CEA Rollercoaster

My dad's cea level has been on a rollercoaster this past year. Diagnosed stage IV in 5/02. His cea prior to June 06 had never been above 18. All of a sudden it went from 5 to 55 in three months. It was 55 in September 06. By October 31, it was 187. Needless to say, we were in shock!! He started treatment (avastin & ?) at the end of October. By the end of November, his cea was 181 (not great but it was going down). By the beginning of January, it was 100 (what an improvement). By the end of January, it was 65 (yeah!!!). As of this week, it is 57.5 (did not go down very much). Does this mean that the drugs are not working as much? Onc does not seem very concerned. Dad will continue treatment until June, take a 6 week break and then get a pet scan. Onc wants a break before pet scan - clearer results. Any thoughts would be appreciated. Thanks!!