| Summary: | Purpose: Treatment planning for x-ray activated Auger electron radiotherapy requires
knowledge of the spatial distribution of Auger electron-producing target atoms in DNA;
iodine is a candidate atom. Because planning uses computed tomography (CT) data to
show anatomy, obtaining the target atoms' distribution with CT methods is an attractive
goal. This study evaluates the ability of two available CT systems to measure the target
atoms' spatial distribution.
Method and Materials: A polychromatic desktop CT scanner and a synchrotron
monochromatic CT system acquired images of iodine concentrations in water, ranging
from 0.03-10 mg/ml. The polychromatic scanner was operated at 40 kVp while the
synchrotron system was operated at 32.5 keV and 33.5 keV. Calibration curves of
Hounsfield units (HU) vs. iodine concentration were obtained from each CT set, with
minimum detectable iodine concentration defined as the smallest concentration
distinguishable from water with contrast-to-noise ratio of 3. K-edge subtraction (KES)
analysis was applied to the synchrotron CT data as another quantification method. To
determine if iodine uptake could be quantified in vitro, Chinese hamster ovary (CHO)
cells grown with iododeoxyuridine (IUdR) were imaged with the synchrotron. Iodine
uptake was measured with the HU calibration curve and KES.
Results: The expected iodine concentration for breast cancer in vivo is estimated to be
0.06 mg/ml for IUdR. The minimum detectable iodine concentration was 0.1 mg/ml for
the 40 kVp polychromatic CT data and 0.1 mg/ml for the synchrotron CT at 33.5 keV;
minimum detectability using KES was 0.25 mg/ml. Thus, these current systems could not
visualize the estimated target concentration. The measured iodine concentration in the
cells was 0.21±0.04 mg/ml using the HU calibration curve and 0.20±0.01 mg/ml using
KES, compared to an expected concentration in DNA of 0.001 mg/ml.
Conclusions: Using the current acquisition methods, these CT systems proved unable to
measure the expected concentration. Improvements may be possible by modifying the
acquisition parameters. From the cell image results, CT imaging for treatment planning
will quantify both DNA-incorporated iodine and intracellular unincorporated iodine; if
the two amounts can be shown to have a stable proportion; CT quantification methods
may be satisfactory for treatment planning.
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