Definition/General
Introduction:
Mucinous carcinoma of the breast, also known as colloid carcinoma, is a rare special type of invasive ductal carcinoma characterized by abundant extracellular mucin production
On FNAC, it shows distinctive cytological features with clusters of epithelial cells floating in abundant mucoid background
This accounts for 1-7% of all breast carcinomas and has a relatively favorable prognosis compared to invasive ductal carcinoma NOS.
Origin:
Pure mucinous carcinoma arises from ductal epithelium and shows >90% mucin component
Mixed mucinous carcinoma contains 30-90% mucinous component with conventional ductal carcinoma
Type A (hypercellular) has increased cellularity and Grade 1 nuclei, while Type B (hypocellular) has lower cellularity and more abundant mucin.
Classification:
WHO classification recognizes pure mucinous carcinoma (>90% mucin) and mixed mucinous carcinoma (30-90% mucin)
Histological subtypes include Type A (hypercellular pattern) and Type B (hypocellular pattern)
Cytological grading follows nuclear morphology assessment.
Epidemiology:
Predominantly affects elderly women with median age 65-71 years
More common in postmenopausal women (85% cases)
Indian studies show slightly younger age distribution (50-60 years)
Associated with favorable hormone receptor status and excellent prognosis when pure type.
Clinical Features
Presentation:
Well-circumscribed, mobile breast mass typically measuring 2-5 cm
Slow-growing tumor with indolent course
Often mimics fibroadenoma on clinical examination
Skin changes and nipple discharge are uncommon.
Symptoms:
Painless breast lump in elderly women
Mass may feel soft and fluctuant due to high mucin content
No associated skin dimpling or nipple retraction
Bilateral occurrence reported in 5-10% cases.
Risk Factors:
Advanced age >65 years
Postmenopausal status
No strong association with BRCA1/2 mutations
Favorable hormone receptor profile (ER+/PR+)
Previous history of breast cancer.
Screening:
Regular mammographic screening in women >50 years
Mammography shows circumscribed mass with mixed density
Ultrasound demonstrates hypoechoic lesion with posterior enhancement
MRI shows high T2 signal intensity.
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Gross Description
Appearance:
Well-circumscribed, gelatinous tumor with smooth borders
Cut surface shows glistening, mucoid appearance
Color ranges from gray-white to translucent
Consistency is soft and slimy due to abundant mucin.
Characteristics:
Tumor size typically 2-5 cm diameter
Encapsulated or pseudo-encapsulated appearance
No necrosis or hemorrhage
Mucoid material may leak from cut surface
Associated DCIS component may be present.
Size Location:
Usually located in upper outer quadrant
Size ranges from 1-10 cm with average 3 cm
Pure type tends to be larger than mixed type
Multifocal occurrence in 15-20% cases.
Multifocality:
Bilateral synchronous occurrence in 5-10% cases
Multifocal disease within same breast in 15-20%
Associated DCIS may extend beyond invasive component
Lymph node metastasis rare in pure type.
Microscopic Description
Histological Features:
Abundant extracellular mucin pools containing clusters and single epithelial cells
Mucin occupies >90% of tumor volume in pure type
Epithelial cells show low-grade nuclear features
Minimal stromal component between mucin lakes.
Cellular Characteristics:
Type A shows increased cellularity with small clusters and micropapillary groups
Type B demonstrates sparse cellularity with single cells and small clusters
Nuclear features are uniformly bland with Grade 1 morphology
Mitotic activity is low.
Architectural Patterns:
Floating clusters of epithelial cells in mucin lakes
Micropapillary architecture in some areas
Cribriform patterns may be present
Absence of significant stromal desmoplasia
Well-defined tumor borders.
Grading Criteria:
Nuclear grade predominantly Grade 1 (low grade)
Architectural score typically 1-2
Mitotic count usually <10 per 10 HPF
Overall histological grade I-II
Nottingham grading system applicable.
Immunohistochemistry
Positive Markers:
ER positive (85-95% cases)
PR positive (70-80% cases)
CK7 positive
CK8/18 positive
E-cadherin positive
EMA positive
Mammaglobin positive (70% cases).
Negative Markers:
HER2/neu negative (90-95% cases)
CK5/6 negative
p63 negative
CK14 negative
GCDFP-15 variable (30-50%)
TTF-1 negative.
Diagnostic Utility:
Hormone receptor positivity confirms epithelial origin
CK7+/CK20- pattern supports breast primary
Mammaglobin helpful for breast origin confirmation
HER2 negativity associated with favorable prognosis.
Molecular Subtypes:
Predominantly Luminal A subtype (ER+/PR+/HER2-/Ki67 low)
Excellent prognosis associated with this profile
Low proliferation index
Triple-negative cases extremely rare.
Molecular/Genetic
Genetic Mutations:
PIK3CA mutations in 30-40% cases
Low frequency of TP53 mutations (<5%)
BRCA1/2 mutations rare
Lower overall mutation burden compared to ductal carcinoma NOS.
Molecular Markers:
High expression of MUC2 and MUC5AC
Low Ki-67 proliferation index (<10%)
p53 expression typically wild-type pattern
Cyclins E and D1 may be expressed.
Prognostic Significance:
Excellent prognosis with 10-year survival >90% for pure type
Mixed type has intermediate prognosis
Node-negative disease in majority of cases
Low recurrence risk.
Therapeutic Targets:
Hormone receptor positivity indicates endocrine therapy responsiveness
Anti-HER2 therapy not indicated due to negativity
CDK4/6 inhibitors may be considered in advanced cases.
Differential Diagnosis
Similar Entities:
Fibroadenoma with myxoid change
Mucinous cystadenocarcinoma
Invasive lobular carcinoma with mucin
Primary mucinous tumors from other sites
Mucocele-like lesions.
Distinguishing Features:
Fibroadenoma: benign epithelial cells, no nuclear atypia, biphasic pattern
Invasive lobular: single file pattern, E-cadherin negative
Metastatic mucinous carcinoma: CK7-/CK20+ pattern, site-specific markers positive.
Diagnostic Challenges:
Distinction from mucinous cystadenocarcinoma requires architectural assessment
Low cellularity may mimic benign lesions
Mixed type may be confused with ductal carcinoma
Metastatic disease exclusion important.
Rare Variants:
Signet ring cell variant with intracytoplasmic mucin
Neuroendocrine differentiation rare
Micropapillary variant with complex architecture
Solid variant with minimal mucin production.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Site and Procedure
Site: [breast location], Procedure: Fine needle aspiration cytology
Adequacy
Adequate for evaluation
Background
Abundant mucoid/colloid material
Cellularity
Low to moderate cellularity relative to abundant mucin
Cellular Features
Cohesive clusters of epithelial cells with bland nuclear morphology floating in mucin
Nuclear Features
Grade 1 nuclei with minimal pleomorphism and rare mitosis
Special Features
Absence of myoepithelial cells, minimal inflammatory cells
Cytological Diagnosis
Suspicious for mucinous carcinoma of breast (Category V)
Recommendation
Histopathological confirmation recommended with cell block/core biopsy