• Enrolled subjects will be asked to undergo 18F-FDG PET/CT imaging (if not performed within the previous 90 days) and 89Zr-trastuzumab PET/CT imaging at baseline prior to the start of monotherapy MT-5111 treatment. Standard of care 18F-FDG PET/CT examinations performed within 90 days of 89Zr-trastuzumab administration can be used in place of a study specific 18F-FDG PET/CT scan. Baseline 89Zr-trastuzumab imaging should be completed no more than 30 days prior to initiating treatment with MT-5111.
• Cold trastuzumab and 89Zr-trastuzumab will be administered at Visit 1 and the 89Zr-trastuzumab PET/CT (Visit 2) will take place 4 days (+/-1 day) after administration of 89Zr-trastuzumab.

-To minimize uptake of the 89Zr-trastuzumab in normal tissues, immediately prior to planned 89Zr-trastuzumab injection patients will receive a 50 mg dose of "cold" unlabeled trastuzumab intravenously (IV). The administration of a fixed small dose of unlabeled antibody to improve tumor-to-normal-tissue (T/N) uptake of the radiolabeled antibody is standard in this type of imaging procedure.

-SUVmax is maximum standardized uptake value