Abstract
CASE: Lynch syndrome (hereditary nonpolyposis colorectal cancer) is associated with extracolonic manifestations, but skeletal tumors are rare. Our patient, a 12-year-old boy with Lynch syndrome, developed osteosarcoma of the left femur. Treatment included cytotoxic chemotherapy, wide resection, and pembrolizumab. Two years later, he developed an aggressive lesion in the contralateral femur that was thought to be metastatic osteosarcoma but which histology revealed to be Langerhans cell histiocytosis. CONCLUSION: This case underscores the importance of advanced testing in patients with osteosarcoma and poor response to chemotherapy, and of tissue sampling when patients with a primary malignancy develop new bone lesions. LEVEL OF EVIDENCE: IV.