Yttrium-90 internal pair production imaging using first generation PET/CT provides high-resolution images for qualitative diagnostic purposes

Voxel-based dosimetry predicting treatment response and related toxicity in HCC patients treated with resin-based Y90 radioembolization: a prospective, single-arm study

BACKGROUND

There is an increasing body of evidence indicating Y90 dose thresholds for tumor response and treatment-related toxicity. These thresholds are poorly studied in resin Y90, particularly in hepatocellular carcinoma (HCC).

PURPOSE

To evaluate the efficacy of prospective voxel-based dosimetry for predicting treatment response and adverse events (AEs) in patients with HCC undergoing resin-based Y90 radioembolization.

MATERIALS AND METHODS

This correlative study was based on a prospective single-arm clinical trial (NCT04172714), which evaluated the efficacy of low/scout (555 MBq) activity of resin-based Y90 for treatment planning. Partition model was used with goal of tumor dose (TD) > 200 Gy and non-tumoral liver dose (NTLD) < 70 Gy for non-segmental therapies. Single compartment dose of 200 Gy was used for segmentectomies. Prescribed Y90 activity minus scout activity was administered for therapeutic Y90 followed by Y90-PET/CT. Sureplan® (MIM Software, Cleveland, OH) was used for dosimetry analysis. Treatment response was evaluated at 3 and 6 months. Receiver operating characteristic curve determined TD response threshold for objective response (OR) and complete response (CR) as well as non-tumor liver dose (NTLD) threshold that predicted AEs.

RESULTS

N = 30 patients were treated with 33 tumors (19 segmental and 14 non-segmental). One patient died before the first imaging, and clinical follow-up was excluded from this analysis. Overall, 26 (81%) of the tumors had an OR and 23 (72%) had a CR. A mean TD of 253 Gy predicted an OR with 92% sensitivity and 83% specificity (area under the curve (AUC = 0.929, p < 0.001). A mean TD of 337 Gy predicted a CR with 83% sensitivity and 89% specificity (AUC = 0.845, p < 0.001). A mean NTLD of 81 and 87 Gy predicted grade 3 AEs with 100% sensitivity and 100% specificity in the non-segmental cohort at 3- and 6-month post Y90, respectively.

CONCLUSION

In patients with HCC undergoing resin-based Y90, there are dose response and dose toxicity thresholds directly affecting outcomes.

CLINICAL TRIAL NUMBER

NCT04172714.

Yttrium-90 quantitative phantom study using digital photon counting PET

BACKGROUND

PET imaging of 90Y-microsphere distribution following radioembolisation is challenging due to the count-starved statistics from the low branching ratio of e+/e- pair production during 90Y decay. PET systems using silicon photo-multipliers have shown better 90Y image quality compared to conventional photo-multiplier tubes. The main goal of the present study was to evaluate reconstruction parameters for different phantom configurations and varying listmode acquisition lengths to improve quantitative accuracy in 90Y dosimetry, using digital photon counting PET/CT.

METHODS

Quantitative PET and dosimetry accuracy were evaluated using two uniform cylindrical phantoms specific for PET calibration validation. A third body phantom with a 9:1 hot sphere-to-background ratio was scanned at different activity concentrations of 90Y. Reconstructions were performed using OSEM algorithm with varying parameters. Time-of-flight and point-spread function modellings were included in all reconstructions. Absorbed dose calculations were carried out using voxel S-values convolution and were compared to reference Monte Carlo simulations. Dose-volume histograms and root-mean-square deviations were used to evaluate reconstruction parameter sets. Using listmode data, phantom and patient datasets were rebinned into various lengths of time to assess the influence of count statistics on the calculation of absorbed dose. Comparisons between the local energy deposition method and the absorbed dose calculations were performed.

RESULTS

Using a 2-mm full width at half maximum post-reconstruction Gaussian filter, the dosimetric accuracy was found to be similar to that found with no filter applied but also reduced noise. Larger filter sizes should not be used. An acquisition length of more than 10 min/bed reduces image noise but has no significant impact in the quantification of phantom or patient data for the digital photon counting PET. 3 iterations with 10 subsets were found suitable for large spheres whereas 1 iteration with 30 subsets could improve dosimetry for smaller spheres.

CONCLUSION

The best choice of the combination of iterations and subsets depends on the size of the spheres. However, one should be careful on this choice, depending on the imaging conditions and setup. This study can be useful in this choice for future studies for more accurate 90Y post-dosimetry using a digital photon counting PET/CT.

Diagnostic Performance of Theranostic Radionuclides Used in Transarterial Radioembolization for Liver Cancer

Primary liver tumor with hepatocellular carcinoma accounting for 75–80% of all such tumors, is one of the global leading causes of cancer-related death, especially in cirrhotic patients. Liver tumors are highly hypervascularized via the hepatic artery, while normal liver tissues are mainly supplied by the portal vein; consequently, intra-arterially delivered treatment, which includes transarterial chemoembolization (TACE) and transarterial radioembolization (TARE), is deemed as a palliative treatment. With the development of nuclear technology and radiochemistry, TARE has become an alternative for patients with hepatic cancer, especially for patients who failed other therapies, or for patients who need tumor downstaging treatment. In practice, some radionuclides have suitable physicochemical characteristics to act as radioactive embolism agents. Among them, ⁹⁰Y emits β rays only and is suitable for bremsstrahlung single photon emission computed tomography (BS SPECT) and positron emission tomography (PET); meanwhile, some others, such as ¹³¹I, ¹⁵³Sm, ¹⁶⁶Ho, ¹⁷⁷Lu, ¹⁸⁶Re, and ¹⁸⁸Re, emit both β and γ rays, enabling embolism beads to play a role in both therapy and single photon emission computed tomography (SPECT) imaging. During TARE, concomitant imaging provide additive diagnostic information and help to guide the course of liver cancer treatment. Therefore, we review the theranostic radionuclides that have been used or could potentially be used in TARE for liver cancer and focus on the clinical benefits of diagnostic applications, including real-time monitoring of embolism beads, evaluating irradiation dose, predicting therapy effects, and corresponding adjustments to TARE.

High quality imaging and dosimetry for yttrium-90 (90Y) liver radioembolization using a SiPM-based PET/CT scanner

PURPOSE

Transarterial radioembolization (TARE) with yttrium-90 (⁹⁰Y) microspheres is a liver-directed treatment for primary and secondary hepatic malignancies. Personalized dosimetry aims for maximum treatment effect and reduced toxicity. We aimed to compare pre-treatment voxel-based dosimetry from ^99mTc macroaggregated albumin (MAA) SPECT/CT with post-treatment ⁹⁰Y PET/CT for absorbed dose values, and to evaluate image quality of ⁹⁰Y SiPM-based PET/CT.

METHODS

Forty-two patients (28 men, 14 women, mean age: 67 ± 11 years) with advanced hepatic malignancies were prospectively enrolled. Twenty patients were treated with glass and 22 with resin microspheres. Radiation absorbed doses from planning ^99mTc-MAA SPECT/CT and post-therapy ⁹⁰Y PET/CT were assessed. ⁹⁰Y PET/CT images were acquired for 20 min and reconstructed to produce 5-, 10-, 15-, and 20-min datasets, then evaluated using the 5-point Likert scale.

RESULTS

The mean administered activity was 3.44 ± 1.5 GBq for glass and 1.62 ± 0.7 GBq for resin microspheres. The mean tumor absorbed doses calculated from ^99mTc-MAA SPECT/CT and ⁹⁰Y PET/CT were 175.69 ± 113.76 Gy and 193.58 ± 111.09 Gy (P = 0.61), respectively for glass microspheres; they were 60.18 ± 42.20 Gy and 70.98 ± 49.65 Gy (P = 0.37), respectively for resin microspheres. The mean normal liver absorbed doses from ^99mTc-MAA SPECT/CT and ⁹⁰Y PET/CT were 32.70 ± 22.25 Gy and 30.62 ± 20.09 Gy (P = 0.77), respectively for glass microspheres; they were 18.33 ± 11.08 Gy and 24.32 ± 15.58 Gy (P = 0.17), respectively for resin microspheres. Image quality of ⁹⁰Y PET/CT at 5-, 10-, 15-, and 20-min scan time showed a Likert score of 3.6 ± 0.54, 4.57 ± 0.58, 4.84 ± 0.37, and 4.9 ± 0.3, respectively.

CONCLUSIONS

^99mTc-MAA SPECT/CT demonstrated great accuracy for treatment planning dosimetry. SiPM-based PET/CT scanner showed good image quality at 10-min scan time, acquired in one bed position. A PET/CT scan time of 5 min showed acceptable image quality and suffices for dosimetry and treatment verification. This allows for inclusion of ⁹⁰Y PET/CT in busy routine clinical workflows. Studies with larger patient cohorts are needed to confirm these findings.

Radiometals for imaging and theranostics, current production, and future perspectives

The aim of this review is to make the reader familiar with currently available radiometals, their production modes, capacities, and quality concerns related to their medical use, as well as new emerging radiometals and irradiation technologies from the perspective of their diagnostic and theranostic applications. Production methods of ¹⁷⁷ Lu serve as an example of various issues related to the production yield, specific activity, radionuclidic and chemical purity, and production economy. Other radiometals that are currently used or explored for potential medical applications, with particular focus on their theranostic value, are discussed. Using radiometals for diagnostic imaging and therapy is on the rise. The high demand for radiometals for medical use prompts investigations towards using alternative irradiation reactions, while using existing nuclear reactors and accelerator facilities. This review discusses these production capacities and what is necessary to cover the growing demand for theranostic nuclides.

Comparison of quantitative Y-90 SPECT and non-time-of-flight PET imaging in post-therapy radioembolization of liver cancer

PURPOSE

Radioembolization with yttrium-90 microspheres may be optimized with patient-specific pretherapy treatment planning. Dose verification and validation of treatment planning methods require quantitative imaging of the post-therapy distribution of yttrium-90 (Y-90). Methods for quantitative imaging of Y-90 using both bremsstrahlung SPECT and PET have previously been described. The purpose of this study was to compare the two modalities quantitatively in humans.

METHODS

Calibration correction factors for both quantitative Y-90 bremsstrahlung SPECT and a non-time-of-flight PET system without compensation for prompt coincidences were developed by imaging three phantoms. The consistency of these calibration correction factors for the different phantoms was evaluated. Post-therapy images from both modalities were obtained from 15 patients with hepatocellular carcinoma who underwent hepatic radioembolization using Y-90 glass microspheres. Quantitative SPECT and PET images were rigidly registered and the total liver activities and activity distributions estimated for each modality were compared. The activity distributions were compared using profiles, voxel-by-voxel correlation and Bland-Altman analyses, and activity-volume histograms.

RESULTS

The mean ± standard deviation of difference in the total activity in the liver between the two modalities was 0% ± 9% (range -21%-18%). Voxel-by-voxel comparisons showed a good agreement in regions corresponding roughly to treated tumor and treated normal liver; the agreement was poorer in regions with low or no expected activity, where PET appeared to overestimate the activity. The correlation coefficients between intrahepatic voxel pairs for the two modalities ranged from 0.86 to 0.94. Cumulative activity volume histograms were in good agreement.

CONCLUSIONS

These data indicate that, with appropriate reconstruction methods and measured calibration correction factors, either Y-90 SPECT/CT or Y-90 PET/CT can be used for quantitative post-therapy monitoring of Y-90 activity distribution following hepatic radioembolization.

Theranostic Imaging of Yttrium-90

This paper overviews Yttrium-90 ((90)Y) as a theranostic and nuclear medicine imaging of (90)Y radioactivity with bremsstrahlung imaging and positron emission tomography. In addition, detection and optical imaging of (90)Y radioactivity using Cerenkov luminescence will also be reviewed. Methods and approaches for qualitative and quantitative (90)Y imaging will be briefly discussed. Although challenges remain for (90)Y imaging, continued clinical demand for predictive imaging response assessment and target/nontarget dosimetry will drive research and technical innovation to provide greater clinical utility of (90)Y as a theranostic agent.

Radioembolization and the Dynamic Role of (90)Y PET/CT

Before the advent of tomographic imaging, it was postulated that decay of (90) Y to the 0(+) excited state of (90)Zr may result in emission of a positron-electron pair. While the branching ratio for pair-production is small (~32 × 10(-6)), PET has been successfully used to image (90) Y in numerous recent patients and phantom studies. (90) Y PET imaging has been performed on a variety of PET/CT systems, with and without time-of-flight (TOF) and/or resolution recovery capabilities as well as on both bismuth-germanate and lutetium yttrium orthosilicate (LYSO)-based scanners. On all systems, resolution and contrast superior to bremsstrahlung SPECT has been reported. The intrinsic radioactivity present in LYSO-based PET scanners is a potential limitation associated with accurate quantification of (90) Y. However, intrinsic radioactivity has been shown to have a negligible effect at the high activity concentrations common in (90) Y radioembolization. Accurate quantification is possible on a variety of PET scanner models, with or without TOF, although TOF improves accuracy at lower activity concentrations. Quantitative (90) Y PET images can be transformed into 3-dimensional (3D) maps of absorbed dose based on the premise that the (90) Y activity distribution does not change after infusion. This transformation has been accomplished in several ways, although the most common is with the use of 3D dose-point-kernel convolution. From a clinical standpoint, (90) Y PET provides a superior post-infusion evaluation of treatment technical success owing to its improved resolution. Absorbed dose maps generated from quantitative PET data can be used to predict treatment efficacy and manage patient follow-up. For patients who receive multiple treatments, this information can also be used to provide patient-specific treatment-planning for successive therapies, potentially improving response. The broad utilization of (90) Y PET has the potential to provide a wealth of dose-response information, which may lead to development of improved radioembolization treatment-planning models in the future.