## Most Common Location of Breast Cancer **Key Point:** The upper outer quadrant (UOQ) is the most common site for breast cancer, accounting for approximately 40–50% of all breast malignancies. This is followed by the upper inner quadrant (15–20%), lower outer quadrant (10–15%), and lower inner quadrant (5–10%). ### Anatomical Distribution of Breast Cancer | Quadrant | Frequency | Anatomical Notes | |----------|-----------|------------------| | Upper outer | 40–50% | Contains the most breast tissue; includes the axillary tail | | Upper inner | 15–20% | Second most common | | Lower outer | 10–15% | Third most common | | Lower inner | 5–10% | Least common | | Central/Nipple | 5–10% | Variable; includes Paget's disease | **High-Yield:** The upper outer quadrant contains the largest volume of breast tissue and the axillary tail of Spence, which extends into the axilla. This anatomical abundance of glandular tissue explains the higher incidence of cancer in this region. **Clinical Pearl:** Tumours in the upper outer quadrant have direct lymphatic drainage to the axillary lymph nodes, making axillary lymph node involvement more common and clinically significant in these cases. This has implications for staging and surgical planning (sentinel lymph node biopsy vs. axillary clearance). **Mnemonic:** **UOQ Rule** — **U**pper **O**uter **Q**uadrant is the most common site because it has the most glandular tissue and the axillary tail. ### Why Upper Outer? 1. **Tissue volume:** The upper outer quadrant contains approximately 50% of the total breast tissue. 2. **Axillary tail:** The tail of Spence extends superolaterally into the axilla, increasing the area of glandular tissue. 3. **Lymphatic drainage:** Direct drainage to axillary nodes may influence both tumour biology and clinical presentation.
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