BACKGROUND Among women, breast cancer (BC) is the most prevalent type of cancer and the top cause of cancer deaths. Although non-Hodgkin lymphoma (NHL) is the most prevalent hematological cancer, it is rarely reported synchronous with BC. Moreover, which malignancy appears first can rarely be explained because they are usually detected incidentally while diagnosing and treating other malignancies. This paper reports a case of invasive ductal carcinoma (IDC) concomitant with NHL. METHODS A 35-year-old woman presented with simultaneous IDC in the left breast and NHL in a lymph node in the neck. The patient underwent a modified radical mastectomy for stage IIIA IDC and received rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) chemotherapy for stage I NHL. Treating BC and NHL remains challenging due to their significantly different management, the lack of guidelines for treating BC and lymphoma simultaneously, and uncertainty about whether synchronous tumors should be treated separately as distinct clinical entities or as one disease with treatment covering both. Therefore, the best approach continues to be focusing on the most biologically aggressive malignancies. CONCLUSIONS The enlargement of lymph nodes not in the lymphatic drainage of the primary tumor should be suspected of indicating multiple primary malignancies until proven otherwise. For patients with luminal-B BC, NHL chemotherapy can involve receiving the R-CHOP regimen, including doxorubicin and cyclophosphamide, which can help to mitigate BC.