Model-based downscatter estimation in simultaneous I-131/In-111 dual-isotope SPECT imaging
Yong Du, Bin He, Richard L. Wahl, and Eric C. Frey
Both In-111/Y-90 ibritumomab tiuexetan and I-131 tositumomab are promising radiolabeled antibody therapy regimens for non-Hodgkin's lymphoma. Due to the different ranges of the beta rays from the Y-90 and I-131 therapeutic radioisotopes, Wahl has proposed combination therapy which has the potential to provide improved efficacy. In such a combined regimen, the ability to perform simultaneous quantitative imaging of the In-111 and I-131 labeled antibodies would greatly ease the dosimetry process. In this work we investigate the effects of downscattered I-131 photons into the In-111 image and propose a method for estimating the downscatter that could serve as the basis for a compensation method.
METHODS AND MATERIALS:
We investigated the importance
of photons downscatter of I-131 photons in the object or collimator-detector
system into In-111 energy windows using a previously validated Monte Carlo
For both phantoms, MC simulation indicated that the total counts of downscatter contamination from I-131 into the In-111 energy windows were almost the same as the total counts in the I-131 energy window, indicating that the contamination could significantly degrade the In-111 image quality. The downscatter contamination estimated using the model-based method was in excellent agreement with the MC simulated results from both phantoms.
In simultaneous I-131/In-111 dual isotope SPECT, the image quality and quantitative accuracy for In-111 is significantly degraded by downscatter from I131. This effect could potentially be minimized by giving higher doses of In111 relative to I-131 in such dosimetry studies. Further, We have developed a model-based method that provides an accurate estimation of the downscatter contamination and can serve as the basis of a contamination compensation method to improve treatment plan accuracy for combined isotope radioimmunotherapy.
These following figures show profiles through the
projections of a point source in a water cylinder in the two In111 photopeak
energy windows with HEGP collimator. Here we compared the model-based
I131 window In111 171 keV window In111 245 keV window
MCS estimated MCS estimated MCS
Projection images from a torso phantom. The images obtained from MC simulation and the contamination estimated using the downscatter estimation method in each of the two In-111 energy windows are shown.
through the above images from the In111 energy windows. The profile was
taken across the liver. The MCS and estimated downscatter are shown. The
agreement is excellent.
Horizontal profiles through the above images from the In111 energy windows. The profile was taken across the liver. The MCS and estimated downscatter are shown. The agreement is excellent.