| 1. |
- Dewaraja, Y K, et al.
(författare)
-
Accurate dosimetry in I-131 radionuclide therapy using patient-specific, 3-dimensional methods for SPECT reconstruction and absorbed dose calculation
- 2005
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 46:5, s. 840-849
-
Tidskriftsartikel (refereegranskat)abstract
- I-131 radionuclide therapy studies have not shown a strong relationship between tumor absorbed dose and response, possibly due to inaccuracies in activity quantification and dose estimation. The goal of this work was to establish the accuracy of I-131 activity quantification and absorbed dose estimation when patient-specific, 3-dimensional (3D) methods are used for SPECT reconstruction and for absorbed dose calculation. Methods: Clinically realistic voxel-phantom simulations were used in the evaluation of activity quantification and dosimetry. SPECT reconstruction was performed using an ordered-subsets expectation maximization (OSEM) algorithm with compensation for scatter, attenuation, and 3D detector response. Based on the SPECT image and a patient-specific density map derived from CT, 3D dosimetry was performed using a newly implemented Monte Carlo code. Dosimetry was evaluated by comparing mean absorbed dose estimates calculated directly from the defined phantom activity map with those calculated from the SPECT image of the phantom. Finally, the 3D methods were applied to a radioimmunotherapy patient, and the mean tumor absorbed dose from the new calculation was compared with that from conventional dosimetry obtained from conjugate-view imaging. Results: Overall, the accuracy of the SPECT-based absorbed dose estimates in the phantom was > 12% for targets down to 16 mL and up to 35% for the smallest 7-mL tumor. To improve accuracy in the smallest tumor, more OSEM iterations may be needed. The relative SD from multiple realizations was < 3% for all targets except for the smallest tumor. For the patient, the mean tumor absorbed dose estimate from the new Monte Carlo calculation was 7% higher than that from conventional dosimetry. Conclusion: For target sizes down to 16 mL, highly accurate and precise dosimetry can be obtained with 3D methods for SPECT reconstruction and absorbed dose estimation. In the future, these methods can be applied to patients to potentially establish correlations between tumor regression and the absorbed dose statistics from 3D dosimetry.
|
|
| 2. |
- Dewaraja, Y K, et al.
(författare)
-
Accuracy of 131I tumor quantification in radioimmunotherapy using SPECT imaging with an ultra-high-energy collimator: Monte Carlo study
- 2000
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 41:10, s. 1760-1760
-
Tidskriftsartikel (refereegranskat)abstract
- Accuracy of 131I tumor quantification after radioimmunotherapy (RIT) was investigated for SPECT imaging with an ultra-high-energy (UHE) collimator designed for imaging 511-keV photons. METHODS: First, measurements and Monte Carlo simulations were carried out to compare the UHE collimator with a conventionally used, high-energy collimator. On the basis of this comparison, the UHE collimator was selected for this investigation, which was carried out by simulation of spherical tumors in a phantom. Reconstruction was by an expectation-maximization algorithm that included scatter and attenuation correction. Keeping the tumor activity constant, simulations were carried out to assess how volume-of-interest (VOI) counts vary with background activity, radius of rotation (ROR), tumor location, and size. The constant calibration factor for quantification was determined from VOI counts corresponding to a 3.63-cm-radius sphere of known activity. Tight VOIs corresponding to the physical size of the spheres or tumors were used. RESULTS: Use of the UHE collimator resulted in a large reduction in 131I penetration, which is especially significant in RIT where background uptake is high. With the UHE collimator, typical patient images showed an improvement in contrast. Considering the desired geometric events, sensitivity was reduced, but only by a factor of 1.6. Simulation results for a 3.63-cm-radius tumor showed that VOI counts vary with background, location, and ROR by less than 3.2%, 3%, and 5.3%, respectively. The variation with tumor size was more significant and was a function of the background. Good quantification accuracy (<6.5% error) was achieved when tumor size was the same as the sphere size used in the calibration, irrespective of the other parameters. For smaller tumors, activities were underestimated by up to -15% for the 2.88-cm-radius sphere, -23% for the 2.29-cm-radius sphere, and -47% for the 1.68-cm-radius sphere. CONCLUSION: Reasonable accuracy can be achieved for VOI quantification of 131I using SPECT with an UHE collimator and a constant calibration factor. Difference in tumor size relative to the size of the calibration sphere had the biggest effect on accuracy, and recovery coefficients are needed to improve quantification of small tumors.
|
|
| 3. |
- Dewaraja, Y K, et al.
(författare)
-
Characterization of scatter and penetration using Monte Carlo simulation in 131I imaging
- 2000
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 41:1, s. 123-130
-
Tidskriftsartikel (refereegranskat)abstract
- In 131I SPECT, image quality and quantification accuracy are degraded by object scatter as well as scatter and penetration in the collimator. The characterization of energy and spatial distributions of scatter and penetration performed in this study by Monte Carlo simulation will be useful for the development and evaluation of techniques that compensate for such events in 131I imaging. METHODS: First, to test the accuracy of the Monte Carlo model, simulated and measured data were compared for both a point source and a phantom. Next, simulations to investigate scatter and penetration were performed for four geometries: point source in air, point source in a water-filled cylinder, hot sphere in a cylinder filled with nonradioactive water, and hot sphere in a cylinder filled with radioactive water. Energy spectra were separated according to order of scatter, type of interaction, and gamma-ray emission energy. A preliminary evaluation of the triple-energy window (TEW) scatter correction method was performed. RESULTS: The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and radial point spread functions. For a point source in air, simulations show that 73% of events in the photopeak window had either scattered in or penetrated the collimator, indicating the significance of collimator interactions. For a point source in a water-filled phantom, the separated energy spectra showed that a 20% photopeak window can be used to eliminate events that scatter more than two times in the phantom. For the hot sphere phantoms, it was shown that in the photopeak region the spectrum shape of penetration events is very similar to that of primary (no scatter and no penetration) events. For the hot sphere regions of interest, the percentage difference between true scatter counts and the TEW estimate of scatter counts was <12%. CONCLUSION: In 131I SPECT, object scatter as well as collimator scatter and penetration are significant. The TEW method provides a reasonable correction for scatter, but the similarity between the 364-keV primary and penetration energy spectra makes it difficult to compensate for these penetration events using techniques that are based on spectral analysis.
|
|
| 4. |
- Dewaraja, Yuni K., et al.
(författare)
-
Monte Carlo evaluation of object shape effects in iodine-131 SPET tumor activity quantification
- 2001
-
Ingår i: European Journal Of Nuclear Medicine. - : Springer Science and Business Media LLC. - 1432-105X .- 0340-6997 .- 1619-7089. ; 28:7, s. 900-906
-
Tidskriftsartikel (refereegranskat)abstract
- In our clinical iodine-131 single-photon emission tomography (SPET) quantification for radioimmunotherapy, calibration and partial volume correction are based on measurements with phantoms containing spheres to simulate patient tumors even though real tumors are frequently nonspherical. In this study, Monte Carlo simulation was used to evaluate how object shape influences "spill-out" and "spill-in", which are major sources of quantification error associated with the poor spatial resolution of 131I SPET. Objects that varied in shape (spheres, cylinders, and an irregular structure) but were identical in activity and volume were simulated. Iterative reconstruction employed both attenuation and triple-energy-window scatter compensation. VOIs were defined in the reconstructed images both using physical boundaries and using expanded boundaries to allow for the limited resolution. When physical boundaries were used, both spill-out and spill-in were more significant for nonspherical structures than for spherical structures. Over the range of object volumes (50-200 ml) and at all background levels, VOI counts in cylinders were lower than VOI counts in spheres. This underestimation increased with decrease in object size (for the cold background -18% at 200 ml and -39% at 50 ml). It also decreased with increase in background activity because spill-in partially compensated for spill-out. It was shown that with a VOI larger than physical size, the results are independent of object shape and size only in the case of cold background. Activity quantification was carried out using a procedure similar to that used in our clinic. Quantification of nonspherical objects was improved by simple sphere-based partial volume correction, but the error was still large in some cases (for example, -39% for a 50-ml cylinder in a cold background and -35% for a 200-ml irregular structure defined on the basis of a typical tumor outlined on an X-ray computed tomography scan of a patient with non-Hodgkin's lymphoma). Partial volume correction by patient-specific Monte Carlo simulation may provide better quantification accuracy.
|
|
