Metastatic ER-positive pleomorphic lobular breast cancer: psoas metastasis and stable disease on eribulin

What happened, in summary

This 78-year-old woman had left breast invasive pleomorphic lobular carcinoma diagnosed 8 years before the current presentation. The primary tumor was grade 2, ER-positive, PR-positive, HER2-negative, with an MIB-1 labeling index of 28%, and was staged pT3N1M0, Stage IIIA. She underwent total mastectomy, then chemotherapy with fluorouracil, epirubicin, cyclophosphamide, and paclitaxel, followed by 5 years of anastrozole. Fifteen months after completing initial treatment, a local recurrence was surgically removed from the skin of the left chest wall. That recurrence had similar ER-positive, PR-positive, HER2-negative staining, and she started letrozole. Five months later, multiple bone metastases were found, leading to tamoxifen plus bisphosphonate therapy. One year after bone metastases were detected, she developed mild throbbing pain in the left hip that worsened with walking and bending. CT and MRI showed a left psoas and iliac muscle abnormality that initially looked like an abscess, but she had normal inflammatory markers and no systemic signs of infection. Because the pain was initially mild and inflammatory markers were normal, she was monitored closely before repeat imaging. The lesion grew by 14 mm over 2 months, and PET-CT showed strong uptake with SUVmax 11.2. CT-guided biopsy confirmed metastatic invasive lobular carcinoma with ER-positive, PR-negative, HER2-negative staining and MIB-1 index of 8%. The diagnosis was malignant psoas syndrome from breast cancer metastasis. Palbociclib plus fulvestrant reduced the metastatic lesions, and tramadol with acetaminophen controlled the pain, so radiotherapy was not needed. Seven months later, a new retroperitoneal mass appeared. Eribulin 1.4 mg/m2 was started, and as of May 2025 she remained under outpatient monitoring with stable disease.