Hypofractionated Radiation Therapy for Progressive Heterotopic Ossification: The Relationship between Dose and Efficacy

Purpose: We sought to evaluate treatment outcomes after hypofractionated short-course radiation therapy (RT) for progressive heterotopic ossification (HO). Methods and Materials: Nine consecutive patients were treated between January 2016 and December 2018. None had received previous preventive RT. RT was performed to prevent further HO progression with one of the following dose-fractionation regimens: 7 Gy × 2 fractions, 9 Gy × 1 fraction, 6 Gy × 2 fractions, 6 Gy × 3 fractions, 8 Gy × 2 fractions, or 7 Gy × 3 fractions. Results: All patients were male, with a median age of 30 years (range, 16-55). Eight patients presented with grade III Brooker classification. The most commonly involved site was the hip, followed by femur and knee. With a median assessment time of 7.1 (range, 5.1-23.1) months, eight patients achieved decreased HO, with five showing a dramatic (≥50%) reduction. Among the five excellent responders, a 21 Gy with 7 Gy per fraction schedule was used in four patients. HO status was maintained at the final median follow-up of 11.6 months (range, 6.2-36.1), and Brooker's grade was improved in three patients. No significant RT-related toxicity was noted. Conclusions: The current RT scheme was effective for the management of progressive HO. It is speculated that higher RT doses could result in the superior efficacy of progressive HO.