With recent more extensive screening, many men are now diagnosed as early stage gleason 6 cancers, with the resulting debate about over diagnosis and over treatment of miniscule amounts of cancer.
Dr Epstein and Johns Hopkins have several studies, and published criteria for "insignificant" cancers, which was defined as PSA density below .15*, no more than two biopsy cores with cancer, no biopsy with more than 50% cancer and no high-grade cancer.
*PSA density is PSA figure divided by the gland size. A larger gland naturally produces more PSA.
Biopsy readings are subjective: what looks like cancer to one person might appear more normal to another, which is why biopsy slides should be sent for a 2nd reading.
12% in one core of 13 does not indicate a high volume of cancer. If your gleason scores are not in the high risk category, it's worth waiting for now to further consider your options if you're doubtful. Best to feel sure you've made the right choice and have selected the best doctor available.
Whether you would want to use active surveillance medium to long term is another question. Not everyone can handle that approach, and many want to have it out yesterday.