Introduction/Background Invasive lobular carcinoma (ILC) with extracellular mucin production is a recently described new variant of ILC with few reported cases in the English literature. We report another case of this rare entity diagnosed in the pathology department of Saint Savvas hospital.

Methodology A 65-year-old patient was admitted to the hospital for a palpable lump of the right breast. Physical examination and imaging studies were consistent with malignancy. Frozen section was positive while sentinel lymph node biopsy was negative for malignancy. A right mastectomy was performed. On gross examination the tumor was soft, pale, grayish blue, gelatin-like and well circumscribed. On microscopic examination the tumor consisted of two foci measuring 13 and 6 mm. It consisted of a non-mucinous ILC component of the classical and solid type. The mucinous area represented about 70% of the tumor. Extracellular mucin production was seen in the form of relatively circumscribed multiple nodular areas. Tumor cells in the mucinous area were arranged in clusters and single cells. Several signet ring cells were identified. Tumor cells showed positive staining for ER and negative for PR, HER-2 and E-Cadherin. Ki-67 stained 25% of tumor nuclei.

Results The diagnosis of ILC with extracellular mucin production was made. Adjuvant radiotherapy and hormonal therapy was administered.

The patient is alive without evidence of recurrence or metastasis six months after surgery.

Conclusion Awareness of this new entity will help us avoid confusion with other mucin-producing neoplasms. Differential diagnosis includes mucinous carcinoma, solid papillary carcinoma with extracellular mucin production and composite mucinous and lobular carcinoma. Correct diagnosis is important because both treatment and prognosis are distinct.

Disclosure Nothing to disclose.