Radiotherapy for prophylaxis of heterotopic ossification (HO) is commonly used in high risk patients following orthopedic surgery. While treatment is effective and can prevent morbidity associated with HO, with any dose of radiation there is a concern of a radiation induced malignancy. Here we a report a case of radiation induced osteosarcoma which developed 11 years after a single fraction of 700 cGy. We performed dosimetric analysis by superimposing the patient's original treatment field on a CT scan performed after the diagnosis. The radiotherapy dose for this patient is lower than classically reported for radiation induced sarcomas. We identified greatest bony destruction that was thought to be the epicenter of the tumor, and this was specially contoured on the diagnostic CT scan. This volume appears to be located at the edge of the radiotherapy field. Fifty percent of the treated volume received 240 cGy, the mean dose was 333 cGy. There was a variation across the treatment volume, between 21.8 cGy and 717 cGy. While a rare complication, we stress the importance of informing regarding the risk of a radiation induced malignancy following HO prophylaxis.