Case 34 Answer 5
This was a mucinous carcinoma. They tend to be round or oval solid masses because they make mucin. Tubular carcinomas present as small spiculated masses because they form spiculated tubules. This could be an adenoid cystic carcinoma, but these are rare compared with mucinous tumors. DCIS rarely presents as a well defined mass.
If Infiltrating ductal carcinoma, NOS had been offered as an answer, this would have been the best choice. Though IDC-NOS is usually spiculated, it can be well-defined. Since IDC-NOS is the most common type, it is the most likely cancer found when a well-defined mass is biopsied.