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in the lower extremities, 5% to 16% in the ribs, 4% to 10% in the upper extremities, 1% to 5% in the scapulae, and 1% to 4% in the sternum & clavicles. A gradual shift of RBM towards the central skeleton occurred with aging. Compared to our measured data, the mathematical model underestimated the %RBM in the cranium & mandible and the sternum & clavicles. However, the model overestimated the %RBM in the upper extremities, the ribs, and the pelvis & vertebrae. Trends and rates of change in %RBM for these sites were consistent between our measured data and the mathematical model. Finally, although our %RBM values were similar to those of the mathematical model for the lower extremities and scapulae, the trends and rates of change differed. Conclusion: In most sub-volumes, distributions of RBM in children measured in our study slightly differed from those estimated by the most commonly accepted mathematical model. Despite the limited number of subjects for this study, we were able to validate or find inconsistencies between measured and modeled trends and rates of RBM changes with age. Further studies of high-resolution pTB-MRI should be performed for a broader spectrum of ages as high-efficiency pTB-MRI acquisition becomes manageable. These measured data will be important for aiding clinicians to consider the risk of secondary hematological malignancies in pediatric radiation therapy survivors.

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Purpose: To evaluate factors associated with increased risk of pulmonary toxicity in pediatric patients after myeloablative conditioning using total body irradiation (TBI) followed by allogeneic hematopoietic stem cell transplantation (HSCT). Methods and materials: The records of 129 consecutive pediatric patients (range, 1-21 years) who underwent TBIbased myeloablative conditioning for hematologic malignancies at our institution between January 2003 and May 2014 were reviewed. Although total TBI dose ranged from 10.5 to 14Gy, lung doses were reduced to 10Gy with partial transmission blocks. The TBI dose rate ranged from 5.57cGy/ min to 20.85cGy/min. Results: Pulmonary toxicity developed in 70.5% of patients, which proved to be fatal in 38.5% of those patients. Patients with any type of infection at any point during the follow-up period were more likely to develop pulmonary toxicity (p¼0.009), and patients with bacterial infection during the follow-up period had the highest incidence of pulmonary toxicity (p¼0.038). The presence of any grade of acute graftversus-host-disease (GVHD) was associated with an increased incidence of pulmonary toxicity (p¼0.034), which developed in 94.4% of patients with grade III-IV GVHD (p¼0.001). TBI dose rate was significantly related to the development of pulmonary toxicity (p¼0.0495). Pulmonary toxicity was 3.51 times more likely to develop in patients receiving a TBI dose rate greater than 15cGy/min (p¼0.017). Overall survival was significantly shorter in patients who developed pulmonary toxicity (p¼0.0053). Conclusions: A high incidence of pulmonary toxicity was noted in this large series of homogeneously treated pediatric patients undergoing TBI for allogeneic HSCT. The presence of high grade acute GVHD and infection were the most significant factors contributing to the development of pulmonary toxicity. TBI dose rate should be aimed to be kept below 15cGy/min to decrease the risk of pulmonary injury.