A Study of T-DXd in Participants With or Without Brain Metastasis Who Have Previously Treated Advanced or Metastatic HER2 Positive Breast Cancer

Inclusion: * Participants should have pathologically documented breast cancer that is: unresectable/advanced or metastatic; confirmed HER2-positive status expression as determined according to American Society of Clinical Oncology/College of American Pathologists guidelines * Participant must have either: no evidence of BM, or untreated BM on screening contrast brain magnetic resonance imaging/ computed tomography (MRI/CT) scan, not needing immediate local therapy or previously-treated stable or progressing BM * Participants with BMs must be neurologically stable * For participants requiring radiotherapy due to BMs, there should be an adequate washout period before day of first dosing: * ≥ 7 days since stereotactic radiosurgery or gamma knife * ≥ 21 days since whole brain radiotherapy * Eastern Cooperative Oncology Group performance status 0-1 * Previous breast cancer treatment: radiologic or objective evidence of disease progression on or after HER2 targeted therapies and no more than 2 lines/regimens of therapy in the metastatic setting * Participant with the following measurable: at least 1 lesion that can be accurately measured at baseline as ≥ 10 mm in the longest diameter with CT or MRI and is suitable for accurate repeated measurements; or following Non-measurable diseases: Non-measurable, bone-only disease that can be assessed by CT or MRI or X-Ray. Lytic or mixed lytic bone lesions that can be assessed by CT or MRI or X-ray in the absence of measurable disease as de…