Here is a link to an abstract about the clinical pharmacology of Herceptin:
http://www.ncbi.nlm.nih.gov/pubmed/18690878
It states the half-life of Herceptin as 28 days. The half-life of a substance is the period of time required for the concentration or amount of drug in the body to be reduced by one-half.
So, at 21 day intervals, you are maintaining a somewhat higher overall concentration of Herceptin in the body than at 28 day intervals. Neither is long enough for the drug to be completely out of the body.
Hope this is helpful.
Hopeful