Postoperative hip irradiation in prevention of heterotopic ossification: causes of treatment failure.

Over 4 years, 33 patients (mean age, 52 years) underwent hip surgery followed by single-fraction 700-cGy radiation therapy (RT). Records, port films, and plain radiographs were reviewed to assign Brooker classification levels for severity of heterotopic ossification (HO) and assess the amount of new HO developing after surgery. Nineteen patients (58%) developed radiographic evidence of new HO after surgery. All five patients with three or more risk factors and none of 12 with postoperative Brooker level 0 (no radiographic HO) developed new HO. Ten of 16 treated on postoperative day 1, six of 10 on postoperative day 2, none of three on day 3, two of three on day 4, and one of one on day 5 developed new HO. New HO developed outside the irradiated volume in 11 patients. Surgeons should remove all heterotopic bone whenever possible; RT should be administered within 3 days after surgery; portals should cover all potentially involved soft tissue; and future studies should evaluate larger single fractions (900-1,000 cGy) for prevention of HO in patients with three or more risk factors and/or radiographic evidence of residual HO.