LAUSR

e13056 Background: Invasive lobular carcinoma (ILC) accounts for only 10-15% of all invasive breast cancers but has distinct clinicopathologic characteristics and genomic profiles. In particular, metastatic lobular cancers (mILC) have unique sites of metastasis and it is unclear if the response to initial endocrine therapy differs from metastatic ductal cancers (mIDC). Therefore we have undertaken a single-institution, retrospective analysis to compare overall outcomes and response to initial endocrine therapy (ET) in patients (pts) with metastatic ILC and IDC. Methods: An IRB approved retrospective review of medical records was conducted evaluating pts treated for metastatic IDC and ILC at The Ohio State University Comprehensive Cancer Center from January 1, 2004 to December 31, 2014. Pts diagnosed with mIDC were matched on age, year of diagnosis, estrogen receptor/progesterone receptor and HER2 status and site of metastasis 2:1 to patients diagnosed with mILC. Overall survival (OS) was defined as the time from metastasis to death or last known follow-up. Progression-free survival (PFS) was defined as time from metastasis to progression on first-line ET. Time to chemotherapy (TTC) was defined as time from starting ET for metastasis to initiation of chemotherapy. Kaplan Meier (KM) methods were used to calculate median OS, PFS and TTC. Results: A total of one hundred sixty one pts with metastatic breast cancer were included in this analysis. The demographic features between the two groups were well balanced and included in the table below. The median OS was 2.6 yrs (95% CI: 1.55, 3.22) for mILC and 2.2 yrs (95% CI: 1.95, 2.62) for mIDC. A subset of 111 patients who started on endocrine therapy were used in the PFS and TTC analyses. The median PFS following first-line ET was 2.2 yrs (95% CI: 0.1.0, 2.7) for mILC and 1.4 yrs (95% CI: 0.91, 1.90) for mIDC. Median TTC was 2.1 yrs (95% CI: 1.71, 4.92) for mILC and 2.4 yrs (95% CI: 1.90, 4.77) for mIDC. There was no statistically significant difference in outcomes between the two groups. Conclusions: Outcomes in patients with ILC and IDC have been varied, with several studies reporting that patients with ILC have worse outcomes and response to chemotherapy. Our retrospective study examining outcomes in mILC in comparison with mIDC showed no difference in OS. Given the concern of resistance to conventional therapies in patients with lobular cancers, it is reassuring to see that the median PFS on first line ET and TTC was similar to metastatic ductal cancers.[Table: see text]