Also known as: Invasive Lobular Carcinoma
Infiltrating lobular carcinoma (ILC) — now more commonly referred to as invasive lobular carcinoma in current clinical practice — is the second most common type of invasive breast cancer after invasive ductal carcinoma. It begins in the milk-producing lobules of the breast and breaks through the lobule walls to invade surrounding breast tissue, with the potential to spread to lymph nodes and other parts of the body.
How Common Is It?
ILC accounts for approximately 10–15% of all invasive breast cancers — a figure that has been revised upward from earlier estimates of 5–8% as pathological classification and diagnostic imaging have improved. It is diagnosed most frequently in women between the ages of 45 and 65, with a peak incidence slightly later in life than invasive ductal carcinoma. It is more commonly diagnosed in postmenopausal women, and its incidence increased significantly during the 1980s and 1990s alongside widespread use of combined hormone replacement therapy (HRT) — a connection that has since been supported by epidemiological data. Following the decline in HRT use after 2002, ILC incidence rates stabilized.
ILC can occur in men, though this is rare.
How ILC Grows — and Why It Is Uniquely Challenging to Detect
Unlike most other breast cancers, which tend to form a distinct, palpable lump with defined borders, ILC grows in a characteristic single-file or linear pattern — cancer cells spread individually through the breast tissue in thin strands, weaving between normal cells rather than displacing them. This growth pattern is a direct consequence of the loss of a protein called E-cadherin, which normally holds cells together. In ILC, the gene encoding E-cadherin (CDH1) is typically inactivated, causing cells to lose cohesion and spread diffusely.
This diffuse infiltration creates several diagnostic challenges:
- Mammographic invisibility: ILC frequently does not produce the calcifications or well-defined mass that standard mammography is optimized to detect. Studies estimate that standard mammography misses ILC in up to 19–30% of cases, compared to much lower miss rates for invasive ductal carcinoma. Rather than a defined lump, ILC may appear on imaging — if visible at all — as a subtle area of architectural distortion or asymmetry.
- Clinical examination findings: Rather than a discrete hard lump, ILC most commonly presents as a vague thickening, fullness, or firmness in the breast tissue, most often in the upper outer quadrant (the area between the nipple and the underarm). Some women describe it as feeling like a subtle change in breast texture rather than a defined mass. This can easily be dismissed or go unnoticed for an extended period.
- Later-stage diagnosis: Partly because of its subtle presentation, ILC is more frequently diagnosed at a larger tumor size or later stage compared to invasive ductal carcinoma, despite similar or better long-term outcomes with appropriate treatment.
Late-presenting signs of a larger tumor area may include:
- Skin retraction or puckering over the affected area
- Nipple inversion or retraction
- A visible change in breast contour or shape
- Skin dimpling in the overlying breast skin
- Fullness or heaviness in one breast compared to the other
Imaging for ILC — Current Recommendations
Because standard mammography has documented limitations in detecting ILC, current guidelines and evidence support supplemental imaging in many cases:
- Breast MRI is the most sensitive imaging tool for ILC and is significantly better at detecting the extent of disease, multifocal involvement, and contralateral (opposite breast) disease. MRI detects ILC with a sensitivity of approximately 93% compared to approximately 79% for mammography. Current NCCN and ACR guidelines recommend MRI for staging newly diagnosed ILC, particularly when the extent of disease is uncertain or when surgery planning is underway.
- Ultrasound is a useful adjunct, particularly for evaluating palpable areas of concern and guiding biopsy, though it has similar limitations to mammography in detecting the full extent of ILC.
- 3D Mammography (Digital Breast Tomosynthesis) improves detection of ILC compared to standard 2D mammography and is now the standard of care at most imaging centers, though MRI remains superior for ILC specifically.
- Contrast-Enhanced Mammography (CEM) is an emerging option that may improve ILC detection at lower cost than MRI.
If you have been diagnosed with ILC, ask your health care team whether breast MRI has been performed or recommended as part of your staging workup.
Bilateral and Multifocal Disease
ILC has a significantly higher tendency than other breast cancer subtypes to involve both breasts and to be present in multiple locations within the same breast:
- Contralateral (opposite breast) involvement: ILC is found in the opposite breast in approximately 20–30% of cases — a substantially higher rate than invasive ductal carcinoma. This can occur simultaneously (synchronous bilateral ILC) or develop years later (metachronous bilateral ILC). Thorough imaging of both breasts at diagnosis is essential for this reason.
- Multifocal disease (multiple tumors in the same breast): ILC is frequently multicentric — present in more than one quadrant of the same breast — which has significant implications for surgical planning. Women with ILC may have a larger area of involved tissue than imaging suggests, and this can affect decisions about lumpectomy versus mastectomy.
The tendency for bilaterality and multicentricity means that long-term surveillance of both breasts is particularly important for women diagnosed with ILC.
Hormone Receptor Status and Molecular Profile
ILC has a distinctive molecular profile that differs from invasive ductal carcinoma and has important treatment implications:
- Hormone receptor positivity: The vast majority of ILC tumors — approximately 90–95% — are estrogen receptor-positive (ER+) and/or progesterone receptor-positive (PR+). This is higher than the already-elevated rate seen in invasive ductal carcinoma, and it means that most ILC tumors are strongly responsive to hormone-blocking (endocrine) therapies such as tamoxifen, aromatase inhibitors (anastrozole, letrozole, exemestane), and CDK4/6 inhibitors in the metastatic setting.
- HER2 status: ILC is HER2-negative in approximately 90–95% of cases, meaning HER2-targeted therapies such as trastuzumab are less commonly applicable. However, HER2 testing is still performed routinely, as exceptions exist.
- Triple-negative ILC (ER-, PR-, HER2-) is rare, occurring in fewer than 5% of ILC cases.
- E-cadherin loss: The absence of E-cadherin expression, identified through immunohistochemistry (IHC) on biopsy, is a defining molecular feature of ILC and is used pathologically to distinguish it from ductal carcinoma in ambiguous cases.
- Genomic testing: Tests such as Oncotype DX and MammaPrint can be applied to ILC to help determine the benefit of chemotherapy in hormone receptor-positive, HER2-negative early-stage disease, though their predictive accuracy specifically for ILC continues to be studied.
Unique Metastatic Patterns
ILC has a distinctive pattern of metastatic spread that differs from invasive ductal carcinoma — an important consideration for surveillance and symptoms awareness:
- While invasive ductal carcinoma most commonly spreads to the lungs, liver, brain, and bones, ILC has an increased tendency to spread to:
- The peritoneum and gastrointestinal tract (stomach, colon, small bowel) — a pattern rarely seen with ductal carcinoma
- The uterus, ovaries, and other gynecological organs
- The meninges (leptomeningeal carcinomatosis)
- Retroperitoneal lymph nodes
- Bone (as with other breast cancer subtypes)
This atypical metastatic pattern means that gastrointestinal symptoms such as nausea, changes in bowel habits, abdominal fullness, or unexplained weight loss should be reported promptly in women with a history of ILC, as these may represent disease recurrence rather than an unrelated gastrointestinal condition. Gynecological symptoms such as abnormal vaginal bleeding, pelvic pain, or ovarian enlargement also warrant evaluation in this context.
Treatment of ILC
Treatment for ILC follows similar general principles to other invasive breast cancers but with several ILC-specific considerations:
Surgery
Both breast-conserving surgery (lumpectomy) and mastectomy are options for ILC, with comparable survival outcomes when adequate surgical margins are achieved. However, ILC-specific factors often influence the surgical decision:
- The diffuse growth pattern and tendency for multicentricity mean that achieving clear surgical margins with lumpectomy can be more challenging in ILC than in ductal carcinoma, and re-excision rates are higher.
- Preoperative MRI is strongly recommended to accurately determine tumor extent before surgery and reduce re-excision rates.
- Because of bilateral disease risk, some women with ILC discuss contralateral prophylactic mastectomy with their surgical team, particularly those with a family history or genetic mutation. This is an individualized decision that should be made with full informed consent and multidisciplinary input.
- Sentinel lymph node biopsy is performed as standard to evaluate axillary lymph node involvement.
Systemic Therapy
- Endocrine (hormone) therapy is the cornerstone of treatment for ILC given its near-universal hormone receptor positivity. This typically involves tamoxifen (for premenopausal women) or an aromatase inhibitor (anastrozole, letrozole, or exemestane, for postmenopausal women), usually for 5–10 years depending on risk stratification. Current NCCN guidelines (2024) support extended endocrine therapy in higher-risk cases.
- CDK4/6 inhibitors (abemaciclib, ribociclib, palbociclib) in combination with endocrine therapy are now standard of care in hormone receptor-positive, HER2-negative metastatic ILC, and abemaciclib has an adjuvant indication for high-risk early-stage disease per monarchE trial data.
- Chemotherapy: ILC is generally considered less chemosensitive than invasive ductal carcinoma, particularly triple-negative ductal carcinoma. The decision to use chemotherapy is guided by tumor size, lymph node status, and genomic testing results. Neoadjuvant chemotherapy (before surgery) achieves a lower pathological complete response (pCR) rate in ILC compared to ductal carcinoma — an important consideration when planning treatment sequence.
- Radiation therapy is used following breast-conserving surgery and in certain mastectomy cases based on standard indications.
Emerging and Investigational Treatments
Given ILC’s distinct biology, there is growing interest in treatments specifically targeted to its molecular profile:
- FGFR pathways: Activating mutations in the FGFR1 and FGFR2 genes are more common in ILC than in ductal carcinoma, making FGFR inhibitors a subject of active investigation.
- AKT/PI3K pathway inhibitors: PIK3CA mutations and PTEN loss are common in ILC. Agents such as capivasertib (approved 2023 for PIK3CA/AKT/PTEN-altered metastatic HR+/HER2- breast cancer) and inavolisib have specific relevance to ILC.
- ILC-specific clinical trials: Advocacy from the Lobular Breast Cancer Alliance (LBCA) has resulted in increased research focus on ILC-specific trials. Women with ILC are encouraged to ask their oncologist whether ILC-specific or lobular-enriched trials are available to them.
Prognosis
The prognosis for ILC is generally similar to or slightly better than invasive ductal carcinoma in the short to medium term, largely due to its strongly hormone receptor-positive profile and responsiveness to endocrine therapy. However, ILC has a notable characteristic worth understanding:
- Long-term recurrence risk: ILC has a late recurrence pattern — recurrences can occur 10, 15, or even 20 years after initial diagnosis, more commonly than with most invasive ductal carcinomas. This means that long-term follow-up and extended endocrine therapy are particularly important for ILC patients.
- Survival outcomes are heavily influenced by stage at diagnosis, lymph node involvement, and hormone receptor status.
- The 5-year relative survival rate for localized ILC (confined to the breast) is approximately 98–99%, similar to other early-stage invasive breast cancers. Regional disease (spread to nearby lymph nodes) carries a 5-year survival rate of approximately 86%, and distant metastatic disease approximately 28%, per SEER data.
Genetic Risk Factors
Most ILC is sporadic (not inherited), but certain genetic factors are associated with increased ILC risk:
- CDH1 gene mutations: Inherited mutations in the CDH1 gene — the same gene whose somatic inactivation drives ILC’s diffuse growth pattern — are associated with hereditary diffuse gastric cancer (HDGC) syndrome, which also significantly elevates lifetime risk of ILC. Women with CDH1 mutations face an estimated 40–60% lifetime risk of ILC. Current guidelines recommend genetic counseling and consideration of risk-reducing mastectomy for CDH1 mutation carriers.
- BRCA1 and BRCA2 mutations also increase risk of ILC, though they are more strongly associated with invasive ductal carcinoma.
- Women with a personal or strong family history of ILC should discuss genetic counseling and high-risk screening with their health care provider.
Questions to Ask Your Health Care Team
If you have been diagnosed with ILC, consider discussing the following with your oncologist or breast surgeon:
- Has a breast MRI been performed to determine the full extent of disease in both breasts?
- What are my surgical options given the size and distribution of my tumor, and what are the re-excision risk and margin considerations?
- What is my tumor’s hormone receptor and HER2 status, and has genomic testing been recommended?
- Am I a candidate for extended endocrine therapy, and for how long?
- Should I be referred for genetic counseling given my diagnosis of ILC?
- Am I eligible for any clinical trials specific to lobular breast cancer?
- What symptoms should prompt me to seek evaluation for recurrence given ILC’s atypical metastatic patterns?
