Purpose: To assess the relative importance of 10 prognostic factors in pT1N0M0 breast cancer (≤2 cm in diameter, node negative).

Experimental design: Women diagnosed with breast cancer in Finland from 1991 to 1992 were identified from the files of the Finnish Cancer Registry, and individual clinicopathological data were collected from the hospital case records of women living in five regions comprising about one-half of the Finnish population. Of the women with minimum required information available (n = 2842), 852 had unilateral pT₁N₀M₀ cancer. The median follow-up time was 9.5 years, and only 5% had received systemic adjuvant therapy. Estrogen receptor (ER), progesterone receptor, erbB2, p53, and Ki-67 expression was determined from tumor tissue microarrays using immunohistochemistry, and the erbB2 (HER-2) amplification status was determined using chromogenic in situ hybridization.

Results: Primary tumor size ≤5 mm and histological grade 1 were associated with 100 and 95% (95% confidence interval, 92–98%) 9-year distant disease-free survival, respectively, whereas strong erbB2 expression or the presence of >20% Ki-67-positive cells was associated with >20% risk. ER and progesterone receptor values obtained from the hospital case records or tumor microarrays showed weaker association with outcome than the erbB2 status. Small (≤10 mm) erbB2-negative cancers were associated with >90% 9-year distant disease-free survival, irrespective of histological grade.

Conclusions: Prognosis of pT₁N₀M₀ breast cancer is generally well defined by the histological grade and primary tumor size. The erbB2 status was superior to ER as a prognostic factor in these tumors.