STOP-HER2: Stopping Trastuzumab in HER2+ MBC

Inclusion Criteria: * Age ≥18 years * Participants must have histologically or cytologically confirmed unresectable locally advanced or metastatic invasive breast carcinoma that is HER2-positive by American Society of Clinical Oncology/College of American Pathologists 2018 criteria, as assessed by standard institutional guidelines (central testing is not required). Both estrogen receptor (ER)-positive/HER2-positive and ER-negative/HER2-positive will be eligible. * Participants with ER-positive disease should continue endocrine therapy. * Participants must be currently receiving first-line anti-HER2 therapy (any regimen) for metastatic disease and must have been on this therapy for at least 3 years without evidence of progressive disease according to RECIST 1.1 criteria. The following exceptions apply: * Patients with history of brain-only progressive disease previously treated with local therapy (surgery and/or radiation therapy) are eligible, provided they meet all the following study criteria: * Asymptomatic * Not requiring anti-convulsant for symptomatic control * Not requiring corticosteroids * No evidence of interim central nervous system (CNS) progression between the completion of CNS-directed therapy and screening radiographic study * Minimum of 2 years (24 months) between completion of CNS-directed therapy and study start * Participants with history of oligo-progression (i.e., progressive disease of a single lesion) outside CNS treated with local treatment…