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Estimation of bone mineral density with computed tomography performed in radiotherapy planning
Julia Götstedt1,2, Frida Ljung1, Jenny Nilsson3,4
1Department of Medical Radiation Sciences, Institute of Clinical Sciences, Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden.
Background And Purpose:
Cancer patients are at increased risk of impaired bone health. The aim of this study was to develop a method for estimating bone mineral density (BMD) from computed tomography (CT) images acquired for radiotherapy (RT) planning.
Materials And Methods:
We calculated BMD (mg/cm3) in trabecular bone specimens (n = 28) with known elemental composition and simulated their radiodensity at 71 keV, corresponding to 120 kV commonly used in CT scanning. A linear translation function from CT number to BMD was derived and validated with phantom measurements.A set of 178 patient scans acquired with two different CT scanners in RT planning was retrieved. Trabecular bone was defined by placing a spherical volume of interest (VOI) centrally in the 9th-12th thoracic vertebrae. Interobserver variability was evaluated for VOIs delineated by three independent observers. Estimation of BMD for patient data using the translation function was validated against BMD estimated by phantom and internal calibration. Scanner differences were assessed by phantom measurements.
Results:
The derived translation function (BMD = 0.7161 × CT number - 1.212; R = 0.99) agreed in the patient data set with the spine phantom calibration (mean difference 8.7 mg/cm3 with confidence interval (CI) 1.4-16, R = 1.0) and internal calibration (mean difference -1.5 mg/cm3, CI -13.1-10.2, R = 0.99). The delineation method showed a high interobserver agreement (intraclass correlation coefficient 0.99, CI 0.99-1).
Conclusions:
A method for reliable BMD assessment from RT CT images was successfully developed and validated. Future studies should establish clinically relevant BMD cut-off values.
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