Comparison of posttherapy 90Y positron emission tomography/computed tomography dosimetry methods in liver therapy with 90Y microspheres

A comparison of methods for in vivo activity and absorbed dose quantification with PET/CT following yttrium-90 radioembolization

BACKGROUND

Yttrium-90 (90Y) positron emission tomography (PET)/computed tomography (CT) imaging is increasingly being used to perform tumor (T) and normal liver (NL) voxel dosimetry after 90Y-radioembolization (90Y-RE). Yet, the accuracy of in vivo 90Y-PET/CT imaging, subject to motion blur and co-registration inaccuracies, and 90Y-PET/CT dose quantification, subject to availability of different voxel dosimetry algorithms, are not well understood.

PURPOSE

The purpose of this study was to investigate the accuracy of 90Y-PET/CT-based activity estimates following 90Y-RE and characterize differences between 90Y-PET/CT-based voxel dosimetry algorithms.

METHODS

Thirty-five patients underwent 90Y-PET/CT imaging after 90Y-RE with TheraSphere. The net administered 90Y activity (Aadmin) was determined using a dose calibrator and pre- and post-procedure exposure rate measurements. The summation of image-based activity (Aimage) was extracted from perfused volume (PV) and 3D-isotropically 2-cm expanded PV contour (PV+2 cm). Absorbed doses were calculated using voxel S-value (VSV), local deposition method (LDM), and LDM with known activity (LDMKA) dosimetry algorithms. Linear regression and Bland-Altman analysis quantified the relationship between Aimage and Aadmin and between mean dose estimates (DLDM, DVSV, DLDM-KA) for PV, T, and perfused NL volumes.

RESULTS

While Aadmin and Aimage in PV were highly correlated (R2 > 0.95), the mean bias ± standard error (SE) and (95% limits of agreement, LOA) was significantly non-zero with -22.7 ± 4.7% (± 28.4%). In PV+2 cm, the mean bias ± SE (± LOA) decreased to 1.3 ± 3.4% (± 18.0%) consistent with zero mean error. DLDM and DVSV were highly correlated (R2 > 0.99) for all volumes of interest (VOIs) and the mean bias ± SE (± LOA) was 2.2 ± 0.2% (± 1.0%), 0.7 ± 0.4% (± 2.8%), and 3.2 ± 0.5% (± 2.8%) for PV, T, and NL, respectively. DLDM-KA and DVSV were correlated with R2 = 0.86, 0.80, and 0.86 for PV, T, and NL, respectively. The mean bias ± SE (± LOA) between DLDM-KA and DVSV was significantly non-zero with -19.6 ± 5.1% (± 31.0%), -20.8 ± 4.4% (± 29.0%), and -18.1 ± 5.3% (± 31.1%) for PV, T, and NL, respectively.

CONCLUSIONS

The summation of Aimage in PV was underestimated relative to Aadmin. Only by accounting for respiratory motion, limited spatial resolution, and PET/CT co-registration errors through VOI expansion was Aimage, on average, equal to Aadmin. The differences between DLDM and DVSV were not clinically relevant, though DLDM-KA was approximately 20% greater than DVSV. Given the high quantitative accuracy of dose calibrators and challenges associated with accurate 90Y-PET/CT quantification, LDMKA is the preferred algorithm for accurate 90Y-PET/CT-based dosimetry following 90Y-RE.

Comparison of 3 Different Therapeutic Particles in Radioembolization of Locally Advanced Intrahepatic Cholangiocarcinoma

Our objective was to compare 3 different therapeutic particles used for radioembolization in locally advanced intrahepatic cholangiocarcinoma. Methods: 90Y-glass, 90Y-resin, and 166Ho-labeled poly(l-lactic acid) microsphere prescribed activity was calculated as per manufacturer recommendations. Posttreatment quantitative 90Y PET/CT and quantitative 166Ho SPECT/CT were used to determine tumor-absorbed dose, whole-normal-liver-absorbed dose, treated-normal-liver-absorbed dose, tumor-to-nontumor ratio, lung-absorbed dose, and lung shunt fraction. Response was assessed using RECIST 1.1 and the [18F]FDG PET-based change in total lesion glycolysis. Hepatotoxicity was assessed using the radioembolization-induced liver disease classification. Results: Six 90Y-glass, 8 90Y-resin, and 7 166Ho microsphere patients were included for analysis. The mean administered activity was 2.6 GBq for 90Y-glass, 1.5 GBq for 90Y-resin, and 7.0 GBq for 166Ho microspheres. Tumor-absorbed dose and treated-normal-liver-absorbed dose were significantly higher for 90Y-glass than for 90Y-resin and 166Ho microspheres (mean tumor-absorbed dose, 197 Gy for 90Y-glass vs. 73 Gy for 90Y-resin and 50 Gy for 166Ho; mean treated-normal-liver-absorbed dose, 79 Gy for 90Y-glass vs. 37 Gy for 90Y-resin and 31 Gy for 166Ho). The whole-normal-liver-absorbed dose and tumor-to-nontumor ratio did not significantly differ between the particles. All patients had a lung-absorbed dose under 30 Gy and a lung shunt fraction under 20%. The 3 groups showed similar toxicity and response according to RECIST 1.1 and [18F]FDG PET-based total lesion glycolysis changes. Conclusion: The therapeutic particles used for radioembolization differed from each other and showed significant differences in absorbed dose, whereas toxicity and response were similar for all groups. This finding emphasizes the need for separate dose constraints and dose targets for each particle.

A single centre intercomparison between commercial treatment planning systems for 90Y radioembolization using virtual and experimental phantoms

INTRODUCTION

Dedicated Treatment Planning Systems (TPSs) were developed to personalize 90Y-transarterial radioembolization. This study evaluated the agreement among four commercial TPSs assessing volumes of interest (VOIs) volumes and dose metrics.

METHODS

A homogeneous (EH) and an anthropomorphic phantom with hot and cold inserts (EA) filled with 99mTc-pertechnetate were acquired with a SPECT/CT scanner. Their virtual versions (VH and VA, respectively) and a phantom with activity inside a single voxel (VK) were generated by an in-house MATLAB script. Images and delineated VOIs were imported into the TPSs to compute voxel-based absorbed dose distributions with various dose deposition approaches: local deposition method (LDM) and dose kernel convolution (DKC) with/without local density correction (LDC). VOI volumes and mean absorbed doses were assessed against their median value across TPSs. Dose-volume histograms (DVHs) and VK-derived dose profiles were evaluated.

RESULTS

Small (<2.1 %) and large (up to 42.4 %) relative volume differences were observed on large (>500 ml) and small VOIs, respectively. Mean absorbed doses relative differences were < 3 % except for small VOIs with steep dose gradients (up to 89.1 % in the VA Cold Sphere VOI). Within the same TPS, LDC negligibly affected the mean absorbed dose, while DKC and LDM showed differences up to 63 %. DHVs were mostly overlapped in experimental phantoms, with some differences in the virtual versions. Dose profiles agreed within 1 %.

CONCLUSION

TPSs showed an overall good agreement except for small VOI volumes and mean absorbed doses of VOIs with steep dose gradients. These discrepancies should be considered in the dosimetry uncertainty assessment, thus requiring an appropriate harmonization.

Activation of Phase Change Contrast Agents Using Ionizing Radiation

The feasibility of activating phase change contrast agents (PCCA) made from Definity (Lantheus Medical Imaging) using X-rays was investigated. A 10 mL of Definity PCCA (0.1 mL PCCA/mL) were injected into gelatin phantoms and irradiated using doses of 0, 30, 50, and 100 Gy. Size distribution and PCCA activation were measured after irradiation. Definity PCCAs were activated at a threshold of 50 Gy. Changes were visible with microscopy, visual inspection of T-flasks, and ultrasound imaging of gelatin phantoms. Moreover, increasing the radiation dose above 50 Gy appeared to further activate PCCA. These results indicate Definity PCCAs may be useful for ultrasound-based radiation dosimetry.

Tumour-to-normal tissue (T/N) dosimetry ratios role in assessment of 90Y selective internal radiation therapy (SIRT)

OBJECTIVE

The purpose of our work is to assess the role of tumour-to-normal tissue (T/N) dosimetry ratios for predicting response in patients undergoing locoregional therapy to the liver with ⁹⁰Y microspheres.

METHODS

A total of 39 patients (7 female:32 male, mean age 68.3 ± 7.6 years), underwent positron emission tomography (PET)/CT imaging after treatment with ⁹⁰Y microspheres. For attenuation correction and localization of the ⁹⁰Y microspheres, the low-dose, non-diagnostic CT images from PET/CT were used. The acquisition took 15 min and the reconstruction matrix size was 200 × 200 × 75 mm and voxel size of 4.07 × 4.07 × 3.00 mm. For dosimetry calculations, the local deposition method with known activity of ⁹⁰Y was used. For each patient, regions of interest for tumour(s) and whole liver were manually created; the normal tissue region of interest was created automatically. mRECIST criteria on MRI done at 1 month post-treatment and subsequently every 3 months after ⁹⁰Y treatment, were used to assess response.

RESULTS

For 39 patients, the mean liver, tumour and normal tissue doses (mean ± SD) were, 55.17 ± 26.04 Gy, 911.87 ± 866.54 Gy and 47.79 ± 20.47 Gy, respectively. Among these patients, 31 (79%) showed complete response (CR) and 8 (21%) showed progression of disease (PD). For patients with CR, the mean T/N dose ratio obtained was 24.91 (range 3.09-80.12) and for patients with PD, the mean T/N dose ratio was significantly lower, at 6.69 (range 0.36-14.75).

CONCLUSION

Our data show that patients with CR have a statistically higher T/N dose ratio than those with PD. Because, the number of PD cases was limited and partial volume effect was not considered, further investigation is warranted.

ADVANCES IN KNOWLEDGE

T/N dosimetry ratios can be used for assessing response in patients undergoing locoregional therapy to the liver with ⁹⁰Y microspheres.

An estimate of 90Y dosimetry for bremsstrahlung SPECT/CT imaging in liver therapy with 90Y microspheres

OBJECTIVE

Bremsstrahlung SPECT/CT (bSPECT/CT) is one of the most common methods for post-therapy imaging in ⁹⁰Y microspheres selective internal radiation therapy (SIRT) of liver cancers. Here, we are proposing a simple approach using bSPECT/CT to estimate mean absorbed dose to the liver in patients undergoing treatment with ⁹⁰Y microspheres.

MATERIALS AND METHODS

In our previous study comparing ⁹⁰Y dosimetry obtained using bSPECT/CT vs PET/CT, we found that there was a large difference between the mean absorbed dose values to the whole-liver. However, there was a high linear correlation between the doses, which presented an opportunity for quantitative assessment using bSPECT/CT ⁹⁰Y imaging. In this study, after treatment with ⁹⁰Y microspheres, 43 patients were immediately imaged on a dual-head Infinia SPECT/CT gamma camera and on a mCT PET/CT system. Images from 25 of the patients, randomly selected, were used to calculate the correlation of mean liver doses obtained from bSPECT/CT vs. PET/CT. For the remaining 18 patients, the calculated correlation was used to estimate doses obtained from bSPECT/CT, and these estimations were then compared to the doses obtained from PET/CT, considered the gold standard for quantitative analysis.

RESULTS

From the 25 selected patients, the calculated linear correlation between bSPECT/CT and PET/CT ⁹⁰Y mean absorbed doses in whole liver was high (r^2 = 0.97), with a slope of 2.80 and an intercept of -0.63. This linear fit was used to calculate the bSPECT/CT doses for the remaining 18 patients. For these patients, the mean whole-liver dose obtained from bSPECT/CT fitted data vs that obtained from PET/CT were 50.59 Gy and 50.81 Gy, respectively. The average dose difference was 0.2 ± 5.4 Gy (range -18.2%-13.0%). The repeatability coefficient was 10.5 (20.8 % of the mean).

CONCLUSION

Although quantitative bremsstrahlung imaging is difficult, it is possible to calculate adequate estimates of whole-liver dosimetry from bSPECT/CT imaging that is calibrated using its correlation with post-therapy PET/CT ⁹⁰Y images.