Dear mjpsmom, A mucocele-like tumor is a benign (non-cancerous) tumor consisting of cysts of mucinous material that have ruptured, leaking secretions into surrounding tissues. The finding of mucocele-like tumor cells has been associated with findings of DCIS, as well as invasive carcinoma, predominantly mucinous carcinoma. This is why re-excision of surrounding tissue is done, to be sure of what is involved and whether or not there are any areas of mucinous carcinoma. Mucinous carcinoma is a rare type of invasive breast cancer. It has a slightly better prognosis and a slightly lower chance of metastasis than invasive ductal or lobular cancers of the same size.
A mucocoele is a collection of mucus, for any of several reasons. It's not a tumor, so "mucocoele-like tumor" is a strange term. However, the issue would be, as you seem to understand, whether there's a mucinous carcinoma there, causing the mucus. Mucinous carcinoma is generally invasive, but is a much less aggressive form of cancer than the usual breast cancers. So it boils down to this: a stereotactic biopsy is a good, but incomplete sample. In order to explain the mucus, they want to more widely excise the area to determine if there might be mucinous cancer there. A mucocoele, per se, is not harmful nor pre-cancerous. It's about being sure what's going on. If there were mucinous cancer, it would need complete excision.