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Invasive ductal carcinoma (IDC), also known as infiltrating ductal carcinoma, is the most common type of breast cancer, accounting for approximately 70–80% of all invasive breast malignancies. It originates in the epithelial lining of the breast ducts and invades the surrounding stroma, with potential to spread to lymph nodes and distant organs.

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Pathophysiology

• Begins as ductal carcinoma in situ (DCIS)
• Invades through the basement membrane into surrounding fibrous and fatty tissue
• Can metastasize via:
  • Lymphatic system (especially axillary nodes)
  • Hematogenous route (lung, liver, brain, bone)

Histological Features

• Irregular nests, cords, or tubules of malignant epithelial cells infiltrating stroma
• Desmoplastic stromal reaction is common
• Graded via the Nottingham Histologic Score (Elston-Ellis modification of Scarff-Bloom-Richardson):
  • Tubule formation
  • Nuclear pleomorphism
  • Mitotic count
  • Grades: 1 (well-differentiated), 2 (moderately), 3 (poorly differentiated)

Receptor status (critical for treatment planning):

• Estrogen receptor (ER)
• Progesterone receptor (PR)
• HER2/neu overexpression

Clinical Features

• Palpable breast mass (hard, irregular, immobile)
• Skin changes: dimpling, tethering, nipple retraction
• Nipple discharge