Irradiation of normal, non-malignant bone during cancer therapy can lead to atrophy and increased risk of fracture at several skeletal sites, particularly the hip. This bone loss has been largely attributed to damaged osteoblasts. Little attention has been given to increased bone resorption as a contributor to radiation-induced osteoporosis. Our aims were to identify if radiation increases bone resorption resulting in acute bone loss and if bone loss could be prevented by administering risedronate.

Twenty-week-old female C57BL/6 mice were either: not irradiated and treated with placebo (NR+PL); whole-body irradiated with 2 Gy x-rays and treated with placebo (IR+PL); or irradiated and treated with risedronate (IR+RIS; 30 μg/kg every other day). Calcein injections were administered 7 and 2 days before sacrifice. Bones were collected 1, 2, and 3 weeks after exposure. MicroCT analysis was performed at 3 sites: proximal tibial metaphysis, distal femoral metaphysis, and the body of the 5th lumbar vertebra (L5). Osteoclasts were identified from TRAP-stained histological sections. Dynamic histomorphometry of cortical and trabecular bone was performed. Circulating TRAP5b and osteocalcin concentrations were quantified.

In animals receiving IR+PL, significant (P<0.05) reduction in trabecular volume fraction relative to non-irradiated controls was observed at all three skeletal sites and time points. Likewise, radiation-induced loss of connectivity and trabecular number relative to NR+PL were observed at all skeletal sites throughout the study. Bone loss primarily occurred during the first week post-exposure. Trabecular and endocortical bone formation was not reduced until week 2. Loss of bone volume was absent in animals receiving IR+RIS. Histology indicated greater osteoclast numbers at week 1 within IR+PL mice. Serum TRAP5b concentration was increased in IR+PL mice only at week 1 compared to NR+PL (P=0.05). Risedronate treatment prevented the radiation-induced increase in osteoclast number, surface, and TRAP5b.

This study demonstrated a rapid loss of trabecular bone at several skeletal sites after whole-body irradiation. Changes were accompanied by an increase in osteoclast number and serum markers of bone loss. Risedronate entirely prevented bone loss, providing further evidence that an increase in bone resorption likely caused this radiation-induced bone loss.