Collision tumor: invasive ductal carcinoma in association with mucosa-associated lymphoid tissue (MALT) lymphoma in the same breast.

Synchronous occurrence of multiple neoplastic processes is uncommon and the relationship between breast cancer with lymphoproliferative diseases is unusual as well. Furthermore, breast involvement by malignant lymphoma is a rare event and primary breast mucosa-associated lymphoid tissue (MALT) lymphoma is even rarer. We report a patient with synchronous occurrence of malignant lymphoma of MALT type and ductal carcinoma of the breast, presenting as "collision tumor," invading each other and occurring as a single mass in the breast.