A surprising complete radiographic response to palbociclib plus endocrine therapy in an HR-positive, HER2-low lung, pleural, and lymph node metastatic breast cancer patient: a case report.

Hormone receptor-positive, human epidermal growth factor receptor 2-low (HER2-low) metastatic breast cancer (MBC) constitutes the predominant subtype of advanced breast cancer, with most patients eventually developing resistance to endocrine therapy (ET) and cyclin-dependent kinase 4/6 inhibitors (CDK4/6i). This case report describes a 64-year-old woman with hormone receptor (HR)-positive, HER2-low MBC involving lung, pleural, and lymph node metastases who achieved a remarkable and sustained radiographic complete response (rCR) to palbociclib plus fulvestrant, followed by letrozole after fulvestrant injection intolerance. The patient exhibited near-complete regression of metastatic lesions within 18 months and maintained remission for over 6 years, far exceeding the typical median progression-free survival observed in clinical trials. The prolonged remission observed underscores the existence of biologically distinct tumor subsets with exceptional sensitivity to CDK4/6 inhibition, warranting investigation into underlying molecular mechanisms.