The patient underwent an ultrasound-guided core needle biopsy and then partial mastectomy and axillary lymph node dissection. The histopathology and IHC evaluations confirmed a mucinous carcinoma of the breast, a subtype of invasive ductal carcinoma (IDC).

They tend to occur in older women, and If palpable, they manifest as soft masses. On imaging, mucinous breast carcinomas usually appear as well-circumscribed masses, and regarding mucin content, they may have low density.

Other malignant breast masses that may present as benign-looking, well-circumscribed lesions include papillary, medullary, and metaplastic carcinomas and malignant phyllodes tumors.