Dosimetry of Y-90 Microspheres Utilizing Tc-99m SPECT and Y-90 PET

Y-90 PET/MR imaging optimization with a Bayesian penalized likelihood reconstruction algorithm

Positron Emission Tomography (PET) imaging after90 Y liver radioembolization is used for both lesion identification and dosimetry. Bayesian penalized likelihood (BPL) reconstruction algorithms are an alternative to ordered subset expectation maximization (OSEM) with improved image quality and lesion detectability. The investigation of optimal parameters for90 Y image reconstruction of Q.Clear, a commercial BPL algorithm developed by General Electric (GE), in PET/MR is a field of interest and the subject of this study. The NEMA phantom was filled at an 8:1 sphere-to-background ratio. Acquisitions were performed on a PET/MR scanner for clinically relevant activities between 0.7 and 3.3 MBq/ml. Reconstructions with Q.Clear were performed varying the β penalty parameter between 20 and 6000, the acquisition time between 5 and 20 min and pixel size between 1.56 and 4.69 mm. OSEM reconstructions of 28 subsets with 2 and 4 iterations with and without Time-of-Flight (TOF) were compared to Q.Clear with β = 4000. Recovery coefficients (RC), their coefficient of variation (COV), background variability (BV), contrast-to-noise ratio (CNR) and residual activity in the cold insert were evaluated. Increasing β parameter lowered RC, COV and BV, while CNR was maximized at β = 4000; further increase resulted in oversmoothing. For quantification purposes, β = 1000-2000 could be more appropriate. Longer acquisition times resulted in larger CNR due to reduced image noise. Q.Clear reconstructions led to higher CNR than OSEM. A β of 4000 was obtained for optimal image quality, although lower values could be considered for quantification purposes. An optimal acquisition time of 15 min was proposed considering its clinical use.

A microscopic model of the dose distribution in hepatocellular carcinoma after selective internal radiation therapy

The dosimetry evaluation for the selective internal radiation therapy is currently performed assuming a uniform activity distribution, which is in contrast with literature findings. A 2D microscopic model of the perfused liver was developed to evaluate the effect of two different 90Y microspheres distributions: i) homogeneous partitioning with the microspheres equally distributed in the perfused liver, and ii) tumor-clustered partitioning where the microspheres distribution is inferred from the patient specific images.

METHODS

Two subjects diagnosed with liver cancer were included in this study. For each subject, abdominal CT scans acquired prior to the SIRT and post-treatment 90Y positron emission tomography were considered. Two microspheres partitionings were simulated namely homogeneous and tumor-clustered partitioning. The homogeneous and tumor-clustered partitionings were derived starting from CT images. The microspheres radiation is simulated by means of Russell's law.

RESULTS

In homogenous simulations, the dose delivery is uniform in the whole liver while in the tumor-clustered simulations a heterogeneous distribution of the delivered dose is visible with higher values in the tumor regions. In addition, in the tumor-clustered simulation, the delivered dose is higher in the viable tumor than in the necrotic tumor, for all patients. In the tumor-clustered case, the dose delivered in the non-tumoral tissue (NTT) was considerably lower than in the perfused liver.

CONCLUSIONS

The model proposed here represents a proof-of-concept for personalized dosimetry assessment based on preoperative CT images.

90Y post-radioembolization clinical assessment with whole-body Biograph Vision Quadra PET/CT: image quality, tumor, liver and lung dosimetry

PURPOSE

Evaluation of 90Y liver radioembolization post-treatment clinical data using a whole-body Biograph Vision Quadra PET/CT to investigate the potential of protocol optimization in terms of scan time and dosimetry.

METHODS

17 patients with hepatocellular carcinoma with median (IQR) injected activity 2393 (1348-3298) MBq were included. Pre-treatment dosimetry plan was based on 99mTc-MAA SPECT/CT with Simplicit90Y™ and post-treatment validation with Quadra using Simplicit90Y™ and HERMIA independently. Regarding the image analysis, mean and peak SNR, the coefficient of variation (COV) and lesion-to-background ratio (LBR) were evaluated. For the post-treatment dosimetry validation, the mean tumor, whole liver and lung absorbed dose evaluation was performed using Simplicit90Y and HERMES. Images were reconstructed with 20-, 15-, 10-, 5- and 1- min sinograms with 2, 4, 6 and 8 iterations. Wilcoxon signed rank test was used to show statistical significance (p < 0.05).

RESULTS

There was no difference of statistical significance between 20- and 5- min reconstructed times for the peak SNR, COV and LBR. In addition, there was no difference of statistical significance between 20- and 1- min reconstructed times for all dosimetry metrics. Lung dosimetry showed consistently lower values than the expected. Tumor absorbed dose based on Simplicit90Y™ was similar to the expected while HERMES consistently underestimated significantly the measured tumor absorbed dose. Finally, there was no difference of statistical significance between expected and measured tumor, whole liver and lung dose for all reconstruction times.

CONCLUSION

In this study we evaluated, in terms of image quality and dosimetry, whole-body PET clinical images of patients after having been treated with 90Y microspheres radioembolization for liver cancer. Compared to the 20-min standard scan, the simulated 5-min reconstructed images provided equal image peak SNR and noise behavior, while performing also similarly for post-treatment dosimetry of tumor, whole liver and lung absorbed doses.

Phantom study for 90Y liver radioembolization dosimetry with a long axial field-of-view PET/CT

PURPOSE

The physical properties of yttrium-90 (90Y) allow for imaging with positron emission tomography/computed tomography (PET/CT). The increased sensitivity of long axial field-of-view (LAFOV) PET/CT scanners possibly allows to overcome the small branching ratio for positron production from 90Y decays and to improve for the post-treatment dosimetry of 90Y of selective internal radiation therapy.

METHODS

For the challenging case of an image quality body phantom, we compare a full Monte Carlo (MC) dose calculation with the results from the two commercial software packages Simplicit90Y and Hermes. The voxel dosimetry module of Hermes relies on the 90Y images taken with a LAFOV PET/CT, while the MC and Simplicit90Y dose calculations are image independent.

RESULTS

The resulting doses from the MC calculation and Simplicit90Y agree well within the error margins. The image-based dose calculation with Hermes, however, consistently underestimates the dose. This is due to the mismatch of the activity distribution in the PET images and the size of the volume of interest. We found that only for the smallest phantom sphere there is a statistically significant dependence of the Hermes dose on the image reconstruction parameters and scan time.

CONCLUSION

Our study shows that Simplicit90Y's local deposition model can provide a reliable dose estimate. On the other hand, the image based dose calculation suffers from the suboptimal reconstruction of the 90Y distribution in small structures.

Predictive Dosimetry and Outcomes of Hepatocellular Carcinoma Treated by Yttrium-90 Resin Microsphere Radioembolization: A Retrospective Analysis Using Technetium-99m Macroaggregated Albumin Single Photon Emission CT/CT and Planning Software

PURPOSE

To characterize estimated mean absorbed tumor dose (ADT), objective response (OR), and estimated target dose of hepatocellular carcinoma (HCC) after resin microsphere yttrium-90 (90Y) radioembolization using partition dosimetry.

MATERIALS AND METHODS

In this retrospective, single-center study, multicompartment dosimetry of index tumors receiving 90Y radioembolization between October 2015 and June 2022 was performed using a commercial software package and pretreatment technetium-99m macroaggregated albumin single photon emission computed tomography (SPECT)/computed tomography (CT). In total, 101 patients with HCC underwent 102 treatments of 127 index tumors. Patients underwent imaging every 2-3 months after treatment to determine best response per modified Response Evaluation Criteria in Solid Tumors (mRECIST). Best response was defined as the greatest response category per mRECIST and categorized as OR or nonresponse (NR). A Cox proportional hazards model evaluated the probability of tumor OR and progression-free survival using ADT.

RESULTS

The median follow-up period was 148 days (interquartile range [IQR], 92-273 days). The median ADT of OR was 141.9 Gy (IQR, 89.4-215.8 Gy) compared with the median ADT of NR treatments of 70.8 Gy (IQR, 42.0-135.3 Gy; P < .001). Only ADT was predictive of response (hazard ratio = 2.79 [95% confidence interval {CI}: 1.44-5.40]; P = .003). At 6 months, an ADT of 157 Gy predicted 90.0% (95% CI: 41.3%-98.3%) probability of OR. At 1 year, an ADT of 157 Gy predicted 91.6% (95% CI: 78.3%-100%) probability of progression-free survival. Partition modeling and delivered activity were predictive of progression (P = .021 and P = .003, respectively).

CONCLUSIONS

For HCC treated with resin microspheres, tumors receiving higher ADT exhibited higher rates of OR. An ADT of 157 Gy predicted 90.0% OR at 6 months.

Using Deep Learning to Predict Treatment Response in Patients with Hepatocellular Carcinoma Treated with Y90 Radiation Segmentectomy

Treatment of hepatocellular carcinoma (HCC) with Y90 radioembolization segmentectomy (Y90-RE) demonstrates a tumor dose-response threshold, where dose estimates are highly dependent on accurate SPECT/CT acquisition, registration, and reconstruction. Any error can result in distorted absorbed dose distributions and inaccurate estimates of treatment success. This study improves upon the voxel-based dosimetry model, one of the most accurate methods available clinically, by using a deep convolutional network ensemble to account for the spatially variable uptake of Y90 within a treated lesion. A retrospective analysis was conducted in patients with HCC who received Y90-RE at a single institution. Seventy-seven patients with 103 lesions met the inclusion criteria: three or fewer tumors, pre- and post treatment MRI, and no prior Y90-RE. Lesions were labeled as complete (n = 57) or incomplete response (n = 46) based on 3-month post treatment MRI and divided by medical record number into a 20% hold-out test set and 80% training set with 5-fold cross-validation. Slice-wise predictions were made from an average ensemble of models and thresholds from the highest accuracy epochs across all five folds. Lesion predictions were made by thresholding all slice predictions through the lesion. When compared to the voxel-based dosimetry model, our model had a higher F1-score (0.72 vs. 0.2), higher accuracy (0.65 vs. 0.60), and higher sensitivity (1.0 vs. 0.11) at predicting complete treatment response. This algorithm has the potential to identify patients with treatment failure who may benefit from earlier follow-up or additional treatment.

Voxel-S-value methods adapted to heterogeneous media for quantitative Y-90 microsphere radioembolization dosimetry

PURPOSE

The absorbed dose estimation from Voxel-S-Value (VSV) method in heterogeneous media is suboptimal as VSVs are calculated in homogeneous media. The aim of this study is to develop and evaluate new VSV methods in order to enhance the accuracy of Y-90 microspheres absorbed dose estimation in liver, lungs, tumors and lung-liver interface regions.

METHODS

Ten patients with Y-90 microspheres SPECT/CT and PET/CT data, six of whom had additional Tc-99m-macroaggregated albumin SPECT/CT data, were analyzed from the Deep Blue Data Repository. Seven existing VSV methods along with three newly proposed VSV methods were evaluated: liver and lung kernel with center voxel scaling (LiLuCK), liver kernel with density correction and lung kernel with center voxel scaling (LiKDLuCK), liver kernel with center voxel scaling and lung kernel with density correction (LiCKLuKD). Monte Carlo (MC) results were regarded as the gold standard. Absolute absorbed dose errors (%AADE) of these methods for the liver, lungs, tumors, upper liver, and lower lungs were assessed.

RESULTS

Liver and tumor's median %AADE of all methods were <3% for three types of imaging data. In the lungs, however, three recently proposed VSV methods provided median %AADEs of less than 7%, whereas the differences exceeded 20% for existing methods that did not use a lung kernel. LiCKLuKD could achieve median %AADE <2% in the liver, upper liver and tumors, and median %AADE <7% in the lungs and lower lungs in three types of data.

CONCLUSION

All methods are consistent with MC in the liver and tumors. Methods with tissue-specific kernel and effective correction achieve smaller errors in lungs. LiCKLuKD has comparable results with MC in absorbed dose estimation of Y-90 radioembolization for all target regions.

Phantom-based evaluation of yttrium-90 datasets using biograph vision quadra

PURPOSE

The image quality characteristics of two NEMA phantoms with yttrium-90 (⁹⁰Y) were evaluated on a long axial field-of-view (AFOV) PET/CT. The purpose was to identify the optimized reconstruction setup for the imaging of patients with hepatocellular carcinoma after ⁹⁰Y radioembolization.

METHODS

Two NEMA phantoms were used, where one had a 1:10 sphere to background activity concentration ratio and the second had cold background. Reconstruction parameters used are as follows: iterations 2 to 8, Gaussian filter 2- to 6-mm full-width-at-half-maximum, reconstruction matrices 440 × 440 and 220 × 220, high sensitivity (HS), and ultra-high sensitivity (UHS) modes. 50-, 40-, 30-, 20-, 10-, and 5-min acquisitions were reconstructed. The measurements included recovery coefficients (RC), signal-to-noise ratio (SNR), background variability, and lung error which measures the residual error in the corrections. Patient data were reconstructed with 20-, 10-, 5-, and 1-min time frames and evaluated in terms of SNR.

RESULTS

The RC for the hot phantom was 0.36, 0.45, 0.53, 0.63, 0.68, and 0.84 for the spheres with diameters of 10, 13, 17, 22, 28, and 37 mm, respectively, for UHS 2 iterations, a 220 × 220 matrix, and 50-min acquisition. The RC values did not differ with acquisition times down to 20 min. The SNR was the highest for 2 iterations, measured 11.7, 16.6, 17.6, 19.4, 21.9, and 27.7 while the background variability was the lowest (27.59, 27.08, 27.36, 26.44, 30.11, and 33.51%). The lung error was 18%. For the patient dataset, the SNR was 19%, 20%, 24%, and 31% higher for 2 iterations compared to 4 iterations for 20-, 10-, 5-, and 1-min time frames, respectively.

CONCLUSIONS

This study evaluates the NEMA image quality of a long AFOV PET/CT scanner with ⁹⁰Y. It provides high RC for the smallest sphere compared to other standard AFOV scanners at shorter scan times. The maximum patient SNR was for 2 iterations, 20 min, while 5 min delivers images with acceptable SNR.

Accuracy and Safety of Scout Dose Resin Yttrium-90 Microspheres for Radioembolization Therapy Treatment Planning: A Prospective Single-Arm Clinical Trial

PURPOSE

To compare the accuracy and safety of 0.56 GBq resin yttrium-90 (⁹⁰Y) (scout⁹⁰Y) microspheres with those of technetium-99m macroaggregated albumin (MAA) in predicting the therapeutic ⁹⁰Y (Rx⁹⁰Y) dose for patients with hepatocellular carcinoma (HCC).

MATERIALS AND METHODS

This prospective single-arm clinical trial (Clinicaltrials.gov: NCT04172714) recruited patients with HCC. Patients underwent same-day mapping with MAA and scout⁹⁰Y. Rx⁹⁰Y activity was administered 3 days after mapping. Using paired t test and Pearson correlation, the tumor-to-normal ratio (TNR), lung shunt fraction (LSF), predicted mean tumor dose (TD), and nontumoral liver dose (NTLD) by MAA and scout⁹⁰Y were compared with those by Rx⁹⁰Y. Bland-Altman plots compared the level of agreement between the TNR and LSF of scout⁹⁰Y and MAA with that of Rx⁹⁰Y. The safety of scout⁹⁰Y was evaluated by examining the discrepancy in extrahepatic activity between MAA and scout⁹⁰Y.

RESULTS

Thirty patients were treated using 19 segmental and 14 nonsegmental (ie, 2 contiguous segments or nonsegmental) therapies. MAA had weak LSF, moderate TNR, and moderate TD linear correlation with Rx⁹⁰Y. Scout⁹⁰Y had a moderate LSF, strong TNR, strong TD, and very strong NTLD in correlation with those of Rx⁹⁰Y. Furthermore, the TNR and LSF of scout⁹⁰Y had a stronger agreement with those of Rx⁹⁰Y than with those of MAA. In the nonsegmental subgroup, MAA had no significant correlation with the TD and NTLD of Rx⁹⁰Y, whereas scout⁹⁰Y had a very strong correlation with both of these factors. In the segmental subgroup, both MAA and scout⁹⁰Y had a strong linear correlation with the TD and NTLD of Rx⁹⁰Y.

CONCLUSIONS

Compared with MAA, scout⁹⁰Y is a more accurate surrogate for Rx⁹⁰Y biodistribution for nonsegmental therapies.

Effect of tumour involvement on activity determination of resin Yttrium-90 in selective internal radiation therapy of metastatic liver cancer

Introduction:

The study was aimed to evaluate the effect of tumour involvement on resin Yttrium-90 (Y90) activity determination for metastatic liver cancer treatment.

Methods:

One hundred and two cases of resin Y90 microsphere treatment were retrospectively studied. Body surface area (BSA) method was used in the calculation of resin Y90 activity. The total activity (TA) was calculated as a summation of activities obtained from BSA-based calculation and tumour involvement (TI). TI and TA of each case were evaluated. The contributions of TI to TA were calculated with the ratio of TI/TA.

Results:

The average contribution of TI to TA was 4·1%. The contributions were < 5·8% in 75% of the cases, < 2·2% in 50% of the cases and < 1·0% in 25% of the cases.

Conclusions:

Overall the effect of tumour involvement on the activity determination was small. The activity calculation could be simplified by neglecting TI in 25% of the cases where the activity contribution from TI was less than 1%. Contouring tumour and liver structures for TI calculation could be avoided in these cases, and the efficiency of the workflow for resin Y90 procedures could be improved.

Radioembolization, Principles and indications

Radioembolization is the selective application of radionuclide-loaded microspheres into liver arteries for the therapy of liver tumours and metastases. In this review, we focused on therapy planning and dosimetry, as well as the main indications of 90Y-glass and resin microspheres and 166Ho-microspheres.

Realized tumor to normal ratios in hepatocellular carcinoma patients undergoing transarterial radioembolization: a retrospective evaluation

OBJECTIVES

To determine the realized tumor to normal ratios (TNRs) in patients undergoing radiation segmentectomies (RS); determine the relationship between TNRs and particle load in transarterial radioembolization (TARE).

METHODS

In total, 148 patients who underwent 184 TARE procedures for hepatocellular carcinoma were evaluated. Post treatment SPECT CT bremsstrahlung imaging was analyzed utilizing Simplicit90y™ to determine realized TNR. A model which normalized activity across all RS treatments to a level that would achieve 400 Gy by unicompartmental dosing was created to determine the affect realized TNR would have on tumor absorbed dose.

RESULTS

The mean TNR in the setting of RS was 2.88 ± 1.60 and was higher for glass as compared to resin microspheres (3.07 ± 1.68 vs 2.24 ± 1.21, p = 0.01). The TNR was significantly greater in the RS as compared to the lobar deliveries (2.88 ± 1.60 vs 2.16 ± 1.12, p < 0.01). When normalizing the activity of RS treatments to the level required to achieve 400 Gy by unicompartmental calculations, there was found to be significant differences in the predicted tumor absorbed dose when separated by the median tumor dose (601.2 ± 133.3 vs 1146.9 ± 297.5, p < 0.01) or median realized TNR (1119.2 ± 341 Gy vs 635.7 ± 160.2 Gy, p < 0.01). Particle load was found to be associated with TNR on univariate (p < 0.01) and multivariate (p < 0.01) analysis.

CONCLUSION

Significant TNRs are seen in RS and perhaps argue for the use of multi-compartmental dosimetry techniques in this setting and particle load may affect TNR.

KEY POINTS

• Tumor to normal ratios were significantly higher in radiation segmentectomies than lobar deliveries. • Tumor to normal ratios were significantly higher when utilizing glass, as compared to resin microspheres. • When creating a model that prescribed the activity required to reach 400 Gy by MIRD, realized tumor dose varied significantly in radiation segmentectomies.

Key Role of Personalized Dosimetry in Dose Adjustment for Selective Internal Radiotherapy: Retrospective Study of Patients Treated With 90Y Resin Microspheres

PURPOSE

Selective internal radiotherapy with 90Y microspheres is widely used for the treatment of patients with liver cancer. A dosimetric analysis using the dosimetry software Simplicit90y (Boston Scientific, Natick, MA) was conducted to define doses to the tumor and healthy liver, and to determine a threshold tumor dose that could predict progression-free survival.

METHODS

Patients experiencing hepatocellular carcinoma and treated with 90Y-labeled resin microspheres were included in a retrospective study. The time-to-progression of the target lesions (TTPLs) and overall survival (OS) were evaluated using Kaplan-Meier tests, and this comparison was based on a log-rank test.

RESULTS

Twenty-four procedures for patients with portal vein thrombosis were realized. Median follow-up was 16 months. A threshold tumor dose of 125 Gy was determined with a sensitivity of 89% and a specificity of 100%. For patients with a tumor dose of less than 125 Gy, the median OS was 7.5 months (95% confidence interval [CI], 5-14 months) and the TTPL was 3 months (95% CI, 2-6 months) versus 33 months (95% CI, 22-39 months) and 23 months (95% CI, 7-38 months), respectively, for those with a tumor dose of 125 Gy or more (P = 0.002 and P = 0.0004).

CONCLUSIONS

Personalized dosimetry based on 99mTc-MAA SPECT/CT is predictive of TTPL and OS in patients with hepatocellular carcinoma. Customized dosimetry software is essential to optimize treatment planning.

The Role of Ablative Radiotherapy to Liver Oligometastases from Colorectal Cancer

Purpose of Review

This review describes recent data supporting locoregional ablative radiation in the treatment of oligometastatic colorectal cancer liver metastases.

Recent Findings

Stereotactic body radiotherapy (SBRT) demonstrates high rates of local control in colorectal cancer liver metastases when a biologically equivalent dose of > 100 Gy is delivered. Future innovations to improve the efficacy of SBRT include MRI-guided radiotherapy (MRgRT) to enhance target accuracy, systemic immune activation to treat extrahepatic disease, and genomic customization. Selective internal radiotherapy (SIRT) with y-90 is an intra-arterial therapy that delivers high doses to liver metastases internally which has shown to increase liver disease control in phase 3 trials. Advancements in transarterial radioembolization (TARE) dosimetry could improve local control and decrease toxicity.

Summary

SBRT and SIRT are both promising options in treating unresectable metastatic colorectal cancer liver metastases. Identification of oligometastatic patients who receive long-term disease control from either therapy is essential. Future advancements focusing on improving radiation design and customization could further improve efficacy and toxicity.

International recommendations for personalised selective internal radiation therapy of primary and metastatic liver diseases with yttrium-90 resin microspheres

PURPOSE

A multidisciplinary expert panel convened to formulate state-of-the-art recommendations for optimisation of selective internal radiation therapy (SIRT) with yttrium-90 (90Y)-resin microspheres.

METHODS

A steering committee of 23 international experts representing all participating specialties formulated recommendations for SIRT with 90Y-resin microspheres activity prescription and post-treatment dosimetry, based on literature searches and the responses to a 61-question survey that was completed by 43 leading experts (including the steering committee members). The survey was validated by the steering committee and completed anonymously. In a face-to-face meeting, the results of the survey were presented and discussed. Recommendations were derived and level of agreement defined (strong agreement ≥ 80%, moderate agreement 50%-79%, no agreement ≤ 49%).

RESULTS

Forty-seven recommendations were established, including guidance such as a multidisciplinary team should define treatment strategy and therapeutic intent (strong agreement); 3D imaging with CT and an angiography with cone-beam-CT, if available, and 99mTc-MAA SPECT/CT are recommended for extrahepatic/intrahepatic deposition assessment, treatment field definition and calculation of the 90Y-resin microspheres activity needed (moderate/strong agreement). A personalised approach, using dosimetry (partition model and/or voxel-based) is recommended for activity prescription, when either whole liver or selective, non-ablative or ablative SIRT is planned (strong agreement). A mean absorbed dose to non-tumoural liver of 40 Gy or less is considered safe (strong agreement). A minimum mean target-absorbed dose to tumour of 100-120 Gy is recommended for hepatocellular carcinoma, liver metastatic colorectal cancer and cholangiocarcinoma (moderate/strong agreement). Post-SIRT imaging for treatment verification with 90Y-PET/CT is recommended (strong agreement). Post-SIRT dosimetry is also recommended (strong agreement).

CONCLUSION

Practitioners are encouraged to work towards adoption of these recommendations.

Retrospective SPECT/CT dosimetry following transarterial radioembolization

Transarterial radioembolization (TARE) effectively treats unresectable primary and metastatic liver tumors through intra‐arterial injection of Yttrium‐90 (⁹⁰Y) beta particle emitting microspheres which implant around the tumor. Current dosimetry models are highly simplistic and there is a large need for an image‐based dosimetry post‐TARE, which would improve treatment safety and efficacy. Current post‐TARE imaging is ⁹⁰Y bremsstrahlung SPECT/CT and we study the use of these images for dosimetry. Retrospective image review of ten patients having a Philips Healthcare^TM SPECT/CT following TARE SIR‐Spheres® implantation. Emission series with attenuation correction were resampled to 3 mm resolution and used to create image‐based dose distributions. Dose distributions and analysis were performed in MIM Software SurePlan^TM utilizing SurePlan^TM Local Deposition Method (LDM) and a dose convolution method (WFBH). We sought to implement a patient‐specific background subtraction prior to dose calculation to make these noisy bremsstrahlung SPECT images suitable for post‐TARE dosimetry. On average the percentage of mean background counts to maximum count in the image across all patients was 9.4 ± 4.9% (maximum = 7.6%, minimum = 2.3%). Absolute dose increased and profile line width decreased as background subtraction value increased. The average value of the LDM and WFBH dose methods was statistically the same. As background subtraction value increased, the DVH curves become unrealistic and distorted. Background subtraction on bremsstrahlung SPECT image has a large effect on post‐TARE dosimetry. The background contour defined provides a systematic estimate to the activity background that accounts for the scanner and patient conditions at the time of the image study and is easily implemented using commercially available software. Using the mean count in the background contour as a subtraction across the entire image gave the most realistic dose distributions. This methodology is independent of microsphere and software manufacturer allowing for use with any available products or tools.

Impact of the dosimetry approach on the resulting 90Y radioembolization planned absorbed doses based on 99mTc-MAA SPECT-CT: is there agreement between dosimetry methods?

BACKGROUND

Prior radioembolization, a simulation using ^99mTc-macroaggregated albumin as ⁹⁰Y-microspheres surrogate is performed. Gamma scintigraphy images (planar, SPECT, or SPECT-CT) are acquired to evaluate intrahepatic ⁹⁰Y-microspheres distribution and detect possible extrahepatic and lung shunting. These images may be used for pre-treatment dosimetry evaluation to calculate the ⁹⁰Y activity that would get an optimal tumor response while sparing healthy tissues. Several dosimetry methods are available, but there is still no consensus on the best methodology to calculate absorbed doses. The goal of this study was to retrospectively evaluate the impact of using different dosimetry approaches on the resulting ⁹⁰Y-radioembolization pre-treatment absorbed dose evaluation based on ^99mTc-MAA images.

METHODS

Absorbed doses within volumes of interest resulting from partition model (PM) and 3D voxel dosimetry methods (3D-VDM) (dose-point kernel convolution and local deposition method) were evaluated. Additionally, a new "Multi-tumor Partition Model" (MTPM) was developed. The differences among dosimetry approaches were evaluated in terms of mean absorbed dose and dose volume histograms within the volumes of interest.

RESULTS

Differences in mean absorbed dose among dosimetry methods are higher in tumor volumes than in non-tumoral ones. The differences between MTPM and both 3D-VDM were substantially lower than those observed between PM and any 3D-VDM. A poor correlation and concordance were found between PM and the other studied dosimetry approaches. DVH obtained from either 3D-VDM are pretty similar in both healthy liver and individual tumors. Although no relevant global differences, in terms of absorbed dose in Gy, between both 3D-VDM were found, important voxel-by-voxel differences have been observed.

CONCLUSIONS

Significant differences among the studied dosimetry approaches for ⁹⁰Y-radioembolization treatments exist. Differences do not yield a substantial impact in treatment planning for healthy tissue but they do for tumoral liver. An individual segmentation and evaluation of the tumors is essential. In patients with multiple tumors, the application of PM is not optimal and the 3D-VDM or the new MTPM are suggested instead. If a 3D-VDM method is not available, MTPM is the best option. Furthermore, both 3D-VDM approaches may be indistinctly used.

Chemorefractory liver metastasis from cervical cancer successfully treated with a combination of yttrium-90 and immunotherapy

Liver metastases in cervical cancer is rare and can be difficult-to-treat. The current guidelines established by the Gynecologic Oncology Group recommend platinum-based systemic chemotherapy in combination with an anti-angiogenic agent such as bevacizumab, however, overall survival remains poor following diagnosis and options for patients who fail chemotherapy are limited. Yttrium-90 (Y90) radioembolization (RE) has shown great promise in the treatment of chemo-refractory colorectal liver metastases. We describe a 30-year-old female with a history of stage IB endocervical adenocarcinoma who later developed metastases to the liver, that were unresponsive to multiple chemotherapeutics and chemoembolization, and was successfully treated with Y90 RE with concurrent systemic Pembrolizumab. The Y90 RE treatment resulted in positive clinical and imaging responses with improvement in her quality of life, all of which continue to persist at the time of writing this manuscript about 8-months into her RE treatment.

Transarterial Radioembolization for Hepatocellular Carcinoma and Hepatic Metastases: Clinical Aspects and Dosimetry Models

Transarterial radioembolization (TARE) with Yttrium-90 (⁹⁰Y) microspheres is a liver-directed therapy for primary and metastatic disease. This manuscript provides a review of the clinical literature on TARE indications and efficacy with overviews of patient-selection and toxicity. Current dosimetry models used in practice are safe, relatively simple, and easy for clinicians to use. Planning currently relies on the imperfect surrogate, ^99mTc macroaggregated albumin. Post-therapy quantitative imaging (⁹⁰Y SPECT/CT or ⁹⁰Y PET/CT) of microspheres can be used to calculate the macroscopic in vivo absorbed dose distribution. Similar to the evolution of other brachytherapy dose calculations, TARE is moving toward more patient-specific dosimetry that includes calculating and reporting nonuniform dose distributions throughout tumors and normal uninvolved liver.