Local irradiation alters bone morphology and increases bone fragility in a mouse model
Introduction
Radiation therapy (RTx) is widely used in the treatment of metastatic tumors to bone and soft tissue tumors adjacent to bone. Unfortunately, late bone fractures coincident with the field of external beam radiotherapy are common, even occurring in cases where there was no prior trauma to the bone (Lin et al., 1998). In fact, the cumulative incidence of insufficiency fracture (IF) following RTx can be upwards of 45%, as seen in women treated for cervical cancer (Kwon et al., 2008).
Low bone mineral density (BMD) is often used to screen for fracture risk in older patients (Griffith and Genant, 2008, Lochmuller et al., 1998). However, in a study of IF’s post-RTx Konski and Sowers (1996) found that BMD at the fracture locations was not reduced. Inoue (2003) reported similar findings in an RTx patient with normal radiographs who complained of pain, but was later found to have suffered an IF. BMD sparing effects have been noted in a focal irradiation rat model as well (Fukuda and Hasegawa, 2000). These discrepancies raise the question of whether irradiation changes the normal relationship between bone density measures and fracture risk. Improvements in our understanding of the effects of radiation therapy on bone from a compositional and mechanical standpoint could aid in the development of methods to detect bones at risk of fracture and contribute to strategies to mitigate the negative effects of radiotherapy.
Animal models provide an attractive platform to study the time and dose response of radiotherapy. Whole body radiation exposure in mice (Bandstra et al., 2008, Hamilton et al., 2006, Willey et al., 2008) results in marked trabecular bone loss, even with low doses of radiation. However, since models involving whole body irradiation may affect the endocrine axis as well as bone, they may not be relevant to the clinical scenario of focal radiotherapy. Studies of hind limb irradiation using high radiation doses have been performed in rats and rabbits (King et al., 1979, Maeda et al., 1988, Nyaruba et al., 1998, Sawajiri and Mizoe, 2003, Sugimoto et al., 1991), but none have investigated the structure–material–strength relationship and used the information to estimate fracture risk.
In this work we evaluated the effect of focal irradiation on bone by quantifying morphology and whole bone strength with time and addressed four research questions: (1) Does a hind limb irradiation mouse model retain bone density but exhibit decreased bone strength? (2) Do changes in bone morphology and density correlate with changes in bone strength after irradiation? (3) Can finite element models that account for changes in bone structure and material distribution, and include provision for failure, better explain observed changes in bone strength after irradiation, compared with morphology measures? (4) Does an embrittled material failure model more accurately predict bone strength compared with an established constant–strain failure model?
Section snippets
Small animal radiation therapy
Under an IACUC-approved protocol, female Balb/c mice (12–14 weeks of age) were anesthetized with an intramuscular injection of Telazol® (45 mg/kg) and xylazine (7.5 mg/kg), and placed in dorsal recumbency with the right hind limb extended onto a Plexiglas positioning jig. Limbs were irradiated with a nominal surface dose of 5 Gy (n=21) or 20 Gy (n=22) using a therapeutic X-ray unit (MGC-30, Phillips Medical Systems) operating at an effective dose rate of 1.6 Gy/min at a source–object distance of 15
Results
Focal radiation therapy was well tolerated by the mice. Although most of the mice developed some degree of alopecia and erythema over the irradiated site, none experienced clinically significant complications from the procedure, and no mice were lost as a result of anesthesia. Body weights were recorded on a weekly basis, and there was no evidence of systemic ill effects in irradiated mice.
Discussion
The focal irradiation model implemented here mimicked clinical observations of trabecular bone loss (Howland et al., 1975), retained total BMD of bone over time (Konski and Sowers, 1996), and increased fracture risk after RTx via reduced bone strength. BMD is commonly used to assess fracture risk in osteoporosis (Szulc et al., 2005) but is not always well correlated with fracture risk. In a study of post-menopausal women, the majority of fractures were found to occur in patients without
Conflict of interest statement
This work was supported by grants (Kenneth A. Mann and Matthew J. Allen) from the Carol M. Baldwin Breast Cancer Foundation. These sponsors were not involved in the design, collection, or analysis of included data.
Acknowledgements
The authors would like to acknowledge the contributions of Erica R. Fischer. This work was supported by grants (K.A.M and M.J.A.) from the Carol M. Baldwin Breast Cancer Foundation. These sponsors were not involved in the design, collection, or analysis of included data.
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