Postoperative irradiation for the prevention of heterotopic bone: analysis of different dose schedules and shielding considerations

Ninety-seven high risk hips were irradiated postoperatively for prevention of heterotopic bone (HTB) in the UCLA Department of Radiation Oncology from 1980 to 1988. Ninety-two hips in 82 patients were eligible for analysis with a minimum follow-up of 2 months and a median follow-up of 10 months. Forty-nine of the hips had porous coated ingrowth prostheses. From 1980 to 1986, 2 Gy fractions were used to deliver 20 Gy (8 hips), 12 Gy (1 hip), and 10 Gy (27 hips). Since December of 1986, 38 hips received 8 Gy in two increments and 18 hips received a single 7 Gy fraction. All porous ingrowth components were shielded with custom blocks. Six out of 92 hips developed clinically significant (Brooker grade 3 or 4 heterotopic bone). There was one clinically significant failure in 78 hips (1.3%) when irradiation was initiated before post-operative day (POD) #6 and shielding was properly placed. One clinical failure occurred in 38 hips which received 8 Gy in two increments. One clinical failure occurred out of the 18 hips treated with 7 Gy in one fraction. This failure could be related to block malposition. There were four clinical failures in the 36 hips treated with 2 Gy fractions to total doses of 10 Gy, 12 Gy, or 20 Gy. Three of these failures were associated with initiation of treatment after POD #5, and the fourth was related to block malposition. Unshielded trochanteric osteotomies resulted in five migrations and seven fibrous unions for a total non-osseous union rate of 12/36 (33%). Shielding of the remaining 28 trochanteric osteotomies resulted in a non-osseous union rate of 7% (0 migrations and 2 fibrous unions). There were no failures of union of components, and the only side effects noted in the series were the five trochanteric migrations. In conclusion, the use of 8 Gy in two increments or 7 Gy in one fraction was found to be as efficacious as conventional 2 Gy fractionation schemes with no increase in side effects. For optimal results, treatment should be implemented prior to POD #5 with shielding of the trochanteric osteotomy. Postoperative irradiation to prevent HTB can be used in hips with porous components using properly placed blocks to shield the porous region.