My Husband Was Diagnosed With Prostate Cancer About 8 Years

my husband was diagnosed with prostate cancer about 8 years ago, and was treated with radiation treatments. Now he just has to go back ever year to get his psa test done. my question is there anyway his doctors can tell if his is cured of this. they say his cancer was a very slow growing cancer with a G score of 6 what does this mean? would there be any test the doctors can do to determine if he has been cured of the disease?