Purpose: The frequency and significance of gene expression profile-derived molecular subtypes of breast cancers found in mammography screening are unknown. Experimental Design: We identified breast cancers diagnosed in women of any age living in defined geographic regions in Finland in 1991 to 1992 and collected clinical and pathologic data. Surrogates for the molecular subtypes were determined for 247 cancers found in organized mammography screening and 989 cancers detected outside of

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... using immunohistochemistry or in situ hybridization. Molecular subtypes were defined as luminal A [estrogen receptor (ER) positive and/or progesterone receptor (PR) positive, HER2-], luminal B (ER+ and/ or PR+, HER2+), basal-like (ER-, PR-, HER2-, cytokeratin 5+, and/or HER1+), HER2+/ER-(ER-, PR-, and HER2+), and unclassified. The median follow-up time was 9.4 years. Results: The luminal type A was common (73.7%) and the HER2+/ERtype is rare (5.7%) in screen-detected cancer, and only 16% were HER2 positive. Women with cancer diagnosed in screening at ages 50 to 69 years had similar molecular subtype distribution as women whose cancer was found outside of screening at age >69 years. In a multivariate model, cancer detection at screening independently predicted favorable distant disease-free survival when the molecular subtype was included as a covariate in addition to age, histologic grade, and cancer size. Women with small (pT 1 N 0 M 0 ) HER2-positive cancer had similar outcome regardless of the method of detection. Conclusions: Molecular subtype distribution of screen-detected breast cancer differs from that of cancers found outside of screening and accounts in part for the better outcome of screen-detected cancer.