## Histological Types of Breast Carcinoma **Key Point:** Invasive ductal carcinoma (IDC), also called invasive carcinoma of no special type (NST), accounts for approximately 70–80% of all invasive breast cancers. ### Frequency Distribution | Histological Type | Frequency | Prognosis | 5-Year Survival | | --- | --- | --- | --- | | Invasive Ductal (NST) | 70–80% | Intermediate | 75–80% | | Invasive Lobular | 10–15% | Intermediate | 75–80% | | Tubular | 2–3% | Favorable | >90% | | Mucinous (Colloid) | 2–3% | Favorable | >90% | | Medullary | 1–2% | Intermediate | 80–85% | | Papillary | <1% | Favorable | >90% | **High-Yield:** IDC/NST is the default diagnosis when no special type features are identified. It presents as a hard, irregular mass with possible nipple retraction, skin dimpling, or axillary lymphadenopathy. ### Why IDC is Most Common 1. **Origin:** Arises from ductal epithelium, which comprises the majority of breast tissue 2. **Lack of special features:** Does not show the organized growth patterns seen in tubular, mucinous, or papillary types 3. **Aggressive behavior:** Higher proliferation rate compared to special types **Clinical Pearl:** The upper outer quadrant is the most common site for breast carcinoma (50% of cases), followed by the upper inner quadrant. This distribution correlates with the greater volume of breast tissue in these regions. **Mnemonic:** IDC = **I**nvasive **D**uctal **C**arcinoma — the "default" diagnosis when no special features are present; think of it as the "catch-all" category for ductal origin tumors without distinctive histology. [cite:Robbins 10e Ch 24]
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