Osteosarcoma following single fraction radiation prophylaxis for heterotopic ossification

Abstract

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.

Figure 1

The original 8 × 15 field was digitally reconstructed and superimposed on a CT scan following the patient’s diagnosis of osteosarcoma, and the treatment plan was subsequently recreated. The region of greatest bony destruction was thought to be the epicenter of the tumor, and was specially contoured on the diagnostic CT scan. The volume appears to be located at the edge of the radiotherapy field, including both areas that received >700 cGy and <100 cGy.

Figure 2

Digitally reconstructed radiograph (DRR) based on original portal images. Portal images from patient’s original treatment were recreated and superimposed on CT scan obtained following the diagnosis of osteosarcoma. An open 8 × 15 field was utilized for patient. Treatment was delivered using 6 MV photons, with a source to skin distance (SSD) or 91.5 cm.

Figure 3

Dose-volume histogram of the contoured volume, including the areas of greatest bony destruction. Fifty percent of the treated volume received 240 cGy, although there was a wide variation across the treatment volume, from a maximum of 717 cGy to a minimum of 21.8 cGy with a mean of 333 cGy.