There is a difference between Peginterferon alfa 2a & Peginterferon alfa 2b
According to the chart it looks like 120 is the amount for your weight:
It is a chart on this link.
Here is a thread
This is from the web:
Interferon is a drug that mimics a protein in the body that the immune system uses to destroy viruses. Pegylated interferon is a modified form of the drug that is able to stay in the body longer, which provides better treatment. Two kinds of pegylated interferons are available -- peginterferon alfa-2a (also called alfa-2a) and peginterferon alfa-2b (also called alfa-2b). The main difference between these two pegylated interferons is in the dosing.
Alfa-2a is used as a fixed weekly dose. Alfa-2b is used as a weekly dose based on the weight of the patient. There are other differences between the two drugs, but they are mostly technical and are used by the prescribing physician. For example, alfa-2a has a relatively constant absorption after injection and is distributed mostly in the blood and organs. But alfa-2b has a rapid absorption and a wider distribution in the body.
Until January 2008, no data directly compared these two medicines. That has changed with a large clinical study known as IDEAL. This should provide interesting results that will allow physicians additional insight into these powerful chronic hepatitis C therapies.