Secondary distal femoral osteosarcoma in a non-irradiated survivor of bilateral retinoblastoma.

Survivors of bilateral retinoblastoma are at significantly increased lifetime risk of secondary malignancies, particularly osteosarcoma. While radiotherapy further amplifies this risk, osteosarcoma may develop in non-irradiated patients due to underlying genetic susceptibility. A 16-year-old female was referred for evaluation of progressive left knee swelling following minor trauma. Her past medical history was significant for bilateral retinoblastoma diagnosed in infancy, managed with right eye enucleation and systemic chemotherapy, without exposure to radiotherapy. She had remained in complete remission for the past 14 years. Radiographs demonstrated heterogeneous distal femoral architecture with multilamellar periosteal reaction and soft tissue thickening. MRI revealed an aggressive metaphyso-epiphyso-diaphyseal lesion with cortical destruction, intra-articular extension, and soft tissue invasion. A separate intramedullary tibial focus suspicious for a skip metastasis was also identified. Staging showed no pulmonary metastases. Histopathological and immunohistochemical analysis confirmed conventional osteosarcoma. This case illustrates the development of secondary osteosarcoma in a non-irradiated survivor of bilateral retinoblastoma. Although genetic testing was not performed, bilateral disease strongly suggests germline RB1 mutation, a well-established risk factor for osteosarcoma. Persistent musculoskeletal symptoms in such patients warrant prompt radiologic evaluation to avoid diagnostic delay.