Invasive lobular carcinoma (ILC) is the second most common invasive breast cancer subtype, accounting for 10-15% of all breast cancers. It originates from lobular epithelial cells and is characterized by E-cadherin (CDH1) gene mutation/loss — this leads to loss of cell-cell adhesion and single-file (Indian file) infiltration pattern. ILC is the most challenging breast cancer on imaging: it can be occult on mammography in 30-40% of cases because it infiltrates in single-file without forming a distinct mass and desmoplastic reaction may be minimal. Ultrasonography sensitivity is higher than mammography but still limited. MRI is the most sensitive modality for ILC detection and extent assessment — more than 90% of lesions occult on mammography and US are detected by MRI. The tendency for bilateral and multifocal/multicentric disease is significantly higher than invasive ductal carcinoma (20-30% bilateral). Typically occurs in women aged 55-65; hormone receptor positivity rate is high (ER+ 90-95%). Metastasis pattern is atypical — may metastasize to areas rarely seen in invasive ductal carcinoma such as bone, peritoneum, gastrointestinal tract, and ovary.
Age Range
45-75
Peak Age
60
Gender
Female predominant
Prevalence
Uncommon
ILC originates from lobular epithelial cells and the key pathogenic event is mutation or epigenetic silencing of the E-cadherin (CDH1) tumor suppressor gene. E-cadherin is a calcium-dependent cell-cell adhesion protein that enables normal epithelial cells to adhere to each other. E-cadherin loss eliminates the ability of tumor cells to adhere → cells infiltrate along the stroma in single-file (Indian file), not forming a cohesive mass. This single-file infiltration pattern is the fundamental reason ILC is occult on imaging: since desmoplastic stromal reaction is minimal, spiculated mass or architectural distortion may not form on mammography. Tumor cells may preserve existing breast structures (ducts, lobules) and form a 'targetoid' (bullseye) pattern around them — concentric cell arrangement around ducts. Enhancement is seen on MRI because neoangiogenesis is present, though vascular structure may be less organized compared to IDC — enhancement pattern is usually heterogeneous or non-mass type. E-cadherin loss also explains the atypical metastasis pattern: single cells easily enter the bloodstream and can settle in unusual areas such as peritoneum, GI tract mucosa, meninges, and retroperitoneal spaces.
Non-mass enhancement in segmental or regional distribution on MRI in a patient with no definite mass on mammography is the most characteristic finding combination of ILC. This scenario results from ILC's single-file infiltration pattern not creating mass effect on mammography but MRI contrast being able to detect micro-infiltrative areas. When this finding combination is present, ILC should be strongly considered and biopsy is indicated.
ILC frequently shows subtle findings on mammography or is completely occult (30-40%). The most common mammographic finding is focal asymmetry or developing asymmetry — density increase without distinct mass margins. Architectural distortion is the second most common finding. Spiculated mass is less frequently observed compared to IDC (30-40% ILC vs 60-70% IDC). Microcalcifications are rare because ILC tends not to contain necrosis or ductal component. Increased breast density, breast size reduction (due to stromal fibrosis), and skin thickening are advanced-stage findings.
Report Sentence
Subtle focal asymmetry/architectural distortion is observed on mammography; although no definite mass is identified, there is a new developing density increase compared to prior examinations; supplemental breast MRI is recommended considering invasive lobular carcinoma.
ILC may demonstrate various appearances on US: (1) ill-defined hypoechoic area — most common US finding, (2) irregular hypoechoic mass with posterior acoustic shadowing, (3) spiculated mass (similar to IDC but less frequent), (4) isoechoic lesion — difficult to distinguish from surrounding parenchyma. Posterior acoustic shadowing reflects the fibrotic stroma of ILC and is usually disproportionately wider than the mass size. Non-parallel orientation may be seen. US sensitivity is higher than mammography (68-98% vs 34-65%) but tends to underestimate tumor size.
Report Sentence
An ill-defined hypoechoic area is seen on US with disproportionately wide posterior acoustic shadowing relative to mass size; this finding pattern is consistent with invasive lobular carcinoma and breast MRI is recommended for extent assessment.
MRI is the most sensitive modality for ILC detection (93-100% sensitivity). The most common MRI finding in ILC is non-mass enhancement (NME) — enhancing areas in segmental, linear, or regional distribution without mass formation. Enhancement pattern may be heterogeneous or stippled. Multifocal/multicentric enhancing areas are frequently observed — additional foci not detected on mammography and US become visible on MRI. Contralateral breast involvement may also be detected by MRI. Kinetic curve usually shows Type II (plateau) or Type III (washout) but Type II pattern is more frequently observed compared to IDC. Diffusion restriction may be seen on DWI but ADC values may be slightly higher than IDC.
Report Sentence
Segmentally distributed non-mass enhancement is seen on contrast-enhanced breast MRI with additional multifocal enhancing foci detected; kinetic curve demonstrates Type II (plateau) pattern; findings are consistent with invasive lobular carcinoma and contralateral breast should also be evaluated.
ILC may demonstrate variable diffusion restriction on DWI. High-grade ILC shows marked diffusion restriction and low ADC values; however in low-grade classic ILC, ADC values may be slightly higher than IDC (1.0-1.2 × 10⁻³ mm²/s). Accompanying diffusion restriction in non-mass enhancement areas strongly supports malignancy. DWI sensitivity is slightly lower in ILC compared to IDC because single-file infiltration may not homogeneously increase cellularity.
Report Sentence
Variable diffusion restriction is observed in enhancement areas on DWI; foci with low ADC values support malignant infiltration and are assessed as consistent with invasive lobular carcinoma.
ILC usually shows hypointense or intermediate signal on T2-weighted sequences — reflecting fibrous stroma and low water content. Low T2 signal compared to surrounding breast parenchyma may help recognize infiltrative areas. Hypointense appearance of non-mass enhancement areas on T2 is an important supportive finding favoring malignancy (benign non-mass enhancement is usually T2 hyperintense). Necrotic areas are rare but may be seen in pleomorphic ILC, showing T2 hyperintensity.
Report Sentence
Enhancement areas demonstrate hypointense signal on T2-weighted sequences; this finding reflects fibrous stromal infiltration and is evaluated in favor of malignancy in differential from benign non-mass enhancement.
Criteria
Most common subtype (70-80%). Small, uniform, round cells showing single-file infiltration. Low mitotic activity, low nuclear grade. E-cadherin loss (CDH1 mutation). ER/PR positive, HER2 negative.
Distinct Features
Type most frequently occult on mammography; ill-defined hypoechoic area on US; non-mass enhancement on MRI; good response to endocrine therapy; late recurrence risk (5-20 years later).
Criteria
Larger, pleomorphic cells with high nuclear grade. More aggressive biology. May contain necrosis. Comprises ~5-15% of all ILCs.
Distinct Features
More frequently forms mass on mammography (similar to IDC); heterogeneous enhancement and Type III kinetic curve more frequent on MRI; marked diffusion restriction on DWI; poor prognosis — aggressiveness similar to IDC.
Criteria
Cells grow in solid sheets instead of single file. Intermediate-high grade. E-cadherin loss is maintained (provides diagnostic distinction from IDC).
Distinct Features
May form spiculated mass similar to IDC on mammography; more prominent mass on US; mass-type enhancement more frequent on MRI; diffusion restriction more prominent on DWI due to solid pattern.
Distinguishing Feature
IDC forms a distinct spiculated mass and is rarely occult on mammography; shows mass-type enhancement and Type III kinetic curve on MRI. ILC is frequently occult, shows non-mass enhancement, and Type II kinetic curve is more common. On immunohistochemistry, E-cadherin loss supports ILC, preserved E-cadherin supports IDC.
Distinguishing Feature
LCIS (lobular carcinoma in-situ) is a non-invasive lesion; usually detected incidentally and rarely shows imaging findings. ILC is invasive and shows enhancement on MRI. Pathologically, LCIS has not breached the basement membrane, ILC has.
Distinguishing Feature
Fibroadenoma is a circumscribed, homogeneous, parallel-oriented benign mass with Type I kinetic curve. ILC is ill-defined, heterogeneous, with malignant kinetic curve. However, small ILCs may resemble fibroadenoma on US — biopsy is indicated when in doubt.
Distinguishing Feature
Diabetic mastopathy may mimic ILC with ill-defined hypoechoic area and posterior acoustic shadowing on US. Type 1 diabetes history and bilateral involvement support diabetic mastopathy. Biopsy may be needed as definitive differentiation by imaging alone is difficult.
Distinguishing Feature
Radial scar may mimic ILC with architectural distortion on mammography; however the center of radial scar is radiolucent and its size changes with different mammographic positions. On MRI, radial scar usually shows no or minimal enhancement; ILC shows prominent enhancement.
Urgency
emergentManagement
surgicalBiopsy
NeededFollow-up
specialist-referralBiopsy is indicated when ILC is suspected — US-guided core biopsy or MRI-guided biopsy (in cases occult on mammography and US). E-cadherin immunohistochemistry is critical in pathological diagnosis (loss = ILC, preserved = IDC). Breast MRI is mandatory for extent assessment before surgical planning because the true size of ILC can be significantly underestimated by mammography and US — MRI changes the surgical plan in 28-50% of cases (wider excision, mastectomy decision, contralateral biopsy). Contralateral breast MRI screening is recommended due to bilateral disease risk (20-30%). Surgical margin assessment is more difficult compared to IDC — positive surgical margin rate and re-excision rate are higher. Endocrine therapy response rate is high (ER+ 90-95%). Chemotherapy response may be lower compared to IDC. Prognosis is generally similar to IDC but late recurrence risk (>5 years) is higher — requires long-term follow-up. Awareness of atypical metastasis sites (peritoneum, GI tract, ovary) and related symptoms is needed.
Since ILC is often occult on mammography, MRI plays a critical role in preoperative staging. Positive surgical margins are more common than in IDC. Mastectomy rate is higher due to multifocality/multicentricity. High ER positivity with good response to hormonal therapy.