Abstract
Invasive mucinous carcinoma (IMC) is an uncommon histologic subtype of breast cancer characterized by abundant extracellular mucin and generally indolent behavior. It occurs more frequently in older women and is often associated with favorable prognostic features.An 84-year-old woman with no prior history of breast malignancy was found to have a suspicious lesion in the upper outer quadrant of the left breast on routine imaging. Ultrasound-guided core needle biopsy demonstrated IMC, Grade I/III, involving all four cores, with the largest contiguous tumor focus measuring 1.3 cm. Immunohistochemistry showed strong estrogen receptor positivity (95%), progesterone receptor positivity, human epidermal growth factor receptor 2 negativity (1+), and an extremely low Ki-67 proliferation index (~1%). Pathologic processing met established fixation standards.This case highlights the importance of histologic subtype and proliferative activity in guiding risk stratification and management considerations for breast cancer in elderly patients. Recognition of indolent tumor biology may help avoid overtreatment while maintaining oncologic safety.