Title : A rare parotid gland metastasis in PIK3CA-mutated HR+/HER2-negative breast cancer
Abstract:
Background : Breast cancer parotid gland metastasis is extremly rare, accounting for less than 0.3% of secondary parotid tumors. Optimal management remains unclear due to limited evidence. We report an unusual case of parotid metastasis occurig in a partient with HR positive HER2 low metastatic breast cancer harboring a PIK3CA mutation, progressing after multiple lines of therapy including trastuzumab deruxtecan (T-Dxd).
Case presentation : We report the case of a 59-year-old woman initially treated in 2004 for left breast invasive ductal carcinoma grade III (pN3), managed with surgery, anthracycline/taxane-based adjuvant chemotherapy, radiotherapy and tamoxifen. Eighteen years later she developed metastatic relapse involving bone,lung, lymph nodes and adrenal gland. Multiplesuccessive systemic therapies were administered including aromatase inhibitor + CDK4/6 inhibitor, fulvestrant, taxanes and trastuzumab deruxtecan in HER2 low context. After five cycles of TDxD, she developed new onset right facial palsy. MRI identified a 15 mm parotid lesion. Cytology from liquid based fine needle aspiration confirmed malignant adenocarcinoma cells with positivity of GATA3, compatible with metastatic dissemination of her knnown breast carcinoma. Immunochemestry subsequently demonstrated complete loss of hormone receptor expression and HER 2 negativity in the parotid metastasis, consistent with receptor conversion under therapeutic pressure. Given disease progression after TDxd and the receptor switch, treatement was initiated with Sacituzumab Govetican. After two cycles, radiological assessement showed stability of the parotid focus.
Conclusion : Parotid gland metastasis is an exceptionally rare metastatic presentation of breast cancer and may reveal molecular evolution. This case highlights an exceptional metastatic pattern of HR+ HER low breast cancer and emphasizes the growing heterogeneity introduced by HER2 low biology in treatement sequencing. Further data are needed to better understand the biological mechanisms underdipping such atypical metastaic tropism and to guide optimal management.
