Optimization of GATE simulations for whole-body planar scintigraphic acquisitions using the XCAT male phantom with 177Lu-DOTATATE biokinetics in a Siemens Symbia T2

doi:10.1016/j.ejmp.2017.07.009

Physica Medica

Volume 42, October 2017, Pages 292-297

https://doi.org/10.1016/j.ejmp.2017.07.009 Get rights and content

Highlights

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Acceleration technics in Monte Carlo simulations are proposed.
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The methods relies on particle tracking.
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Particle tracking in voxelized geometry and detector is kept minimum.
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CPU-time is decreased by a factor of 167.

Abstract

Simulations of planar whole body acquisitions in therapeutic procedures are often extensively time-consuming and therefore rarely used. However, optimising tools and variance reduction techniques can be employed to overcome this problem. In this paper, a variety of features available in GATE are explored and their capabilities to reduce simulation time are evaluated. For this purpose, the male XCAT phantom was used as a virtual patient with ¹⁷⁷Lu-DOTATATE pharmacokinetic for whole body planar acquisition simulations in a Siemens Symbia T2 model. Activity distribution was divided into 8 compartments that were simulated separately. GATE optimization techniques included reducing the amount of time spent in both voxel and detector tracking. Some acceleration techniques led to a decrease of CPU-time by a factor of 167, while image statistics were kept constant. In that context, the simulation of therapeutic procedure imaging would still require 46 days on a single CPU, but this could be reduced to hours on a dedicated cluster.

Introduction

Simulations of planar whole body acquisitions are often extensively time-consuming, especially when dealing with therapeutic procedures, which may render it unfeasible for practical reasons. Hence, it is important to have an estimation of the time needed to obtain the results and, if necessary, employ optimization tools and variance reduction techniques to achieve simulation times that are low enough to permit its usage in clinical routines.

Simulations of planar whole body acquisitions are often extensively time-consuming, especially when therapeutic procedures are involved, which may render it unfeasible for practical reasons. Hence, it is important to have an estimation of the time needed to obtain results and, when necessary, implement optimization tools and variance reduction techniques to decrease simulation time.

An example of time-consuming simulations is the imaging of NeuroEndocrine Tumours (NETs). NETs are small and disseminated tumour lesions that over-express a cellular receptor called somatostatin [1]. Peptide Receptor Radionuclide Therapy (PRRT) of NETs is carried out by the infusion of a synthetic analogue of somatostatin labelled with ¹⁷⁷Lu, ¹⁷⁷Lu-DOTATATE [2]. The goal is to keep irradiation of healthy tissues below the threshold of deterministic effects, while delivering an optimised absorbed dose to the tumour [3], [4]. ¹⁷⁷Lu is both a gamma- and beta-emitter, the former allowing the acquisition of images during therapy. These images are suitable for quantification, and therefore may allow patient-specific dosimetry. However, as there is not a standard dosimetry method for PRRT, different Clinical Dosimetry Protocols (CDP) using SPECT/CT and planar images have been proposed [3], [5]. The present work is part of the DosiTest [6] project that aims at evaluating CDP variability using a simulated reference. DosiTest requires the modelling of whole-body acquisitions based on the XCAT phantom and a hypothetical ¹⁷⁷Lu-DOTATATE pharmacokinetics. The aim of this work is to explore a variety of tools and techniques to reduce simulation time in order to generate whole body planar images that are realistic enough when compared to real acquisitions.

Section snippets

Materials & methods

Simulated acquisitions were performed using the male XCAT model [7], [8] as a virtual patient. The XCAT software creates voxelised human geometry by generating images of attenuation maps and activity distributions. Fig. 1 shows an attenuation map and four examples of activity distributions.

Activity distribution was based on a ¹⁷⁷Lu-DOTATATE pharmacokinetic model established in a previous study [9]. In that study, the model was created using Time-Activity Curves (TAC) of ¹⁷⁷Lu-DOTATATE and

Results

The sensitivity of the modelled gamma camera is 3.65 per 10,000. Among those photons, only 2.59 per 10,000 had energies between 192.4 keV and 239.2 keV (15% energy window at 208 keV) and are considered as counts in the image. This number agrees with the System Planar Sensitivity given by the manufacturer: 565 cpm/Ci represent 2.54 per 10,000 at 247 keV with a 20% energy window.

When comparing voxel and mathematical phantoms, it was found that, when using the same number of photons, ARF spends 20%

Discussion

In our study, a rectangular (21 × 23) ROI centred on the liver was used to define the acceleration factor. The remaining compartments were not evaluated because the time needed to perform the same verification for every functional compartment would be extensively long. Obviously, choosing a different region of interest (ROI) may impact the acceleration factor. However, since the factor was obtained on the same ROI, this gives at least an indication of the relative performances of ARF vs. more

Conclusion

In this work, classical methods for decreasing simulation time were applied, by considering emission angles and parametrised voxels (sGATE). New techniques and approaches were also tested that shown promising results.

A time-consuming aspect of GATE is tracking particles inside voxelised phantoms. Two approaches were implemented. They considered a minimal tracking energy in the phantom, as well as the use of a reduced emission spectrum that contained only energies that can contribute to image

Acknowledgement

This work was developed at the Cancer Research Center of Toulouse (CRCT) and was partly funded by the Brazilian National Counsel of Technological and Scientific Development (CNPq) and the French National Institute of Health and Medical Research (INSERM).

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Clinical dosimetry in molecular radiotherapy (MRT) is a multi-step procedure, prone to uncertainties at every stage of the dosimetric workflow. These are difficult to assess, especially as some are complex or even impossible to measure experimentally.
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Citation Excerpt :
This work highlights one specific aspect of the DosiTest project, i.e. improving quantitative imaging by modelling scintigraphic imaging in MRT. Scintigraphic imaging used in clinical dosimetry includes 2D planar imaging, modelling of which has already been addressed by Costa et al. [5]. In this paper, we consider 3D scintigraphic imaging (Single Photon Emission Computed Tomography, or SPECT) modelling.
Monte Carlo modelling of SPECT imaging in Molecular Radiotherapy can improve activity quantification. Until now, SPECT modelling with GATE only considered circular orbit (CO) acquisitions. This cannot reproduce auto-contour acquisitions, where the detector head moves close to the patient to improve image resolution. The aim of this work is to develop and validate an auto-contouring step-and-shoot acquisition mode for GATE SPECT modelling.
¹⁷⁷Lu and ¹³¹I SPECT experimental acquisitions performed on a Siemens Symbia T2 and GE Discovery 670 gamma camera, respectively, were modelled. SPECT projections were obtained for a cylindrical Jaszczak phantom and a lung and spine phantom. Detector head parameters (radial positions and acquisition angles) were extracted from the experimental projections to model the non-circular orbit (NCO) detector motion. The gamma camera model was validated against the experimental projections obtained with the cylindrical Jaszczak (¹⁷⁷Lu) and lung and spine phantom (¹³¹I). Then, ¹⁷⁷Lu and ¹³¹I CO and NCO SPECT projections were simulated to validate the impact of explicit NCO modelling on simulated projections.
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This work thereby justifies the need for auto-contouring modelling for isotopes with high septal penetration.
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Gamma index passing rate (2% − 1 pixel criteria) between 95 and 98% and average gamma between 0.28 and 0.35 among different time points revealed high similarity between simulated and experimental images. Image reconstruction of the simulated projections was successful on HERMES and Xeleris workstations, a major step forward for the initiation of a multicentric virtual clinical dosimetry trial based on simulated SPECT/CT images.
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Furthermore, Uribe et al. [17] applied the TEW and the analytical photon distribution interpolated (APDI) techniques as two different SC methods and found that in absence of background, the TEW technique causes under/overestimation of the activity for small/large spheres and recommended the APDI SC method for lesions in non-uniform attenuation areas. In addition to the experimental measurements, Monte Carlo simulations are used to validate quantitative SPECT/CT imaging, to evaluate the sensitivity of modelling, to optimize acquisition parameters, and to estimate the 3D distribution of radioactivity in organs [24–30]. Physical factors such as CDR, attenuation and scatter compensation can be studied by modeling their effects in the reconstruction algorithm [31,32].
The objective of this study was to evaluate the image degrading factors in quantitative ¹⁷⁷Lu SPECT imaging when using both main gamma photopeak energies.
Phantom measurements with two different vials containing various calibrated activities in air or water were performed to derive a mean calibration factor (CF) for large and small volumes of interest (VOIs). In addition, Monte Carlo simulations were utilized to investigate the effect of scatter energy window width, scatter correction method, such as effective scatter source estimation (ESSE) and triple energy window (TEW), and attenuation map on the quantification of ¹⁷⁷Lu. Results: The measured mean CF using large and small VOIs in water was 4.50 ± 0.80 and 4.80 ± 0.72 cps MBq⁻¹, respectively. Simulations showed a reference CF of 3.3 cps MBq⁻¹ for the water-filled phantom considering all photons excluding scattered events. By using the attenuation map generated for 190 keV photons, the calculated CFs for 113 keV and 208 keV are 10% lower than by using the weighted mean energy of 175 keV for ¹⁷⁷Lu. The calculated CF using the TEW correction was 17% higher than using the ESSE method for a water-filled phantom. However, our findings showed that an appropriate scatter window combination can reduce this difference between TEW and ESSE methods.
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Physica Medica

Highlights

Abstract

Introduction

Section snippets

Materials & methods

Results

Discussion

Conclusion

Acknowledgement

References (19)

DosiTest: a virtual intercomparison of clinical dosimetry trials in nuclear medicine based on Monte Carlo modeling

Phys Med

Peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors with somatostatin analogues

Eur Rev Med Pharmacol Sci

Results of individual patient dosimetry in peptide receptor radionuclide therapy with 177Lu DOTA-TATE and 177Lu DOTA-NOC

Cancer Biother Radiopharm

MIRD pamphlet no. 26: joint EANM/MIRD guidelines for quantitative 177Lu SPECT applied for dosimetry of radiopharmaceutical therapy

J. Nucl. Med.

Assessment of MIRD data for internal dosimetry using GATE Monte Carlo code

Radiat Environ Biophys

Radiopharmaceuticals for nuclear cardiology: radiation dosimetry, uncertainties, and risk

J. Nucl. Med.

Realistic CT simulation using the 4D XCAT phantom

Med Phys

4D XCAT phantom for multimodality imaging research

Med. Phys.

Cited by (10)

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Generation of clinical <sup>177</sup>Lu SPECT/CT images based on Monte Carlo simulation with GATE

Improving individualised dosimetry in radioiodine therapy for hyperthyroidism using population biokinetic modelling

The effect of attenuation map, scatter energy window width, and volume of interest on the calibration factor calculation in quantitative <sup>177</sup>Lu SPECT imaging: Simulation and phantom study

A Monte Carlo model for the internal dosimetry of choroid plexuses in nuclear medicine procedures

Original paperOptimization of GATE simulations for whole-body planar scintigraphic acquisitions using the XCAT male phantom with 177Lu-DOTATATE biokinetics in a Siemens Symbia T2

Highlights

Abstract

Introduction

Section snippets

Materials & methods

Results

Discussion

Conclusion

Acknowledgement

Phys Med

Peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors with somatostatin analogues

Eur Rev Med Pharmacol Sci

Results of individual patient dosimetry in peptide receptor radionuclide therapy with 177Lu DOTA-TATE and 177Lu DOTA-NOC

Cancer Biother Radiopharm

MIRD pamphlet no. 26: joint EANM/MIRD guidelines for quantitative 177Lu SPECT applied for dosimetry of radiopharmaceutical therapy

J. Nucl. Med.

Assessment of MIRD data for internal dosimetry using GATE Monte Carlo code

Radiat Environ Biophys

Radiopharmaceuticals for nuclear cardiology: radiation dosimetry, uncertainties, and risk

J. Nucl. Med.

Realistic CT simulation using the 4D XCAT phantom

Med Phys

4D XCAT phantom for multimodality imaging research

Med. Phys.

Original paper
Optimization of GATE simulations for whole-body planar scintigraphic acquisitions using the XCAT male phantom with ¹⁷⁷Lu-DOTATATE biokinetics in a Siemens Symbia T2