Heterogeneity in EGFR Mutations: Explaining a "Mixed Response" to EGFR TKIs in Patients with a Reported Mutation

Article

One of the questions that comes up fairly frequently is what to make of a "mixed response" to systemic therapy: after several weeks or months of treatment, a scan shows some areas of known disease shrinking, but others are growing.  Why might this happen? What does it mean? And what should it lead us to do?