ERBB2 and neoplasm: Some avascular metastases of the brain may be outside the BBB and therefore have poor access to trastuzumab, while larger lesions with active angiogenesis and blood vessel support may have a blood-tumor barrier (BTB) with greater trastuzumab delivery, however we have shown that microregional distribution of trastuzumab can remain highly heterogeneous and incomplete even when delivered by vessels within HER2-positive lesions.