Quantification of the trabecular bone volume fraction for bone marrow dosimetry in molecular radiotherapy by using a dual-energy (SPECT/)CT.

A complete characterization of spongiosa (bone marrow plus trabecular bone) is required to calculate the absorbed dose to active bone marrow. Due to the complex microanatomy, it is necessary to apply non-conventional imaging methods in nuclear medicine. The aim of this study is validating a phantomless quantification method using dual-energy quantitative computed tomography (DEQCT) for the quantification of trabecular bone volume fraction for bone marrow dosimetry in molecular radiotherapy. First, a phantomless quantification method (mass fraction method) based on x-ray beam and detector sensitivity was validated in an integrated dual energy SPECT/CT and in a dual source computed tomography (DSCT) system for comparison. The validation was performed in a phantom consisting of different water, fat and hydroxyapatite compositions. Moreover, the European spine phantom (ESP) was used to simulate the spine geometry. Bone mineral content (BMC) of the whole vertebra and bone mineral density (BMD) in the spongiosa region of each phantom vertebra were measured using DEQCT and dual energy x-ray absorptiometry (DEXA). Lastly, BMC was measured in a patient using DEQCT and DEXA. Measured values of hydroxyapatite fraction and nominal values in the homemade phantom showed a good correlation. The relative error remained below 14.2%. Quantification of BMC (in whole vertebra) and BMD (in spongiosa) in the ESP showed a good agreement between measured values and nominal values. The relative error remained between 0.7% and 7.5% for BMC, 1.1% and 7.7% for BMC, 5.4% and 32.0 for BMD, and 59.4% and 10.0% for BMD. Quantification of BMC in lumbar vertebrae 1 and 2 of a patient showed relative errors of 7.6% and -8.4% between DEXA and DSCT. Our study shows that DEQCT using a mass fraction method (phantomless) enables quantification of hydroxyapatite in a clinical nuclear medicine setting. An overestimation of the hydroxyapatite volume fraction was observed in all quantified regions, in particular in the spongiosa region of ESP. This result might be related to insufficient information about the x-ray spectra and the detector sensitivity function.