Frequency of Herceptin dose vs. cardiac risk

I read Bubblesmom's question and have read that several participants receive Herceptin every three weeks after their other treatments are finished. My doctor will be giving me Herceptin every week after my radiation for 6 to 12 months. He said the only time he has given Herceptin every three weeks under these conditions is for women who had a long distance to travel to the clinic. He said that when he had given it every three weeks it was at an increased dosage and that the cardiac toxicity risks were becoming unacceptable. He highly recommends Herceptin weekly, even post-treatment, to lessen the cardiac risks (I received Herceptin weekly with Taxol and Carboplatin 3 mos prior to surgery and 3 mos post-surgery also).

For those of you who have either discussed Herceptin every three weeks or have received it every three weeks, what has been the rationale given for this frequency? I almost replied to Bubblesmom with a suggestion for her to ask for Herceptin every week to lessen the cardiac toxicity risks, but I am unsure why others have gotten this dose frequency.