Osteoblastic Osteosarcoma Arising beneath an Osteochondroma in an 11-Year-Old Male with Multiple Hereditary Exostoses

INTRODUCTION: Multiple hereditary exostoses (MHE) is a rare autosomal dominant disorder characterized by the presence of multiple skeletal deformities. They are painless slow-growing lesions. Malignant transformation tends to occur later in adulthood and has only been seen in 1–5% of patients. OBJECTIVE: We describe the clinical, radiological, and pathological characteristics of a child with MHE who developed osteoblastic osteosarcoma beneath an osteochondroma. CASE PRESENTATION: An 11-year-old male Rwandan presented to our hospital with a two-week history of a dull persistent pain in his left distal femur and loss of weight and appetite. There was no relief with pain killers. He was a known case of multiple hereditary exostoses diagnosed at age 3. He began experiencing mild symptoms 6 months prior to admission which worsened in the last two weeks prior to his admission. On examination, he had multiple palpable bony swellings bilaterally on the proximal humeri and distal femurs. X-rays showed multiple exostoses and MRI showed a lesion with heterogeneous signal intensities that suggested malignant transformation. At surgery, a necrotic lesion beneath the exostosis was excised and sent for histopathological analysis which confirmed osteochondroma with an osteoblastic osteosarcoma in the marrow cavity. Chemotherapy and limb-salvaging surgery were done and he has recovered well. CONCLUSION: Osteosarcomas arising at sites of MHE have not been previously reported in Africa. These tumors rarely undergo malignant transformation.