 75 invasive carcinomas, and 20 noninvasive carcinomas. We recorded the interval between screening and detection, and noted the tumor's appearance on the prior screening mammogram and at the time of diagnosis; these data were correlated with histologic tumor type and the patients' mortality. The doubling time for tumor volume of the invasive carcinomas was estimated.

Excluding the sarcoma, 72 carcinomas (75%) were detected within 18 months of screening. Retrospective review of the available preceding screening mammograms (94 cases) indicated that 10 tumors were missed (observer's error), 63 studies showed no tumor (true interval carcinomas), and 21 studies showed subtle signs of malignancy, mostly nonspecific densities or asymmetries (unrecognized sign). Of 66 invasive carcinomas in which doubling times for tumor volume could be calculated, 27 (41%) had doubling times of less than 100 days. At the end of the study, 20 of the 96 patients had died of breast cancer.

Interval carcinomas in this series were dominated by comedo, medullary, and mucinous carcinomas that often had a nonspecific appearance (when present) on prior screening mammograms. The interval carcinomas also contained a subset of rapidly growing tumors with a grave prognosis.