Metastatic breast cancer with a blood condition that mimicked multiple myeloma

What happened, in summary

A woman in her 80s sought medical care for lower-back pain that felt different from pain she had experienced before. The burning discomfort radiated from her right hip to her ankle, did not improve with simple pain medicines and disrupted her sleep. She also noticed increasing fatigue and poor appetite, although she remained physically well enough to continue her usual activities.

Imaging showed multiple lytic lesions in her bones. Because blood testing also found a paraprotein, multiple myeloma initially appeared to be a possible explanation for both findings. A bone-marrow assessment instead established monoclonal gammopathy of undetermined significance, or MGUS, which did not fully account for the destructive bone lesions. Further investigation, including another biopsy and a breast biopsy, identified hormone receptor-positive breast cancer that had already spread to bone. Her oestrogen-receptor score was 7 out of 8.

She began letrozole, an aromatase inhibitor, together with monthly zoledronic acid to protect the bones and reduce complications from metastatic disease. The treatment controlled her pain and stabilised the breast cancer. She also remained under haematology follow-up for MGUS, with no evidence that it developed into multiple myeloma.

She lived for approximately 5 years after the metastatic breast-cancer diagnosis. During that period, the cancer remained stable and the bone-related pain was controlled. She later died from causes unrelated to cancer. Her experience shows how metastatic breast cancer and a separate blood-protein abnormality can exist at the same time and initially create a misleading clinical picture.