Do you recommend using a ctDNA assay for a patient with HER2+ metastatic breast cancer in a continued CR to guide decision about whether to stop HER2-directed therapy?

This is a good question and is not a rare situation in the management of HER2+ metastatic breast cancer. Radiographic complete response occurs with first-line treatment in 6-15% (CLEOPATRA, GIM-14, and other trials). Currently, the standard of care is to continue anti-Her2 therapy indefinitely. However, many patients/physicians elect to stop their anti-HER2 therapy at some point. ctDNA has a very high positive predictive value for the presence of disease, and therefore, I would continue therapy. Whether I would modify treatment is an open question. The caveat is the possibility of CHiP, so if a DNMT or a p53 mutation is seen, it may not reflect tumor DNA, particularly in an older individual. It is unknown what the negative predictive value ...