What are the risks of cancer returning within 12 months of original treatment if taking tamoxifen?

My Mum has got her first mammogram, 12 months after originally being treated for cancer. The cancer was picked up the first time through a mammogram, so this is why I'm concerned. They managed to catch it early, it hadn't spread to the lymph nodes, but she did end up having to have a mastectomy after an initial lumpectomy, as they discovered rogue cells around the original tumor. Because of the mastectomy she didn't have to have chemo, but she has been on tamoxifen ever since, as they discovered that it was a hormone receptive tumor that she'd had. What are the statistical chances of the cancer coming back now 12 months later? Opinions from those with medical or first hand experience appreciated.