AIMS: Invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) are predominant and well-documented types of invasive breast cancer (IBC). We investigated the clinical outcomes of other types of IBC (i.e. uncommon IBC), which collectively account for Σ20% of all IBC cases, as these are largely unknown.
METHODS AND RESULTS: We identified all IBC cases diagnosed in 2004-2006 (n = 159 293) and 2010-2011 (n = 118 822) from the Surveillance, Epidemiology and End Results (SEER) database. Uncommon IBCs included mixed IDC and ILC (MDLC), IDC mixed with other types of carcinoma, ILC mixed with other types of carcinoma, and other-type breast cancers (OCs). We estimated overall survival (OS) and cancer-specific survival in multivariate regression models. As compared with IDC, MDLC was associated with an increased OS [adjusted hazard ratio (aHR) = 0.92, P < 0.001 at Σ10 years of follow-up; aHR = 0.88, P = 0.01 at Σ4 years of follow-up], whereas OCs were associated with a decreased OS (aHR = 1.06, P = 0.005 at Σ10 years of follow-up; aHR = 1.23, P < 0.001 at Σ4 years of follow-up). Women with other uncommon IBCs had an OS similar to those with IDC. Heterogeneity in survival was observed for some subtypes of OC, with better OS for women with MDLC and tubular carcinoma. Radiotherapy extended OS for all types of IBC in older women (≥50 years). For younger women (<50 years), radiotherapy improved OS in women with IDC, but not in those with ILC or uncommon IBC. Radiotherapy did not change cancer-specific survival of younger women with any IBC.
CONCLUSIONS: Uncommon IBCs have distinct patterns of prognosis and survival. The effectiveness of radiotherapy in women with uncommon IBC may differ by age. The underlying mechanisms warrant further studies.