TITLE:

Model-based downscatter estimation in simultaneous I-131/In-111 dual-isotope SPECT imaging

 

AUTHORS:

Yong Du, Bin He, Richard L. Wahl, and Eric C. Frey

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ABSTRACT

 

PURPOSE:

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 (MC) simulation code. Projection data were simulated for a GE HEGP collimator using a 20% I-131 energy window centered at 364 keV and two 14% In-111 energy windows centered at 171 keV and 245 keV. We simulated a point source in a water-filled cylinder and an extended source in the 3D NCAT mathematical phantom. We have developed a method for modeling the downscatter that models object scatter using the effective source scatter estimate method, including contributions from all the I-131 gamma photon emissions. Photon interactions with collimator-detector system, including the penetration and scatter components due to high-energy I-131 photons, were estimated using a collimator-detector response calculated from MC simulations of point sources in air at various distances from the face of the collimator. The method was validated by comparing estimated downscatter into the In-111 energy windows with that obtained from the above-described MC simulations.

 

RESULTS:

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.

 

CONCLUSIONS:

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.

 

FUNDING SOURCES:

R01 Eb000288

 

The Monte Carlo simulated energy spectrum of I131 point source in a water cylinder measured with HEGP collimator. Contributions from the most abundant I131 gamma photon emissions were simulated. The imaging energy windows of In111 and I131 are marked in the image. From this spectrum we can see there is a significant amount of downscatter contamination in In111 windows from I131 emissions.

 

 

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 estimation and Monte Carlo simulated (MCS) results. We can see the good agreement even in the tails far away from the source position.

 

 

 

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.

 

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.