What happened, in summary

A 69-year-old woman sought care for a left breast lump that had enlarged over 6 months. Examination found a large mass in the upper outer breast, while the right breast, both axillae, and systemic review were unremarkable. Despite adequate core samples, the biopsy did not establish malignancy, creating uncertainty between the clinical findings, imaging, and pathology.

Because cancer remained strongly suspected, she underwent wide local excision with partial breast reconstruction using an anterior intercostal artery perforator flap. A sentinel lymph-node biopsy was performed during the same operation.

Final pathology found a 15 mm grade II micropapillary variant of mucinous breast carcinoma. It was associated with extensive grade II intracystic papillary ductal carcinoma in situ, bringing the total affected area to 66 mm. The tumor showed micropapillary structures, hobnail cells, and psammomatous microcalcifications. It was negative for estrogen receptor, progesterone receptor, and HER2. There was no lymphovascular invasion, the surgical margins were clear, and the sentinel nodes contained no malignancy.

This rare subtype had resembled a necrotic papillary lesion on core biopsy, so the diagnosis became clear only after the full mass was removed and examined.