Disseminated Carcinomatosis of the Bone Marrow from Occult Breast Cancer Responding to a Sequence of Endocrine Therapy.

Patients with cancer of unknown primary (CUP) are generally treated with chemotherapy. Bone marrow involvement suggests an advanced stage, and CUP with disseminated carcinomatosis of the bone marrow (DCBM) appears to have a dismal prognosis. However, our case of CUP with DCBM was successfully treated with a sequence of endocrine therapy over a long period. A woman presenting with low back pain was found to have multiple bone metastasis without an identifiable primary tumor on imaging studies. Blood tests revealed anemia and thrombocytopenia. A bone marrow biopsy was performed and showed relatively uniform small cells, strongly positive for estrogen receptor and progesterone receptor expression. We considered chemotherapy to be risky due to bicytopenia and an aromatase inhibitor, letrozole, was initiated. The patient's symptoms and laboratory findings gradually improved and bone lesions almost disappeared on FDG-PET/CT after 1 year of treatment. After 2 years on letrozole, hemoglobin levels and platelet counts had been gradually decreasing. Although she had no symptoms and no significant changes were observed on a CT scan, disease progression was highly likely. Thus, second-line treatment with fulvestrant and palbociclib was commenced, and hemoglobin levels and platelet counts were restored to within the normal ranges. She currently continues to receive fulvestrant and palbociclib over a year later. CUP complicated with DCBM might be metastatic occult breast cancer, and endocrine therapy can be a valuable treatment option if tumors express hormone receptors.