Bone density changes after radiation for extremity sarcomas: exploring the etiology of pathologic fractures.

PURPOSE: The incidental irradiation (RT) of adjacent bone that takes place during treatment of soft tissue extremity sarcomas is generally presumed to "weaken" the bone by decreasing its density, which subsequently increases the risk for pathologic fracture. This investigation intended to assess the relative effects on bone density of both RT and diminished mechanical loading secondary to tumor-induced and therapy-induced functional extremity impairment. METHODS AND MATERIALS: 19 patients treated with surgical excision and RT for soft tissue extremity sarcomas had bone density measured using dual energy X-ray absorptiometry at four sites: the irradiated (A) and contralateral (B) bone, and an uninvolved bone (C) in the treated extremity and its contralateral counterpart (D). Analysis included (1) [A-B], (2) [C-D], (3) [(A-B), - (C-D)], and (4) [(A-B)/B - (C-D)/D].
RESULTS: The mean bone density for all irradiated sites was increased 0.08 ± 0.22 g/cm(2) (variance) compared to the contralateral unirradiated side when corrected for weight-bearing effects (3). An average increase in bone density of 9 ± 22% (p = 0.08) was also seen when the differences were divided by individual control densities to normalize variation in density of different anatomic sites (4).
CONCLUSIONS: RT does not routinely decrease bone density when corrected for weight bearing or mechanical effects. The pathogenesis for the known increased risk of pathologic fracture in irradiated bones is likely multifactorial, including possible alterations in bone remodeling that can result in stable, or even increased, bone density. Further clinical and basic studies are needed to confirm our unexpected findings.