ObjectiveTo determine whether infiltrating lobular carcinoma (ILC) is associated with high positive-margin rates for single-stage lumpectomy procedures, and to define clinical, mammographic, or histologic characteristics of ILC that might influence the positive-margin rate, thereby affecting treatment decisions. Summary Background DataInfiltrating lobular cancer represents approximately 10% of all invasive breast carcinomas and is often poorly defined on gross examination. MethodsA group of 47 patients with biopsy-proven ILC undergoing breast-conservation therapy (BCT) at the University of Virginia Health Sciences Center between 1975 and 1999 was compared with a group of 150 patients with infiltrating ductal cancer undergoing BCT during the same time period. The pathology of the lumpectomy specimen was reviewed for each patient to confirm surgical margin status. Office and surgical notes as well as mammography reports were examined to determine whether the lesions were deemed palpable before and during surgery. Patients were stratified according to age, family history, tumor size, tumor location, and histologic features of the tumor. ResultsThe incidence of positive margins was greater in the ILC group compared with the infiltrating ductal cancer group. Patient age, family history, and preoperative palpability of the tumor did not correlate with surgical margin status. Of the mammographic features identified, including spiculated mass, calcifications, architectural distortion, and other densities, only architectural distortion predicted positive surgical margin status. Tumor grade, tumor size, lymph node status, and receptor status were not predictive of surgical margin status. ConclusionsFor patients with ILC, BCT is feasible, but these patients are at high risk of tumor-positive resection margins (51% incidence) after the initial resection. Only the mammographic finding of architectural distortion was identified as a preoperative marker reliably identifying a subgroup of ILC patients at especially high risk for a positive surgical margin. For all patients with ILC considering BCT, careful counseling about the potential need for a second procedure to treat the positive margin should be included in the treatment discussion.Infiltrating lobular cancer (ILC) was first described in 1865 by Cornil 1 as a diffusely infiltrative tumor composed of small, round, and regular cells that form single lines throughout a desmoplastic stroma. In 1946, Foote and Stewart 2 developed the criteria now accepted for the diagnosis of classic ILC. Using their strict definition, ILC accounted for approximately 3% to 5% of all breast cancers. In the 1970s, solid, alveolar, mixed, and pleomorphic variants of ILC were described and the definition was broadened to include these subtypes. [3][4][5] This modern, more broadly accepted ILC category currently represents approximately 10% of all breast cancers. 4 During the past decade, attention has focused on comparing the treatment strategies used for ILC