NCI Definition: A lymphoma that arises from the breast. There is no history of extramammary breast lymphoma and ipsilateral axillary lymph node involvement does not exclude the diagnosis of primary breast lymphoma. Most patients present with a painless breast lump. The vast majority of cases are B-cell non-Hodgkin lymphomas. Diffuse large B-cell lymphoma, follicular lymphoma, and extranodal marginal zone B-cell lymphoma of mucosa associated lymphoid tissue are the most common types of primary non-Hodgkin lymphoma of the breast. Primary Hodgkin lymphoma of the breast is rare. 
There is 1 clinical trial for breast lymphoma, of which 1 is open and 0 are completed or closed. Of the trial that contains breast lymphoma as an inclusion criterion, 1 is phase 2 (1 open).
Pembrolizumab is the most common intervention in breast lymphoma clinical trials.
2. The AACR Project GENIE Consortium. AACR Project GENIE: powering precision medicine through an international consortium. Cancer Discovery. 2017;7(8):818-831. Dataset Version 6. This dataset does not represent the totality of the genetic landscape; see paper for more information.