Medullary breast carcinoma is a rare but clinically important invasive breast carcinoma subtype, accounting for 1-5% of all breast cancers. Histologically characterized by syncytial growth pattern, dense lymphoplasmacytic stromal infiltration, high nuclear grade (G3), and well-defined (pushing) borders. As an interesting paradox, despite high-grade histology, prognosis is better than IDC — this paradox is explained by the contribution of intense immune response to tumor control. Significantly more common in BRCA1 mutation carriers (up to 13% BRCA1-associated). Typically presents in young/middle-aged women under 50 years. On imaging, appears as a well-circumscribed, round, rapidly growing mass that can mimic benign lesions such as fibroadenoma or cyst. Shows triple-negative phenotype (ER-, PR-, HER2-).
Age Range
30-55
Peak Age
45
Gender
Female predominant
Prevalence
Rare
Medullary carcinoma is of ductal epithelial origin and shows a syncytial (sheet-like, adherent) growth pattern. High nuclear grade (pleomorphic, large nuclei) and high mitotic index reflect aggressive histology; however, dense lymphoplasmacytic infiltration (predominantly CD8+ cytotoxic T-lymphocytes and plasma cells) surrounding and within the tumor represents a strong immune response against tumor cells. This immune response modulates the tumor's biological behavior and forms the basis of favorable prognosis. The 'pushing' borders of the tumor indicate expansile growth rather than stromal invasion — pseudocapsule formation is the reason for well-defined margins on imaging. High cellularity (tightly packed tumor cells + dense lymphocytic infiltrate) leads to marked diffusion restriction on DWI — the minimal intercellular space restricts water molecule movement. Due to high cellularity and metabolic activity, the tumor grows rapidly and shows intense enhancement on MRI. BRCA1 association is explained by genomic instability and high mutational burden due to DNA repair mechanism defects — this leads to increased neoantigen expression → strong immune response cycle.
The combination of a well-circumscribed, round, benign-appearing mass on US or mammography + marked diffusion restriction (very low ADC) + intense homogeneous enhancement on MRI is the signature finding triad of medullary carcinoma. This paradoxical coexistence of benign morphology with aggressive MR signals distinguishes medullary carcinoma from all other breast lesions.
A well-circumscribed, round or oval, homogeneous hypoechoic mass is seen on US. Margins are sharp and smooth — no spiculated edges. Posterior acoustic enhancement is frequently seen (despite highly cellular tissue, homogeneous cell distribution improves sound transmission). Size is usually >20 mm because it shows rapid growth. This appearance can be confused with fibroadenoma (BI-RADS 3), but a history of rapid growth should raise suspicion.
Report Sentence
A well-circumscribed, round, homogeneous hypoechoic mass with posterior acoustic enhancement is seen in the breast parenchyma; despite benign appearance, biopsy is recommended due to history of rapid growth.
Prominent, homogeneous, early enhancement is demonstrated on contrast-enhanced MRI. Washout (Type III) or plateau (Type II) kinetic pattern may be seen — although Type I (progressive) has also been reported in some cases. Enhancement intensity results from high vascularity and increased capillary permeability of dense lymphoplasmacytic infiltration. Necrotic areas may lead to heterogeneous enhancement in large tumors.
Report Sentence
The mass demonstrates prominent, homogeneous enhancement in the early phase with washout pattern in the late phase on contrast-enhanced MRI; this enhancement pattern suggests a highly vascular malignant process.
Demonstrates markedly high signal on DWI and low signal on ADC map (restricted diffusion). ADC values are typically <1.0 × 10-3 mm2/s. This finding reflects the high cellularity of medullary carcinoma (syncytial tumor cells + dense lymphocytic infiltrate). Shows one of the lowest ADC values among all breast malignancies. Homogeneous low ADC distribution reflects the tumor's homogeneous syncytial structure.
Report Sentence
The mass demonstrates markedly high signal on DWI and low signal on ADC map; the marked diffusion restriction suggests a highly cellular malignant process.
A well-circumscribed or mildly lobulated, round, high-density mass is seen on mammography. Spiculated margin is typically absent. Microcalcifications are rare. Size is usually >20 mm. This appearance resembles fibroadenoma, cyst, or phyllodes tumor. History of well-circumscribed but rapidly growing mass (comparison with prior mammography) is a critical clue.
Report Sentence
A well-circumscribed, round, high-density mass is seen on mammography without microcalcifications; despite benign appearance, biopsy is recommended due to rapid growth.
Intermediate-to-high signal intensity on T2-weighted images. Despite high cellularity, T2 signal is higher than expected — this results from inflammatory edema created by dense lymphoplasmacytic infiltration and free water in intratumoral micronecrosis areas. Homogeneous T2 signal distribution reflects the syncytial structure of the tumor. Not cyst-like bright signal but higher than most solid tumors.
Report Sentence
The mass demonstrates intermediate-to-high signal intensity on T2-weighted sequences with homogeneous signal distribution; this T2 signal is consistent with a cellular process containing inflammatory infiltration.
Criteria
Meets all five classic criteria: (1) syncytial growth >75%, (2) well-defined (pushing) borders, (3) dense lymphoplasmacytic infiltration, (4) high nuclear grade, (5) no glandular/tubular differentiation.
Distinct Features
Best prognosis. Triple-negative phenotype dominant. BRCA1 association strongest in this group. 10-year survival >80%. Low axillary metastasis rate.
Criteria
Does not meet one or two of the five criteria (usually lacking: syncytial pattern <75% or insufficient lymphocytic infiltration or focal infiltrative border).
Distinct Features
Prognosis worse than classic type but better than IDC. Imaging findings similar but margins may be less smooth. Axillary metastasis slightly more common.
Criteria
In WHO 2019 classification, classic and atypical distinction was removed and unified as 'invasive carcinoma with medullary features'. Encompasses all tumors showing medullary pattern.
Distinct Features
Encompasses a broader spectrum. Clinical management and prognosis are individualized according to specific histological features. BRCA testing is recommended.
Distinguishing Feature
Fibroadenoma shows low/intermediate DWI signal and preserved ADC on MRI (low cellularity). Medullary carcinoma shows markedly low ADC and intense enhancement. Fibroadenoma is more common at young age (<30), stable size; medullary carcinoma grows rapidly.
Distinguishing Feature
Phyllodes tumor also presents as well-circumscribed, rapidly growing mass. However, internal cystic areas and 'leaf-like' intratumoral clefts are more prominent on US in phyllodes tumor. Shows heterogeneous signal and enhancement on MRI. Core biopsy is distinguishing.
Distinguishing Feature
IDC typically shows irregular/spiculated margins, posterior shadowing, non-parallel orientation. Medullary carcinoma shows well-circumscribed margins, parallel orientation, and posterior enhancement. However, both show DWI restriction and intense enhancement — definitive distinction is made by biopsy.
Distinguishing Feature
Breast lymphoma can also show well-circumscribed margins, low ADC and intense enhancement. However, lymphoma is frequently bilateral and multifocal with skin thickening. Systemic lymphoma findings (splenomegaly, diffuse LAP) and biopsy are distinguishing.
Urgency
urgentManagement
surgicalBiopsy
NeededFollow-up
6-monthMedullary carcinoma requires surgical treatment (lumpectomy/mastectomy + sentinel lymph node biopsy). Pure type has excellent prognosis (5-year survival >90%). Hormonal therapy is not applied due to triple-negative phenotype; chemotherapy (anthracycline-based) is generally administered. BRCA1 testing is recommended for all patients. If BRCA1 positive, prophylactic contralateral mastectomy may be discussed. Radiation therapy is standard after breast-conserving surgery. Follow-up with annual MRI and mammography. Biopsy should not be delayed for rapidly growing, well-circumscribed mass.
Medullary carcinoma is a malignant tumor despite its well-defined appearance. High tendency to be triple-negative (ER-/PR-/HER2-). More common in BRCA1 mutation carriers. Despite well-defined margins, considered to have relatively favorable prognosis due to dense lymphocytic infiltration. Treatment is surgery + chemotherapy.