Metastasis of differentiated thyroid cancer in the subchondral bone of the femoral head: a case report

BACKGROUND Differentiated thyroid cancer (DTC) is relatively rare and can metastasize to both the lungs and bones. The great majority of bone metastases occur in red marrow regions where blood flow is high. Only one patient has been described with direct DTC metastasis to the subchondral bone of the femoral head. CASE PRESENTATION The patient was a 68-year-old Japanese female who had presented with left hip joint pain at age 63 years. At age 51 years, she had been diagnosed with DTC and underwent partial excision. X-rays showed partial femoral head collapse, suggesting osteoarthritis or idiopathic necrosis of the left femoral head. Three years later, a (131) I whole-body scan showed accumulation in the left femoral head, resulting in a diagnosis of DTC metastasis to the left femoral head. Bipolar hip arthroplasty was performed. Examination of the excised femoral head resulted in a final diagnosis of metastasis of follicular thyroid cancer, which was limited histopathologically to the subchondral bone of the femoral head. CONCLUSION Tumor metastasis to the subchondral bone of the femoral head is exceedingly rare. Overall survival of patients with bone metastasis is improved by complete resection. Differential diagnosis of patients with a previous history of DTC who present with femoral head collapse should include bone metastasis of DTC.