Outcomes of surveillance mammography after treatment of primary breast cancer: a population-based case series

Abstract

To ascertain outcomes of surveillance mammography (SM) following treatment of early stage unilateral primary breast cancer (PBC) in a population based case series. Random samples from all 12,279 women having breast surgery within 4 months after diagnosis of PBC, between July 1991 and December 1993 in Ontario, were drawn from a database created by deterministic linkage of PBC files from the Ontario Cancer Registry (OCR) with episodes of breast surgery extracted from the hospital Discharge Abstract Database (DAD), and mammography from the Ontario physician billings database (OHIP). Among women having >or=1 episode(s) of breast surgery subsequent (SBS) to the date of diagnosis up to December 2000, a sample of 1,200/5,064 (23.7%) was drawn, and among women with no SBS, a sample of 400/7,215 (5.5%). Among these two samples, operative, pathology, and mammography reports were abstracted from original charts. Treatments were abstracted and categorized. Women with complete data for Stages 1 and 2 unilateral PBC were included. From the subsequent surgery sample, 609/1,200 (50.8%) were excluded because of simultaneous or sequential bilateral breast cancers or mastectomies within 6 months, missing stage information, Stage 3 or 4 PBC, or missing primary charts. From the no subsequent surgery sample, 90/400 (22.5%) were excluded by the same criteria. Episodes of bilateral 2-view X-ray mammography, beginning >or=6 months after the diagnosis of unilateral PBC, and if multiple, at least 11 months apart, and not prompted by a clinical concern or symptom, were classified as SM. We confirmed episodes of cancer recurrence within the ipsilateral conserved breast (CRICB) and metachronous contralateral primary breast cancer (CPBC) >or=6 months after the diagnosis of the unilateral PBC from original operative and pathology reports. We used Cox models to describe the association of exposure to >or=1 episode(s) of SM with the risk of death from breast cancer among the study population, and separately among women experiencing CRICB or CPBC. Eligible women comprising 591/1,200 and 310/400 produced a combined case series of 901/1,600 (56.3%). Women with >or=1 episode(s) of SM numbered 721/901 (80.0%). We confirmed 84 CRICB events among 584 women initially treated by lumpectomy (14.4%), and 49 CPBC events among all 901 women in the study population (5.4%). Among women having >or=1 episode(s), the 25th percentile of observed follow up was 1,631 days, the 50th, 4,287 days, and the 75th 5,011 days. Among women without any SM, the 25th percentile of observed follow-up was 440 days, the 50th, 891 days, and the 75th, 1,849 days. Hazard ratio (HR) for death due to breast cancer associated with >or=1 episode of SM was 0.28 (95% CI 0.22-0.37), adjusted for age, stage, type of surgery, adjuvant chemotherapy, and tamoxifen. Among 84/584 women with CRICB, unadjusted HR = 0.36 (95%CI 0.13, 1.00) and among 49/901 women with CPBC, unadjusted HR = 0.86 (0.20-3.77). SM was associated with a significant reduction in the hazard for breast cancer death. Among women who experienced CRICB, the reduction was of borderline significance, and the reduction was not significant among women who experienced CPBC.