Impact of dead time on quantitative 177Lu-SPECT (QSPECT) and kidney dosimetry during PRRT

Desy, Alessandro; Bouvet, Guillaume F.; Frezza, Andrea; Després, Philippe; Beauregard, Jean-Mathieu

doi:10.1186/s40658-020-00303-0

Cited by 12 publications

(8 citation statements)

References 15 publications

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“…We previously demonstrated the necessity to correct for dead time in post-therapeutic 177 Lu imaging with NaI(Tl)-crystal cameras [ 5 ]. Here, we comprehensively investigated the divergent detector behaviour of System A at high count rate, which can compromise the accuracy of quantification and dosimetry, as well as the image quality (Fig.…”

Section: Discussionmentioning

confidence: 99%

“…It is of particular utility in dosimetry-driven protocols of personalized radiopharmaceutical therapy, in which the administered activity can be highly variable [ 1 – 4 ]. We have previously shown the importance of dead-time correction in a 177 Lu post-therapeutic setting [ 5 ]. However, using 177 Lu and 99m Tc, we previously observed divergent behaviours between the two detectors of a SPECT/CT system at high count rate, which lead to image artifacts and limitations in QSPECT accuracy [ 6 , 7 ].…”

Section: Introductionmentioning

confidence: 99%

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Quantitative SPECT (QSPECT) at high count rates with contemporary SPECT/CT systems

et al. 2021

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Background Accurate QSPECT is crucial in dosimetry-based, personalized radiopharmaceutical therapy with 177Lu and other radionuclides. We compared the quantitative performance of three NaI(Tl)-crystal SPECT/CT systems equipped with low-energy high-resolution collimators from two vendors (Siemens Symbia T6; GE Discovery 670 and NM/CT 870 DR). Methods Using up to 14 GBq of 99mTc in planar mode, we determined the calibration factor and dead-time constant under the assumption that these systems have a paralyzable behaviour. We monitored their response when one or both detectors were activated. QSPECT capability was validated by SPECT/CT imaging of a customized NEMA phantom containing up to 17 GBq of 99mTc. Acquisitions were reconstructed with a third-party ordered subset expectation maximization algorithm. Results The Siemens system had a higher calibration factor (100.0 cps/MBq) and a lower dead-time constant (0.49 μs) than those from GE (75.4–87.5 cps/MBq; 1.74 μs). Activities of up to 3.3 vs. 2.3–2.7 GBq, respectively, were quantifiable by QSPECT before the observed count rate plateaued or decreased. When used in single-detector mode, the QSPECT capability of the former system increased to 5.1 GBq, whereas that of the latter two systems remained independent of the detectors activation mode. Conclusion Despite similar hardware, SPECT/CT systems’ response can significantly differ at high count rate, which impacts their QSPECT capability in a post-therapeutic setting.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Quantitative SPECT (QSPECT) at high count rates with contemporary SPECT/CT systems

et al. 2021

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“…A vial with a known activity of Lu was considered as a point source and then gradually moved away from the detector to vary the count rate as proposed in [ 29 , 30 ]. For later acquisition, dead time was considered as negligible [ 31 ] and not accounted for. Note that the exact time duration between the end of injection and the acquisition was determined according to the acquisition hours stored in the DICOM header of the images and was used for calculations.…”

Section: Methodsmentioning

confidence: 99%

Patient-specific dosimetry adapted to variable number of SPECT/CT time-points per cycle for $$^{177}$$Lu-DOTATATE therapy

et al. 2022

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Background The number of SPECT/CT time-points is important for accurate patient dose estimation in peptide receptor radionuclide therapy. However, it may be limited by the patient’s health and logistical reasons. Here, an image-based dosimetric workflow adapted to the number of SPECT/CT acquisitions available throughout the treatment cycles was proposed, taking into account patient-specific pharmacokinetics and usable in clinic for all organs at risk. Methods Thirteen patients with neuroendocrine tumors were treated with four injections of 7.4 GBq of $$^{177}$$ 177 Lu-DOTATATE. Three SPECT/CT images were acquired during the first cycle (1H, 24H and 96H or 144H post-injection) and a single acquisition (24H) for following cycles. Absorbed doses were estimated for kidneys (LK and RK), liver (L), spleen (S), and three surrogates of bone marrow (L2 to L4, L1 to L5 and T9 to L5) that were compared. 3D dose rate distributions were computed with Monte Carlo simulations. Voxel dose rates were averaged at the organ level. The obtained Time Dose-Rate Curves (TDRC) were fitted with a tri-exponential model and time-integrated. This method modeled patient-specific uptake and clearance phases observed at cycle 1. Obtained fitting parameters were reused for the following cycles, scaled to the measure organ dose rate at 24H. An alternative methodology was proposed when some acquisitions were missing based on population average TDRC (named STP-Inter). Seven other patients with three SPECT/CT acquisitions at cycles 1 and 4 were included to estimate the uncertainty of the proposed methods. Results Absorbed doses (in Gy) per cycle available were: 3.1 ± 1.1 (LK), 3.4 ± 1.5 (RK), 4.5 ± 2.8 (L), 4.6 ± 1.8 (S), 0.3 ± 0.2 (bone marrow). There was a significant difference between bone marrow surrogates (L2 to L4 and L1 to L5, Wilcoxon’s test: p value < 0.05), and while depicting very doses, all three surrogates were significantly different than dose in background (p value < 0.01). At cycle 1, if the acquisition at 24H is missing and approximated, medians of percentages of dose difference (PDD) compared to the initial tri-exponential function were inferior to 3.3% for all organs. For cycles with one acquisition, the median errors were smaller with a late time-point. For STP-Inter, medians of PDD were inferior to 7.7% for all volumes, but it was shown to depend on the homogeneity of TDRC. Conclusion The proposed workflow allows the estimation of organ doses, including bone marrow, from a variable number of time-points acquisitions for patients treated with $$^{177}$$ 177 Lu-DOTATATE.

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“…SPECT imaging also includes key technical factors that are important for accurate dosimetry but are not universally available, such as dead times, conversion factors, and calibration of the sources and cameras. For 177 Lu-SPECT quantitation, for example, dead times may impact dose estimates by up to about 22%, requiring corrections (136,137).…”

Section: Technical Aspects Of Imagingmentioning

confidence: 99%

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

et al. 2021

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Impact of dead time on quantitative 177Lu-SPECT (QSPECT) and kidney dosimetry during PRRT

Cited by 12 publications

References 15 publications

Quantitative SPECT (QSPECT) at high count rates with contemporary SPECT/CT systems

Quantitative SPECT (QSPECT) at high count rates with contemporary SPECT/CT systems

Patient-specific dosimetry adapted to variable number of SPECT/CT time-points per cycle for $$^{177}$$Lu-DOTATATE therapy

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

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