Erwann Rault

Development and evaluation of an improved quantitative 90Y bremsstrahlung SPECT method

PURPOSE Yttrium-90 ((90)Y) is one of the most commonly used radionuclides in targeted radionuclide therapy (TRT). Since it decays with essentially no gamma photon emissions, surrogate radionuclides (e.g., (111)In) or imaging agents (e.g., (99m)Tc MAA) are typically used for treatment planning. It would, however, be useful to image (90)Y directly in order to confirm that the distributions measured with these other radionuclides or agents are the same as for the (90)Y labeled agents. As a result, there has been a great deal of interest in quantitative imaging of (90)Y bremsstrahlung photons using single photon emission computed tomography (SPECT) imaging. The continuous and broad energy distribution of bremsstrahlung photons, however, imposes substantial challenges on accurate quantification of the activity distribution. The aim of this work was to develop and evaluate an improved quantitative (90)Y bremsstrahlung SPECT reconstruction method appropriate for these imaging applications. METHODS Accurate modeling of image degrading factors such as object attenuation and scatter and the collimator-detector response is essential to obtain quantitatively accurate images. All of the image degrading factors are energy dependent. Thus, the authors separated the modeling of the bremsstrahlung photons into multiple categories and energy ranges. To improve the accuracy, the authors used a bremsstrahlung energy spectrum previously estimated from experimental measurements and incorporated a model of the distance between (90)Y decay location and bremsstrahlung emission location into the SIMIND code used to generate the response functions and kernels used in the model. This improved Monte Carlo bremsstrahlung simulation was validated by comparison to experimentally measured projection data of a (90)Y line source. The authors validated the accuracy of the forward projection model for photons in the various categories and energy ranges using the validated Monte Carlo (MC) simulation method. The forward projection model was incorporated into an iterative ordered subsets-expectation maximization (OS-EM) reconstruction code to allow for quantitative SPECT reconstruction. The resulting code was validated using both a physical phantom experiment with spherical objects in a warm background and a realistic anatomical phantom simulation. In the physical phantom study, the authors evaluated the method in terms of quantitative accuracy of activity estimates in the spheres; in the simulation study, the authors evaluated the accuracy and precision of activity estimates from various organs and compared them to results from a previously proposed method. RESULTS The authors demonstrated excellent agreement between the experimental measurement and Monte Carlo simulation. In the XCAT phantom simulation, the proposed method achieved much better accuracy in the modeling (error in photon counts was -1.1 %) compared to a previously proposed method (errors were more than 20  %); the quantitative accuracy of activity estimates was excellent for all organs (errors were from -1.6 % to 11.9 %) and comparable to previously published results for (131)I using the same collimator. CONCLUSIONS The proposed (90)Y bremsstrahlung SPECT reconstruction method provided very accurate estimates of organ activities, with accuracies approaching those previously observed for (131)I. The method may be useful in verifying organ doses for targeted radionuclide therapy using (90)Y.

Fast simulation of yttrium-90 bremsstrahlung photons with GATE

PURPOSE Multiple investigators have recently reported the use of yttrium-90 (Y90) bremsstrahlung single photon emission computed tomography (SPECT) imaging for the dosimetry of targeted radionuclide therapies. Because Monte Carlo (MC) simulations are useful for studying SPECT imaging, this study investigates the MC simulation of Y90 bremsstrahlung photons in SPECT. To overcome the computationally expensive simulation of electrons, the authors propose a fast way to simulate the emission of Y90 bremsstrahlung photons based on prerecorded bremsstrahlung photon probability density functions (PDFs). METHODS The accuracy of bremsstrahlung photon simulation is evaluated in two steps. First, the validity of the fast bremsstrahlung photon generator is checked. To that end, fast and analog simulations of photons emitted from a Y90 point source in a water phantom are compared. The same setup is then used to verify the accuracy of the bremsstrahlung photon simulations, comparing the results obtained with PDFs generated from both simulated and measured data to measurements. In both cases, the energy spectra and point spread functions of the photons detected in a scintillation camera are used. RESULTS Results show that the fast simulation method is responsible for a 5% overestimation of the low-energy fluence (below 75 keV) of the bremsstrahlung photons detected using a scintillation camera. The spatial distribution of the detected photons is, however, accurately reproduced with the fast method and a computational acceleration of ∼17-fold is achieved. When measured PDFs are used in the simulations, the simulated energy spectrum of photons emitted from a point source of Y90 in a water phantom and detected in a scintillation camera closely approximates the measured spectrum. The PSF of the photons imaged in the 50-300 keV energy window is also accurately estimated with a 12.4% underestimation of the full width at half maximum and 4.5% underestimation of the full width at tenth maximum. CONCLUSIONS Despite its limited accuracy, the fast bremsstrahlung photon generator is well suited for the simulation of bremsstrahlung photons emitted in large homogeneous organs, such as the liver, and detected in a scintillation camera. The computational acceleration makes it very useful for future investigations of Y90 bremsstrahlung SPECT imaging.

Accurate Monte Carlo modelling of the back compartments of SPECT cameras

Today, new single photon emission computed tomography (SPECT) reconstruction techniques rely on accurate Monte Carlo (MC) simulations to optimize reconstructed images. However, existing MC scintillation camera models which usually include an accurate description of the collimator and crystal, lack correct implementation of the gamma cameras back compartments. In the case of dual isotope simultaneous acquisition (DISA), where backscattered photons from the highest energy isotope are detected in the imaging energy window of the second isotope, this approximation may induce simulation errors. Here, we investigate the influence of backscatter compartment modelling on the simulation accuracy of high-energy isotopes. Three models of a scintillation camera were simulated: a simple model (SM), composed only of a collimator and a NaI(Tl) crystal; an intermediate model (IM), adding a simplified description of the backscatter compartments to the previous model and a complete model (CM), accurately simulating the materials and geometries of the camera. The camera models were evaluated with point sources ((67)Ga, (99m)Tc, (111)In, (123)I, (131)I and (18)F) in air without a collimator, in air with a collimator and in water with a collimator. In the latter case, sensitivities and point-spread functions (PSFs) simulated in the photopeak window with the IM and CM are close to the measured values (error below 10.5%). In the backscatter energy window, however, the IM and CM overestimate the FWHM of the detected PSF by 52% and 23%, respectively, while the SM underestimates it by 34%. The backscatter peak fluence is also overestimated by 20% and 10% with the IM and CM, respectively, whereas it is underestimated by 60% with the SM. The results show that an accurate description of the backscatter compartments is required for SPECT simulations of high-energy isotopes (above 300 keV) when the backscatter energy window is of interest.

Physics process level discrimination of detections for GATE: Assessment of contamination in SPECT and spurious activity in PET

The GEANT4 application for tomographic emission (GATE) is one of the most detailed Monte Carlo simulation tools for SPECT and PET. It allows for realistic phantoms, complex decay schemes, and a large variety of detector geometries. However, only a fraction of the information in each particle history is available for postprocessing. In order to extend the analysis capabilities of GATE, a flexible framework was developed. This framework allows all detected events to be subdivided according to their type: In PET, true coincidences from others, and in SPECT, geometrically collimated photons from others. The framework of the authors can be applied to any isotope, phantom, and detector geometry available in GATE. It is designed to enhance the usability of GATE for the study of contamination and for the investigation of the properties of current and future prototype detectors. The authors apply the framework to a case study of Bexxar, first assuming labeling with 124I, then with 131I. It is shown that with 124I PET, results with an optimized window improve upon those with the standard window but achieve less than half of the ideal improvement. Nevertheless, 124I PET shows improved resolution compared to 131I SPECT with triple-energy-window scatter correction.

Optimization of Yttrium-90 Bremsstrahlung Imaging with Monte Carlo Simulations

Targeted Radionuclide Therapy is an emerging cancer treatment. It uses the properties of some radiopharmaceuticals to fix in specific areas of the organism to deliver a lethal dose of radiation to the surrounding tissues. Yttrium-90 (90Y) is one of the most popular isotopes for this therapy. Its β- emission delivers a high, and well located dose to the targeted areas. However, the verification of the isotope’s position in the body is difficult due to its low gamma emission. The bremsstrahlung interactions between the emitted electrons and the body give rise to a continuous spectrum of photons that can be used for imaging. The aim of this work was to optimize the imaging of the bremsstrahlung photons.

Abstract ID: 222 Clinical implementation of a Monte Carlo based QA platform for validation of Tomotherapy and Cyberknife treatment plans

INTRODUCTION This work describes the clinical implementation of a Monte Carlo based platform for treatment plan validation for Tomotherapy and Cyberknife, including a semi-automatic plan evaluation module based on dose constraints for organs-at-risk (OAR). METHODS The Monte Carlo-based platform Moderato [1] is based on BEAMnrc/DOSXYZnrc and allows for automated re-calculation of doses planned with Tomotherapy and Cyberknife techniques. The Prescription/Validation module generates a set of dose constraints based on the anatomical region and fractionation scheme considered. Upon achievement of the planning, dose results are displayed with visual warnings in case of constraint violation. The system was tested on 83 patient cases in order to evaluate the influence of difference in calculation algorithms on OAR constraints. RESULTS The first results with the Tomotherapy plans allowed for detecting and correcting a problem with the CT Hounsfield units when using a large reconstruction diameter (a CT artifact that lead to air voxels with an overestimated density). The Cyberknife results also showed some dose differences associated with different energy thresholds between Moderato and the Monte Carlo algorithm used in the Treatment Planning Station. Regarding OAR constraints, re-calculation generated few violations in thoracic, pelvic and abdominal cases. However, in spinal and head cases, significant differences can appear (-11% to +6%) on optic pathways and spinal cord, leading to doses above the limits. CONCLUSIONS The Moderato platform constitutes a promising tool for the validation of plan quality, offering both dose re-calculation and OAR constraints evaluation. First results show the importance of this verification for some specific regions. Further work is ongoing to optimize the quantity and relevance of the information displayed, before fully introducing the system in clinical routine.