86 Background: Sentinel lymph node biopsy (SLNB) is not routinely recommended for women diagnosed with ductal carcinoma in situ (DCIS); however, when the pathologist cannot rule out microinvasion on core biopsy (CB), the surgeon must decide whether to perform a SLNB at the time of surgical excision. Up to 10% of patients with T1mi are found to have breast cancer cells in the axillary lymph nodes, but less than 2% contain macrometastasis making the utility of SLNB in this setting unclear. Methods: The University of Chicago pathology database was queried for patients whose CB showed DCIS suspicious for microinvasion (Smic) or definite microinvasion (Mic) from 2000 to 2012. We analyzed histology, imaging, nodal status, core needle size, and the use of myoepithelial immunohistochemistry (IHC) markers to identify microinvasion. Results: We identified 60 women with Smic and 19 women with Mic on CB. In the Smic group, 33% had infiltrating ductal carcinoma (IDC) in the surgical specimen (35, 30, 30, 1 and 0% for T1mi, T1a, T1b, T1c, and T2 respectively compared to 58, 21, 0, 10.5 and 10.5% in the Mic group). A SLNB was performed at the initial surgery in 38/60 (63%) of Smic and in all Mic biopsies; they were positive in 3/42 (7%) and 2/19 (11%) respectively (p=0.64). When N1mic was excluded, the incidence of macroscopic nodal disease was 1/42 (2.4%) and 1/19 (5.3%) p=0.53. Of those with Smic, we observed a higher proportion of IDC associated with a lesion size ≥ 14 mm on imaging (75% versus 15% for < 14 mm, p<0.02); smaller CB needle size (69% with 11, 12 and 14 gauge versus 0% with 9 gauge, p<0.01); and the use of IHC on the CB to identify microinvasion (58% versus 22% without IHC, p<0.01). There was no association to IDC with respect to grade, necrosis, mass on imaging, or biopsy guidance. Conclusions: The incidence of clinically positive nodal disease in patients with Smic on CB is extremely low. However, those with tumors ≥ 14 mm, the use of smaller gauge CB needles, and Smic based on myoepithelial IHC may be at higher risk for IDC. Of the Smic group with DCIS after excision, none harbored nodal disease; therefore, the use of SLNB only after a definitive diagnosis of IDC may prevent overtreatment.