Although the cure of breast cancer by "early detection" and prompt treatment rests on the belief that all breast cancers grow at the same rate, many cancers have been shown to grow rapidly and others slowly. In particular, mammography screening may often detect the slow-growing, nonaggressive tumors that might not be found until much later, if at all.
We reviewed the medical records of a natural cohort of 233 patients. The cohort comprised all women who received their first antineoplastic treatment for breast cancer at Yale–New Haven Hospital during the period from January 1 through December 31, 1988, and had a median follow-up thereafter of 82.4 months.
The mammography screen–detected group (MSDG) contained 97 (42%) of the 233 breast cancers. The rates of subsequent freedom from cancer deaths or recurrences were 95% (92 patients) in the MSDG and 79% (107 patients) in all other patients (log-rank 2P<.001). This superiority occurred partly because 90 (93%) of the MSDG were in the good prognosis TNM stages 0, I, and IIA, compared with 92 (68%) of the non-MSDG (χ2 2P=.001). Of the 31 patients with stage 0 (carcinoma in situ), all of whom had disease-free survival, 24 (77%) were found by mammography screening. Even within similar TNM stages, however, the MSDG had distinctly better disease-free survival results than the non-MSDG. For patients in TNM stages I and IIA, the "failure events" had respective rates of 2% and 13% (log-rank 2P=.02).
The results suggest that many of the breast cancers found by mammography screening have excellent prognosis not just because of early detection, but also because many of the cancers are relatively benign, requiring minimal therapy.