Improved dose–volume histogram estimates for radiopharmaceutical therapy by optimizing quantitative SPECT reconstruction parameters

Abstract

In radiopharmaceutical therapy, an understanding of the dose distribution in normal and target tissues is important for optimizing treatment. Three-dimensional (3D) dosimetry takes into account patient anatomy and the nonuniform uptake of radiopharmaceuticals in tissues. Dose-volume histograms (DVHs) provide a useful summary representation of the 3D dose distribution and have been widely used for external beam treatment planning. Reliable 3D dosimetry requires an accurate 3D radioactivity distribution as the input. However, activity distribution estimates from SPECT are corrupted by noise and partial volume effects (PVEs). In this work, we systematically investigated OS-EM based quantitative SPECT (QSPECT) image reconstruction in terms of its effect on DVHs estimates. A modified 3D NURBS-based Cardiac-Torso (NCAT) phantom that incorporated a non-uniform kidney model and clinically realistic organ activities and biokinetics was used. Projections were generated using a Monte Carlo (MC) simulation; noise effects were studied using 50 noise realizations with clinical count levels. Activity images were reconstructed using QSPECT with compensation for attenuation, scatter and collimator-detector response (CDR). Dose rate distributions were estimated by convolution of the activity image with a voxel S kernel. Cumulative DVHs were calculated from the phantom and QSPECT images and compared both qualitatively and quantitatively. We found that noise, PVEs, and ringing artifacts due to CDR compensation all degraded histogram estimates. Low-pass filtering and early termination of the iterative process were needed to reduce the effects of noise and ringing artifacts on DVHs, but resulted in increased degradations due to PVEs. Large objects with few features, such as the liver, had more accurate histogram estimates and required fewer iterations and more smoothing for optimal results. Smaller objects with fine details, such as the kidneys, required more iterations and less smoothing at early time points post-radiopharmaceutical administration but more smoothing and fewer iterations at later time points when the total organ activity was lower. The results of this study demonstrate the importance of using optimal reconstruction and regularization parameters. Optimal results were obtained with different parameters at each time point, but using a single set of parameters for all time points produced near-optimal dose-volume histograms.

Impact of SPECT corrections on 3D‐dosimetry for liver transarterial radioembolization using the patient relative calibration methodology

“…The influence of PVE on cDVHs for large volumes such as the whole liver, is noticeable even though correction for a collimator-detector response is applied. 48 However, D50% values in Fig. 2(a) are within 3 Gy/GBq, i.e., with a dispersion of less than 10%.…”

“…The uniform liver distribution does not mimic a clinical situation, but it is useful for comparisons with the literature. 48 Liver's segment is representative of a selective treatment of a liver portion, very common in the clinical prac- tice. Nonuniform liver accounts for a heterogeneous activity distribution, typical for this treatment.…”

“…Cheng et al 48 used an absolute calibration, whereas a relative calibration was used here, as in most clinical papers. The influence of PVE on cDVHs for large volumes such as the whole liver, is noticeable even though correction for a collimator-detector response is applied.…”

MIRD Pamphlet No. 26: Joint EANM/MIRD Guidelines for Quantitative 177Lu SPECT Applied for Dosimetry of Radiopharmaceutical Therapy

“…However, the decision about filtering should be based on the specific objectives of the study (34,35).…”

“…When the distribution of absorbed doses for individual voxel locations is calculated within a VOI for further analysis using dose-volume histograms, one should be aware of the fact that CDR correction may result in creation of false dose-volume information (35). Figure 6 illustrates 4 types of reconstructed images of the kidney region.…”