Introduction/Background Breast conserving surgery(BCS) has become a treatment standard for patients with early-stage breast cancer. One of the key tenets of BCS is achieving clear margins, as positive margins are associated with a high rate of local recurrence.we aim to investigate the predictive factors of residual disease(RD) in mastectomy after BCS with positive margins to carcinoma in situ(CIS)

Methodology We retrospectively reviewed the clinical record of 47 patients diagnosed with stage I-II breast cancer who underwent BCS with margins containing foci of CIS (2007–2020) in Salah Azaiez Institute

Results the patients‘ mean age was 53.54 years old presenting with a stage I and II breast cancer in respectively 18.8% and 79.2% of cases.

All the patients underwent a BCS associated with axillary lymph node dissection in 89.36% of cases.

In the final pathological report, most of tumors(95.7%) were unifocal invasive breast carcinoma of non-specific type, of histological grade II(53.2%) with a mean size of 21mm .CIS was associated in 68% of cases and lymph node involvement was noted in 44.7% of cases.

All the patients had positive margins containing foci of CIS with micro-infiltration noted in only 14.9% of cases.

A completion mastectomy(CM) was performed for all the patients. RD was stated in 27.7% of patients. Of them, invasive carcinoma was noted in 61.53%.

We studied the impact of age, initial tumor size, disease stage, histological grade, presence of lympho-vascular invasion and presence of CIS with micro-infiltration in the margins on the presence of residual disease in the CM but no statistical significance was found.

Conclusion This study failed to identify predictive factors of the presence of RD in the CM after BCS with positive margins to CIS.

Further studies including a bigger population are needed to identify the burden of the additional surgery, disease outcome and help guide the surgeon in the decision of CM.

Disclosures the authors have nothing to disclose.