HER2-amplified primary mucinous cystadenocarcinoma of the breast: surgery and adjuvant treatment

What happened, in summary

This 64-year-old woman was admitted on March 7, 2016 after noticing a left breast mass 1 week earlier. Mammography showed a high-density irregular mass measuring about 22 × 20 mm, classified as BI-RADS 4C. She underwent left breast-conserving surgery with sentinel lymph node biopsy on March 11, 2016. Grossly, the tumor measured 2.2 × 1.8 × 1.7 cm and had a gray-white gelatinous cut surface. Microscopy showed a cystic solid tumor lined by tall columnar cells with abundant mucin, plus a small focus of high-grade ductal carcinoma in situ. The tumor was positive for CK7, E-cadherin, GATA-3, mammaglobin, MUC1, MUC2, and MUC5AC. It was ER-negative and PR-negative, with HER2 IHC 3+ and clustered HER2 amplification by FISH. Ki-67 was about 50%, and p53 showed strong diffuse nuclear staining in about 90% of cells. Margins were negative, and all 9 sentinel lymph nodes were negative. Imaging found no ovarian, pancreatic, lung, or gastrointestinal primary, supporting primary mucinous cystadenocarcinoma of the breast rather than metastasis from another organ. The final stage was pT2pN0cM0, Stage IIA. She received EC-T chemotherapy: 4 cycles of epirubicin plus cyclophosphamide, followed by 4 cycles of docetaxel. Breast-conserving intensity-modulated radiotherapy delivered 52.2 Gy to the ipsilateral breast and 63.8 Gy to the tumor bed region over 29 fractions. Anti-HER2 therapy was recommended but declined because of economic limits. Later tumor sequencing found TP53, LATS1, and NFE2L2 missense mutations plus copy number gains including ERBB2, CDK12, GRB7, RARA, AXIN2, PRKAR1A, SOX9, ZNF217, and GNAS; microsatellite status was stable and no clinically significant germline mutation was found. As of November 2024, she had more than 104 months of disease-free survival, with no recurrence or metastasis.