Longitudinal Effects of Single Hindlimb Radiation Therapy on Bone Strength and Morphology at Local and Contralateral Sites

Abstract

Radiation therapy (RTx) is associated with increased risk for late-onset fragility fractures in bone tissue underlying the radiation field. Bone tissue outside the RTx field is often selected as a "normal" comparator tissue in clinical assessment of fragility fracture risk, but the robustness of this comparison is limited by an incomplete understanding of the systemic effects of local radiotherapy. In this study, a mouse model of limited field irradiation was used to quantify longitudinal changes in local (irradiated) and systemic (non-irradiated) femurs with respect to bone density, morphology, and strength. BALB/cJ mice aged 12 weeks underwent unilateral hindlimb irradiation (4 Â 5 Gy) or a sham procedure. Femurs were collected at endpoints of 4 days before treatment and at 0, 1, 2, 4, 8, 12, and 26 weeks post-treatment. Irradiated (RTx), Contralateral (non-RTx), and Sham (non-RTx) femurs were imaged by microcomputed tomography and mechanically tested in three-point bending. In both the RTx and Contralateral non-RTx groups, the longer-term (12-to 26-week) outcomes included trabecular resorption, loss of diaphyseal cortical bone, and decreased bending strength. Contralateral femurs generally followed an intermediate response compared with RTx femurs. Change also varied by anatomic compartment; post-RTx loss of trabecular bone was more profound in the metaphyseal than the epiphyseal compartment, and cortical bone thickness decreased at the mid-diaphysis but increased at the metaphysis. These data demonstrate that changes in bone quantity, density, and architecture occur both locally and systemically after limited field irradiation and vary by anatomic compartment. Furthermore, the severity and persistence of systemic bone damage after limited field irradiation suggest selection of control tissues for assessment of fracture risk or changes in bone density after radiotherapy may be challenging.

Limited field radiation therapy results in decreased bone fracture toughness in a murine model

“…Numerous animal models have been used to study the effects of irradiation (total body, limited field, and focal) on changes to bone architecture and tissue density [ 11 – 22 ]. Human pathologic responses to radiation therapy (RTx), including osteocyte death, trabecular bone resorption, marrow adiposity, and increased bone fragility, are all replicated in mouse models of fractionated limited field RTx [ 12 , 14 , 21 – 23 ].…”

“…Human pathologic responses to radiation therapy (RTx), including osteocyte death, trabecular bone resorption, marrow adiposity, and increased bone fragility, are all replicated in mouse models of fractionated limited field RTx [ 12 , 14 , 21 – 23 ]. A pattern of metaphyseal trabecular bone resorption and diaphyseal cortex thinning has been observed as well [ 11 , 21 , 22 ]. Irradiation induced an early and persistent (4–26 weeks) decrease in mouse femur bending strength and stiffness [ 11 ].…”

“…A pattern of metaphyseal trabecular bone resorption and diaphyseal cortex thinning has been observed as well [ 11 , 21 , 22 ]. Irradiation induced an early and persistent (4–26 weeks) decrease in mouse femur bending strength and stiffness [ 11 ]. Furthermore, material strength of cortical bone (flexural strength) was also diminished following RTx [ 11 ].…”

“…Irradiation induced an early and persistent (4–26 weeks) decrease in mouse femur bending strength and stiffness [ 11 ]. Furthermore, material strength of cortical bone (flexural strength) was also diminished following RTx [ 11 ]. These findings suggest that changes to both structural geometry (femur diaphyseal size) and tissue properties (cortical bone strength and resistance to fracture) contribute to the overall loss of femur bending strength post-RTx.…”

Bench to Bedside: Animal Models of Radiation Induced Musculoskeletal Toxicity

“…Multiple investigators have reported increased osteoclast number and activity post-exposure [43,46,49,[52][53][54], that can lead to early bone loss [54]. Oest et al investigated osteoclast numbers and mineral apposition rate (MAR) over time after focally irradiating the hind-limbs of BALBc/J mice with 20 Gy in five fractions [55]. Histologic samples were analyzed at 1, 2, 4, 8, 12, and 26 weeks after RT, and it was found that early osteoclast numbers were transiently increased by week 2 but then at later time points, a persistent osteoclast depletion was observed.…”