Why would a doctor want a core needle biopsy? What does this mean they are thinking?

Definitive Diagnosis. In this day and age, we have many noninvasive tests that can get us "in the ballpark" when trying to make a diagnosis. However, when it comes to cancer diagnosis, nothing replaces looking at tissue under the microscope; this requires a biopsy. Thankfully, needle biopsies can usually be done under local anesthesia alone (depending on site) without the need for surgery. Good luck!