Mucinous cystadenocarcinoma (MCA) is a rare breast cancer. The present study reports a case of primary MCA of the breast with a comprehensive evaluation of this rare tumour. A 51‑year‑old woman sought medical attention for a mass in the left breast. A core needle biopsy revealed an infiltrating adenocarcinoma with mucus secretion and papillary formation. The macroscopic appearance was of a greyish‑white, tough and well‑circumscribed solid mass, without a notable cyst. Microscopically, the tumour consisted of ducts and cysts of varying sizes. Varying degrees of branching papillary structures were observed in the lumen and cyst cavities. The tumour cells were highly columnar in shape, with high‑grade nuclei arranged in a single‑layer. Immunohistochemistry revealed that the tumour was a basal‑like triple‑negative breast cancer with a high proliferation index and tumour protein p53 diffuse strong expression. Mutations in breast cancer 1‑associated RING domain 1 (BARD1), kinase domain containing receptor (KDR), mucin‑6 (MUC6), tumour protein 53 (TP53) and breast cancer 1‑interacting protein C‑terminal helicase 1 (BRIP1) were identified using DNA analysis. The patient was followed up for 26 months and showed no signs of recurrence or metastasis. In conclusion, the current study presents a case of MCA of breast accompanied by mutations in the BARD1, KDR, MUC6, TP53 and BRIP1 genes, with no recurrence after a 26‑month follow‑up. Combining this case with a review of the literature helps us to better understand the clinicopathological and genetic characteristics of MCA, and guide treatment.

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