Abstract Background: The need for surgical resection of brest lesions of uncertain malignant potential diagnosed at core biopsy is not well established. The aim of our study was to determine the incidence of these lesions on core biopsy and to find the rate of upgrade at surgery and during follow-up.Material and Methods :All breast biopsies at Maisonneuve-Rosemont hospital were prospectively collected from November 1999 to December 2008. We reviewed the records of all patients diagnosed with lobular carcinoma in situ (LCIS), atypical lobular neoplasia (ALH), atypical ductal hyperplasia (ADH) and radial scar during that period. Patients were excluded if there was also presence of invasive carcinoma or ductal carcinoma in situ on the biopsy. An upgrade was defined as the presence of invasive carcinoma or ductal carcinoma in situ on the surgical specimen.Results: During that period, 9325 breast biopsies were done. The women were aged from 37 to 89 years with a mean age of 56 years and were followed for an average of 27 months .There were 20 patients with LCIS (0.2%), 36 ALH (0.4%), 60 radial scars (0.6%) and 110 ADH (1.2%). For lobular neoplasia, 85% of LCIS patients had surgical resection with an upgrade rate of 29% (5/17). On follow-up no patients developed a breast cancer at the site biopsied. 28% of ALH had surgical resection with only one patient presenting an upgrade. For radial scars 58% had surgical resection with an upgrade rate of 3/35 (8%). In all three patients with an upgrade, the biopsy was done on an hypoechogenic lesion. No patients with a radial scar visible only on mammogram presented an upgrade and no patients on follow-up developed a cancer at the site of biopsy. For patients with ADH, 48% had a surgical excision (53/110) with an upgrade rate of 18.9% (9/53). Nine patients of the 49 who did not undergo a surgical excision developed a cancer at the site of biopsy during follow-up (18%). Also, during follow-up, 51% of the patients with ADH who did not have surgery had to be rebiosied around the site of the previous biopsy.Conclusion: The important rate of upgrade in LCIS and ADH mandates a surgical excision when these lesions are found on biopsy results. Also, surgical excision for women with ADH might spare the stress of repeat investigation and biopsy. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4012.