Carcinoma of the breast is a histologically heterogeneous disease. Invasive lobular carcinoma (ILC) accounts for 8-14% of all breast cancers [1,2]. Data from a recent epidemiologic study [3] indicate that for unknown causes the incidence of this type of breast cancer is increasing, especially among postmenopausal women.The morphologic features of lobular carcinoma differ from those of ductal carcinoma. ILC is characterized by small, round cells that are bland in appearance and have scant cytoplasm, which infiltrate the stroma in single file and surround benign breast tissues in a targeted manner [1,4]. Infiltration typically does not destroy anatomic structures or incite a substantial connective tissue response. By virtue of their distinctive growth pattern and biology, lobular carcinomas often fail to form distinct masses that can easily be diagnosed by palpation or mammography. This can make early diagnosis challenging [5,6] and breast conservation approaches more difficult. Lobular carcinomas may have a substantially increased propensity for multifocal and multicentric distribution and for bilaterality [5,[7][8][9][10][11]. Metastatic spread with an uncommon pattern of involvement has been reported [12,13]. CNS = central nervous system; DFS = disease-free survival; EGFR = epidermal growth factor receptor; ER = estrogen receptor; IDC = infiltrating ductal carcinoma; ILC = infiltrating lobular carcinoma; OS = overall survival; PgR = progesterone receptor. AbstractIntroduction: Invasive lobular carcinoma (ILC) comprises approximately 10% of breast cancers and appears to have a distinct biology. Because it is less common than infiltrating ductal carcinoma (IDC), few data have been reported that address the biologic features of ILC in the context of their clinical outcome. In the present study we undertook an extensive comparison of ILC and IDC using a large database to provide a more complete and reliable assessment of their biologic phenotypes and clinical behaviors.

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