preventing cancer reoccurence

I just completed 5 years on arimidex and have been asked to be part of a study and go on it for another 5 years.  Has cancer reoccurred in patients while they have been on arimidex, not due to the drug, but due to the cancer.  In other words, does taking arimidex prevent cancer from reoccurring while a patient is on it?

[Cleveland Clinic]

Dear baba_g:  The purpose of arimidex (an other aromatase inhibitors) is to try to prevent cancer recurrence – not only for the duration that the drug is used but in years beyond.  The purpose of the clinical trial is to ask the question if 10 years is better than 5.  While there are recurrences in people who are taking arimidex and other medications, the data is looked at not individually but in large numbers to see if overall, 10 years would be better than 5.  

[baba_g]

what is the percentage of reccurrence while on medication and, statistically, how much less than not being on it?