Quantitative comparison of pre-therapy ^99mTc-macroaggregated albumin SPECT/CT and post-therapy PET/MR studies of patients who have received intra-arterial radioembolization therapy with ⁹⁰Y microspheres

Highlights

PET/MRI post-therapy ⁹⁰Y studies are feasible and useful for ⁹⁰Y dosimetry calculations.

Due to better MRI soft tissue contrast, liver contours are usually better seen in PET/MRI images than in MAA SPECT/CT images.

The two main contributors to the difference between ⁹⁰Y dosimetry calculations using MAA versus ⁹⁰Y PET/MRI can be attributed to the changes in catheter positioning as well as the liver ROIs used for the calculations.

⁹⁰Y dosimetry values obtained from pre-therapy MAA SPECT/CT scans and PET/MRI post-therapy ⁹⁰Y studies were not significantly different.

Abstract

Objective

The aim of our study was to compare yttrium -90 (⁹⁰Y) dosimetry obtained from pre-therapy ^99mTc-macroaggregated albumin (MAA) SPECT/CT versus post-therapy PET/MRI imaging among patients with primary or metastatic hepatic tumors.

Materials and methods

Prior to ⁹⁰Y radioembolization (RE), 32 patients underwent a scan using MAA mimicking ⁹⁰Y distribution. After RE with ⁹⁰Y microspheres, the patients were imaged on a PET/MRI system. Reconstructed images were transferred to a common platform and used to calculate ⁹⁰Y dosimetry. The Passing-Bablok regression scatter diagram and the Bland and Altman method were used to analyze the difference between dosimetry values.

Results

For MAA and PET/MRI modalities, the mean liver doses for all 32 subjects were 43.0 ± 20.9 Gy and 46.5 ± 22.7 Gy, respectively, with a mean difference of 3.4 ± 6.2 Gy. The repeatibility coefficient was 12.1 (27.0% of the mean). The Spearman rank correlation coefficient was high (ρ = 0.92). Although, there was a substantial difference in the maximum doses to the liver between the modalities, the mean liver doses were relatively close, with a difference of 24.0% or less.

Conclusions

The two main contributors to the difference between dosimetry calculations using MAA versus ⁹⁰Y PET/MRI can be attributed to the changes in catheter positioning as well as the liver ROIs used for the calculations. In spite of these differences, our results demonstrate that the dosimetry values obtained from pre-therapy MAA SPECT/CT scans and PET/MRI post-therapy ⁹⁰Y studies were not significantly different.

Introduction

Hepatocellular carcinoma (HCC) affects over 500,000 people worldwide; it is the sixth most prevalent cancer and the third leading cause of cancer deaths [1,2]. Although surgical resection is the gold standard in treatment, only about 20% of patients are eligible primarily due to extensive disease or other complications. Unresectable HCC treatment options include, systematic chemotherapy (e.g., Sorafenib), transarterial chemoembolization (TACE) [3], thermal radiofrequency (RFA) [4] or microwave ablation, external beam radiation, and yttrium-90 (⁹⁰Y) radioembolization (RE) [5].

Radioembolization with ⁹⁰Y microspheres for patients with advanced HCC demonstrates very good toxicity profile -even in patients with advanced liver cirrhosis-, as well as encouraging data for time-to-progression and overall survival [6].

Prior to ⁹⁰Y RE, a mapping scan using ^99mTc-macroaggregated albumin (MAA) SPECT/CT, mimicking ⁹⁰Y distribution is performed, in order to assess lung shunting, exclude extrahepatic deposition, and assess intrahepatic distribution. In addition, some centers are using MAA scans for personalized dosimetry calculations, assuming perfect matching between MAA and ⁹⁰Y distribution [7]. However, there are several factors which can alter these dosimetry calculations, such as, differences in catheter positioning between MAA and ⁹⁰Y studies, differences between prepared and prescribed activities, and differences between prescribed and administrated activities [8,9]. In addition, ^99mTc MAA particles and ⁹⁰Y-microspheres have different flow characteristics due to differing particle size, dissociation, amount of particles and embolizing effect. The MAA particles range from 10 μm to 150 μm in size, with the vast majority (90%) between 10 μm to 40 μm. In comparison, ⁹⁰Y microspheres are 20 μm to 60 μm in size. Therefore, the true ⁹⁰Y distribution and dosimetry can only be obtained post-therapy using bremsstrahlung SPECT (bSPECT) or PET imaging [10]. The importance of post-therapy ⁹⁰Y imaging is twofold. First, it is used to detect possible extrahepatic activity, which can cause serious complications, such as ulceration and GI bleeds [[11], [12], [13]]. Second, post-therapy imaging provides an estimate of the regional liver absorbed dose on the image-based microsphere distribution, which is considered to be an important predictor of treatment efficacy [14]. Quantitative bSPECT imaging is challenching due to scatter, septal penetration, the continuous nature of the bremsstrahlung energy spectrum, and inefficient bremsstrahlung production [15]. Post-therapy PET/CT or PET/MRI ⁹⁰Y images are far superior, both qualitatively and quantitatively, to bSPECT ⁹⁰Y images [16].

In this retrospective study, the aim was to compare dosimetry estimates from pre-therapy ^99mTc MAA SPECT/CTstudies versus post-therapy ⁹⁰Y PET/MRI studies. PET/MRI studies were used as a gold standard, because they respresent the true post-therapy ⁹⁰Y distribution.