PIK3CA mutation detection by circulating tumor cell sequencing guides the effective PI3K inhibitor treatment of a patient with hormone receptor-positive breast cancer transformed into triple-negative breast cancer: A case report

RATIONALE: Most patients with hormone receptor-positive breast cancer (BC) prefer endocrine therapy and chemotherapy. However, some HR+ BC patients often exhibit resistance to CDK4/6 inhibitors and even undergo molecular subtyping changes during disease progression. Therefore, precise detection and treatment of these patients after disease progression are crucial. PATIENT CONCERNS: In October 2021, a 51-year-old Chinese female (Han ethnicity) with a palpable mass in her left breast was diagnosed with HR+ BC. After endocrine therapy and CDK4/6 inhibitor treatment, the patient's disease still progressed and transformed into triple-negative BC. DIAGNOSES: Peripheral blood circulating tumor cells were isolated from the patient and performed targeted sequencing by using next-generation sequencing, revealing the presence of a PIK3CA mutation in the patient. INTERVENTIONS: The patient's condition was effectively controlled after treatment with PI3K inhibitors. OUTCOMES: The patient's progression free survival was prolonged to 7 months after treatment with the PI3K inhibitor. All adverse reactions were tolerated after symptomatic treatment. Her adverse reactions remained tolerable until further progression. LESSONS: Our case fully demonstrates the importance of the early detection of PIK3CA mutations in treatment strategies. In addition, circulating tumor cells-targeted sequencing technology could be used for the concomitant diagnosis of advanced BC and evaluate gene mutations under treatment pressure.