Background: Reports demonstrate improved survival of stage IV breast cancer patients with primary cancer resection. This may result from selection for surgery, rather than biological processes.
Methods: We performed matched-pair analysis that minimized potential bias in selecting surgery for primary cancer. Chart review was also performed of 5-year survivors to assess selection bias affecting breast surgery.
Results: 19,464 breast cancer patients were identified; 808 (4.2%) were stage IV: 622 were analyzed after eliminating wrong diagnoses or staging, and limiting patients to Massachusetts residents. Matched-pair analysis narrowed or eliminated apparent survival benefit associated with primary site surgery in several comparisons. When the impact of the sequence of systemic and surgical treatments was studied in stage IV patients, 90% 2-year survival occurred in patients receiving chemotherapy first, in contrast to receiving chemotherapy simultaneously with or after surgery, suggesting selection for delayed surgery after excellent response to initial chemotherapy. In bone metastases, the 2-year survival advantage occurred with chemotherapy before surgery; no difference in survival with or without surgery occurred when these treatments were simultaneous. Among 5-year survivors, frequency of primary site surgery after excellent response to systemic therapy, breast surgery in stage III patients incorrectly classified as stage IV, and frequency of oligo metastases all indicated selection bias.
Conclusions: Case selection bias in primary breast cancer resection in state IV patients may explain most, if not all, the apparent survival advantage of such surgery.