Standardised quantitative radioiodine SPECT/CT Imaging for multicentre dosimetry trials in molecular radiotherapy

Abstract

The SEL-I-METRY trial (EudraCT No 2015-002269-47) is the first multicentre trial to investigate the role of 123I and 131I SPECT/CT-based tumour dosimetry to predict response to radioiodine therapy. Standardised dosimetry methodology is essential to provide a robust evidence-base for absorbed dose–response thresholds for molecular radiotherapy (MRT). In this paper a practical standardised protocol is used to establish the first network of centres with consistent methods of radioiodine activity quantification. Nine SPECT/CT systems at eight centres were set-up for quantitative radioiodine imaging. The dead-time of the systems was characterised for up to 2.8 GBq 131I. Volume dependent calibration factors were measured on centrally reconstructed images of 123I and 131I in six (0.8–196 ml) cylinders. Validation of image quantification using these calibration factors was performed on three systems, by imaging a 3D-printed phantom mimicking a patient’s activity distribution. The percentage differences between the activities measured in the SPECT/CT image and those measured by the radionuclide calibrator were calculated. Additionally uncertainties on the SPECT/CT-based activities were calculated to indicate the limit on the quantitative accuracy of this method. For systems set-up to image high 131I count rates, the count rate versus activity did not peak below 2.8 GBq and fit a non-paralysable model. The dead-times and volume-dependent calibration factors were comparable between systems of the same model and crystal thickness. Therefore a global calibration curve could be fitted to each. The errors on the validation phantom activities’ were comparable to the measurement uncertainties derived from uncertainty analysis, at 10% and 16% on average for 123I and 131I respectively in a 5 cm sphere. In conclusion, the dead-time and calibration factors varied between centres, with different models of system. However, global calibration factors may be applied to the same system model with the same crystal thickness, to simplify set-up of future multi-centre MRT studies.

Investigation of post-therapeutic image-based thyroid dosimetry using quantitative SPECT/CT, iodine biokinetics, and the MIRD’s voxel S values in Graves’ disease

“…Therefore, we considered that count loss minimally affected this situation, and we did not need to perform the dead time correction. However, in the most recent paper by Gregory et al [28], they measured the resolving time τ with the same type of SPECT/CT system used in this study (3.796 × 10 −6 s). Taking into account that the mean count rate of projection right above the thyroid in each patient was 10.7 ± 3.6 kcps in the 15% main window centered at 364 keV, the dead time correction factor for the non-paralyzable model, D τ , calculated from Eq.…”

SPECT/CT: Standing on the Shoulders of Giants, It Is Time to Reach for the Sky!

“…Multiple collaborative groups have started ambitious projects aimed at standardizing quantitative SPECT/CT and creating protocols to determine the accuracy and uncertainties of specific dosimetry platforms (11,44). However, these have been hampered by a tendency to sacrifice user control of application settings and transparency in closed-source workstation solutions using proprietary algorithms in favor of easy-to-use interfaces.…”

Setting up a quantitative SPECT imaging network for a European multi-centre dosimetry study of radioiodine treatment for thyroid cancer as part of the MEDIRAD project

“…On two systems, Intevo 1 and Intevo 2, the methodology used here was validated against the dead-time measurement methodology presented by Gregory et al [24]. The authors determined dead time by incrementally adding 131 I to a Jaszczak phantom and performing 100 kcounts static images for each activity level.…”

“…An activity of 1000 MBq in the field-of-view (FOV) was associated with true count rates of 30 ± 3 kcps. Figure 4 shows the comparison between dead-time factors obtained using the method proposed by Gregory et al [24], which involves a series of acquisitions with increasing activity in a large volume uniform phantom, and the method used in the present study. An overall good agreement is found between the two methodologies on both systems assessed.…”

“…An example of a multi-centre clinical trial involving dosimetry for [ 123 I]NaI and [ 131 I]NaI is SEL-I-METRY [23], a phase II clinical trial using quantitative SPECT imaging to investigate the potential of selumetinib to resensitise advanced iodine refractory differentiated thyroid cancer (DTC) to radioiodine. As part of SELIMETRY, a quantitative SPECT imaging network was set up in the UK [24].…”