ER/PR-positive breast cancer with separate solitary rib plasmacytoma: disease-free after breast and plasmacytoma treatment

What happened, in summary

This 64-year-old woman sought a second opinion after a recent diagnosis of left breast cancer. Nine days earlier, she had undergone partial mastectomy and sentinel lymph node dissection at another institution. Her family history was notable for breast cancer in her sister and a first cousin, both diagnosed at age 45. Pathology review showed grade 2 moderately differentiated infiltrating ductal carcinoma associated with ductal carcinoma in situ, with lymphatic and perineural invasion. One sentinel lymph node contained macrometastasis with capsular invasion. Tumor markers showed ER positivity, PR positivity, low Ki-67 expression of 8%, and HER2 negativity. A staging CT scan found a lytic lesion in the left 4th rib. This was initially concerning for breast cancer metastasis, even though she had no rib symptoms. To complete breast cancer treatment, she underwent radical axillary dissection, and a rib biopsy was taken during the same procedure. The rib lesion was not breast cancer. It showed immature plasma cells, with CD138 positivity and Ki-67 above 20%, consistent with a solitary plasmacytoma. Whole-body MRI showed no additional lytic lesions, no target-organ damage, and no bone marrow involvement, excluding multiple myeloma. She received adjuvant chemotherapy, radiotherapy, and tamoxifen for breast cancer. Hematology managed the solitary plasmacytoma with 25 sessions of radiotherapy totaling 50 Gy, using a field that overlapped the breast radiation area. More than 5 years after treatment, she remained disease-free, with no signs of recurrence of either breast cancer or plasmacytoma during annual follow-up. This distinction is important clinically because the rib lesion changed from suspected breast metastasis to a separate hematologic diagnosis.