INTRODUCTION Acute lymphoblastic leukemia (ALL) accounts for over 25% of childhood cancer cases. Over 80% of patients are cured, but up to 75% of survivors experience chronic health conditions from chemo and radiation therapies, including osteoporosis and an almost doubled fracture risk. It is known that radiation exposure causes bone marrow cell death and induces osteoclastic bone resorption, but animal models for treatment-induced bone loss in ALL patients do not exist. MAJOR AIMS The overall goal of this research is to characterize bone loss after radiation therapy, chemotherapy, and bone marrow transplantation in ALL for treatment development and bone loss prevention. Toward this goal, this study models bone loss in young mice after a marrow-ablating dose of radiation, with aims to quantify bone loss and changes in mechanical properties acutely after exposure, provide evidence of the roles of osteoblasts and osteoclasts post-exposure, and suggested a timeframe for intervention. HYPOTHESIS We hypothesized that a marrow ablating dose of radiation would cause significant amounts of bone loss observable within days of radiation exposure due to elevated osteoclast activity, with significant effects on the mechanical properties of bone. METHODS Sixty female 5-week-old C57Bl6 mice were either exposed to an 8Gy whole body dose of x-rays or kept as non-irradiated controls. Bones were collected at 2-, 4-, and 6-days post-irradiation. Microcomputed tomography (microCT) analysis was performed at the proximal tibia to assess microarchitectural parameters, and finite element analysis (FEA) was performed on the same section to assess structural parameters. Blood serum was analyzed for concentrations of bone formation marker osteocalcin and bone resorption marker TRAP-5b using enzyme-linked immunosorbent assays (ELISAs). RESULTS At 2-days after exposure, serum osteocalcin and trabecular bone volume fraction, thickness, and number were significantly elevated in the irradiated group compared to controls. By 6-days post-irradiation, trabecular bone volume fraction, volumetric bone mineral density, trabecular number, and connectivity density were all significantly reduced in irradiated mice compared to controls, while TRAP-5b levels were significantly elevated. Whole bone stiffness and efficiency remained stable throughout the duration of the study, but cortical bone volume and stiffness both increased at 6-days post-irradiation, while trabecular bone volume and stiffness were dramatically reduced. CONCLUSIONS Radiation exposure in young mice causes an acute increase in bone mass followed by rapid loss. At 2-days post-irradiation, the opposite of our hypothesis was correct. Trabecular bone volume fraction was elevated, coupled with elevated osteocalcin levels and steady structural parameters in irradiated mice compared to non-irradiated controls. Our hypothesis was correct by 6-days post-irradiation, with significant losses of trabecular bone volume fraction and mineral density coupled with elevated TRAP-5b levels, increased cortical volume and stiffness, and loss of trabecular stiffness in irradiated mice. This suggests there is less than a one-week window after irradiation to prevent or mitigate bone loss due to radiation therapy. Future work is needed to explain the increases at 2-days and determine the effects of added bone marrow transplantation and chemotherapy on pediatric cancer patients.