Is Invasive Lobular Carcinoma of the Breast Breast Cancer?
Yes, invasive lobular carcinoma (ILC) is definitively a form of breast cancer—specifically, it is the second most common histologic subtype of invasive breast cancer after invasive ductal carcinoma, accounting for 10-15% of all breast malignancies. 1, 2, 3
Classification as Breast Cancer
ILC is classified as an invasive breast carcinoma requiring full oncologic management including surgical excision, staging, and systemic therapy considerations. 4 The American College of Radiology, American College of Surgeons, College of American Pathologists, and Society of Surgical Oncology explicitly categorize invasive lobular carcinoma as a "special histologic type of invasive breast cancer" that must be specified in pathology reports and managed according to invasive breast cancer treatment protocols. 4
Key Distinguishing Features
ILC has unique biological and clinical characteristics that distinguish it from other breast cancers:
Loss of E-cadherin function drives the tumor's characteristic discohesive growth pattern, with cells arranged in single file and dispersed throughout the stroma rather than forming cohesive masses. 1, 5, 6
Typically luminal molecular subtype with estrogen and progesterone receptor positivity and HER2 negativity, making it hormone-responsive. 1, 5
Originates in the breast lobules and is more commonly bilateral compared to invasive ductal carcinoma. 1
Requires more accurate diagnostic imaging due to its diffuse growth pattern that can be challenging to detect on standard mammography. 1
Critical Distinction from LCIS
A common pitfall is confusing invasive lobular carcinoma with lobular carcinoma in situ (LCIS), which are managed completely differently. 7, 8 LCIS is not considered a malignant lesion requiring surgical excision but rather a marker of increased risk for subsequent breast cancer in both breasts. 4 In contrast, ILC is an invasive malignancy requiring definitive cancer treatment including surgical excision with negative margins, lymph node assessment, and consideration for adjuvant therapies. 4, 7
Management as Invasive Breast Cancer
ILC requires comprehensive oncologic management:
Surgical pathology reports must document tumor size, histologic type (specifying ILC), grade, margins of excision, lymph node status, and presence of angiolymphatic invasion—the same requirements as for any invasive breast carcinoma. 4
Patients with ILC are candidates for breast-conserving surgery and radiation if the tumor can be completely excised with negative margins and is not diffuse throughout the breast. 4
Systemic therapy decisions are based on hormone receptor status, HER2 status, tumor size, and nodal involvement, following standard invasive breast cancer treatment paradigms. 7, 2
Post-mastectomy radiation therapy is indicated for large tumors (>5 cm) regardless of nodal status, consistent with invasive breast cancer guidelines. 7
Clinical Recognition as Distinct Entity
While ILC is treated using similar paradigms as invasive ductal carcinoma, recent research increasingly recognizes it as a distinct clinical entity with unique molecular features. 2, 3, 5 Studies show ILC and invasive ductal carcinoma have different genomic landscapes, clinical behaviors, and potentially different responses to neoadjuvant therapies, though both remain forms of invasive breast cancer requiring oncologic treatment. 1, 3, 6