Post-treatment Dosimetry Research Articles

Comparison of perfused volume segmentation between cone-beam CT and 99mTc-MAA SPECT/CT for treatment dosimetry before selective internal radiation therapy using 90Y-glass microspheres

PurposeTo compare the reliability and accuracy of the pre-treatment dosimetry predictions using cone-beam computed tomography (CBCT) versus 99mTc-labeled macroaggregated albumin (MAA) SPECT/CT for perfused volume segmentation in patients with hepatocellular carcinoma treated by selective internal radiation therapy (SIRT) using 90Y-glass microspheres. Materials and methodsFifteen patients (8 men, 7 women) with a mean age of 68.3±10.5 (SD) years (range: 47–82 years) who underwent a total of 17 SIRT procedures using 90Y-glass microspheres for unresectable hepatocellular carcinoma were retrospectively included. Pre-treatment dosimetry data were calculated from 99mTc-MAA SPECT/CT using either CBCT or 99mTc-MAA SPECT/CT to segment the perfused volumes. Post-treatment dosimetry data were calculated using 90Y imaging (SPECT/CT or PET/CT). The whole liver, non-tumoral liver, and tumor volumes were segmented on CT or MRI data. The mean absorbed doses of the tumor (DT), non-tumoral liver, perfused liver (DPL) and perfused non-tumoral liver were calculated. Intra- and interobserver reliabilities were investigated by calculating Lin's concordant correlation coefficients (ρc values). The differences (biases) between pre- and post-treatment dosimetry data were assessed using the modified Bland–Altman method (for non-normally distributed variables), and systematic bias was evaluated using Passing–Bablok regression. ResultsThe intra- and interobserver reliabilities were good-to-excellent (ρc: 0.80–0.99) for all measures using both methods. Compared with 90Y imaging, the median differences were 5.8Gy (IQR: −12.7; 16.1) and 5.6Gy (IQR: −13.6; 10.2) for DPL-CBCT and DPL-99mTc-MAA SPECT/CT, respectively. The median differences were 1.6Gy (IQR: −29; 7.53) and 9.8Gy (IQR: −28.4; 19.9) for DT-CBCT and DT-99mTc-MAA SPECT/CT respectively. Passing–Bablok regression analysis showed that both CBCT and 99mTc-MAA SPECT/CT had proportional biases and thus tendencies to overestimate DT and DPL at higher post-treatment doses. ConclusionCBCT may be a reliable segmentation method, but it does not significantly increase the accuracy of dose prediction compared with that of 99mTc-MAA SPECT/CT. At higher doses both methods tend to overestimate the doses to tumors and perfused livers.

Dosimetric methodology for 131I therapy for benign thyroid diseases

Treatment with 131I is one of the standard treatments for benign thyroid disease. In most cases, the prescribed dose is specific for the same disease. Inspired by external radiotherapy, the emergence of personalized pre-therapeutic dosimetry in nuclear medicine departments makes it possible to adapt to the patient's diseases and physiology; more specific to its kinetics of iodine distribution. By personalizing the prescribed activity, dosimetry makes it possible to comply with the ALARA principle in metabolic radiotherapy while at the same time preserving the therapeutic objective. Post-treatment dosimetry permits to validate the methodology and, by recording the dose actually delivered to the organs surrounding the target volume, to contribute to the understanding of the dose-effect relationship. A re-reading of the MIRD formalism guidelines is important as this document lists the procedure to be followed in thyroid dosimetry. Defining the sensitivity, calculating the S factors, estimating the cumulative activity and residence times and then calculating the dose are the different steps presented. The need to switch from fixed dose prescription to personalized prescription seems to be evident. Several studies in progress as well as the emergence of new software compiling all the information essential for its implementation will permit nuclear medicine departments to involve themselves more easily into the process.

Analysis of differences between 99mTc-MAA SPECT- and 90Y-microsphere PET-based dosimetry for hepatocellular carcinoma selective internal radiation therapy

BackgroundThe aim of this study was to compare predictive and post-treatment dosimetry and analyze the differences, investigating factors related to activity preparation and delivery, imaging modality used, and interventional radiology.MethodsTwenty-three HCC patients treated by selective internal radiation therapy with 90Y glass microspheres were included in this study. Predictive and post-treatment dosimetry were calculated at the voxel level based on 99mTc-MAA SPECT/CT and 90Y-microsphere PET/CT respectively. Dose distribution was analyzed through mean dose, metrics extracted from dose-volume histograms, and Dice similarity coefficients applied on isodoses. Reproducibility of the radiological gesture and its influence on dose deviation was evaluated.Results90Y delivered activity was lower than expected in 67% (16/24) of the cases mainly due to the residual activity. A mean deviation of − 6 ± 11% was observed between the delivered activity and the 90Y PET’s FOV activity. In addition, a substantial difference of − 20 ± 8% was measured on 90Y PET images between the activity in the liver and in the whole FOV. After normalization, 99mTc-MAA SPECT dosimetry was highly correlated and concordant with 90Y-microsphere PET dosimetry for all dose metrics evaluated (ρ = 0.87, ρc = 0.86, P = 3.10−8 and ρ = 0.91, ρc = 0.90, P = 7.10−10 for tumor and normal liver mean dose respectively for example). Besides, mean tumor dose deviation was lower when the catheter position was identical than when it differed (16 Gy vs. 37 Gy, P = 0.007). Concordance between predictive and post-treatment dosimetry, evaluated with Dice similarity coefficients applied on isodoses, significantly correlated with the distance of the catheter position from artery bifurcation (P = 0.04, 0.0004, and 0.05, for 50 Gy, 100 Gy, and 150 Gy isodoses respectively).ConclusionsDiscrepancies between planned activity and activity measured on 90Y PET images were observed and seemed to be mainly related to clinical hazards and equipment issues. Predictive vs. post-treatment comparison of relative dose distributions between tumor and normal liver showed a good correlation and no significant difference highlighting the predictive value of 99mTc MAA SPECT/CT-based dosimetry. Besides, the reproducibility of catheter tip position appears critical in the agreement between predictive and actual dose distribution.

59 Comparison between predictive and post-treatment dosimetry for hepatocellular carcinoma patients treated with [formula omitted]Y-glass microsphere radioembolization

Introduction The aim of this study was to analyze the differences between 99 m Tc-MAA SPECT and 90 Y-microsphere PET dosimetry investigating imaging (spatial resolution, partial volume effect, noise) and clinical factors (catheter positioning, modified vascularization, MAA/microsphere distribution). Methods Nineteen 90 Y-glass microsphere radioembolizations of hepatocellular carcinoma patients were analyzed in this study. For each treatment, predictive and post-treatment doses to tumor and normal liver (NL) were calculated with a dedicated software (PLANET® Dose, DOSIsoft, Cachan, France) applying a convolution method based on voxel-S factors. The mean ( ± standard deviation) administered activity was 3.5 GBq ( ± 1.2) for a mean tumor volume of 546 mL ( ± 408mL). In addition to the tumor average dose ( D avg ) , dose metrics extracted from dose volume histograms were analyzed: minimum dose to 70% and 50% of the tumor volume ( D 70 and D 50 ) and percentage of the tumor volume receiving at least 205 Gy ( V 205 ) , as recommended in the literature for glass-microsphere treatments [1] . Difference and correlation between predictive and post-treatment doses were assessed using a paired Student’s t test and Pearson’s correlation coefficient. Spatial concordance analysis at the voxel level between MAA and microsphere distributions is ongoing. This should provide additional information to better understand the part of dose differences related to clinical factors. Results Dose results showed a good correlation between 99 m Tc-MAA SPECT and 90 Y-microsphere PET based dosimetry (Table 1). However, difference between them was significant for all tumor and NL dose metrics considered. A 90 Y-microsphere PET tendency to underestimate 99 m Tc-MAA SPECT based doses was observed. Preliminary results indicate that the largest dose discrepancies can be explained by clinical differences in terms of administration procedure and particle distribution. Download : Download high-res image (249KB) Download : Download full-size image Conclusions The overall good correlation observed between predictive and post-treatment dosimetry confirm the MAA predictive value. A tendency of 90 Y-microsphere PET to underestimate 99 m Tc-MAA SPECT doses was observed.

Comparison of the 90Y-labelled glass microspheres liver radioembolisation dosimetry with the estimated dosimetry obtained from pre-treatment 99mTc-MAA SPECT images reconstructed with and without attenuation correction

ABSTRACT90Y-labelled glass microspheres liver radioembolisation is an effective treatment for unresectable hepatic tumours. Its treatment planning is based on 99mTc-macro-aggregated albumin (MAA) single photon emission computed tomography (SPECT) imaging. However, photons interaction with human tissues may significantly compromise the accuracy of radioactivity quantification in nuclear medicine. The aim of this work is to evaluate the impact of a patient-specific attenuation correction on pre-treatment 99mTc-MAA SPECT images and, consequently, on the estimation of radioembolisation dosimetry. The methodology developed for attenuation correction was firstly validated on phantom models. Following this, a data set of eight patients was retrospectively selected from our database and the pre-treatment images were reconstructed with and without attenuation correction. Then, the liver dosimetry maps were estimated and compared with the liver dosimetry maps estimated from the post-treatment 90Y positron emission tomography images. Results show that the attenuation corrected phantom images were greatly improved, originating higher contrast, signal-to-noise ratio and contrast-to-noise ratio comparatively to non-attenuation corrected images. Attenuation corrected clinical pre-treatment images significantly increase the agreement, measured with the intraclass correlation coefficient, between predictive and post-treatment dosimetry maps comparatively to the non-attenuation corrected ones (Wilcoxon test, p = 0.008). In conclusion, a statistically significant increase in the agreement was achieved; however, poor correlation is still observed at the single-voxel level in some patients. Thus, future research should be addressed to investigate the reasons of this and manage to optimise the agreement between the predictive and post-treatment dosimetry.

Outcomes and toxicity following Yttrium-90 radioembolization for hepatic metastases from neuroendocrine tumors-a single-institution experience.

The prognosis of patients with hepatic metastases from neuroendocrine tumors (NET) is generally good, and radioembolization with Yttrium-90 microspheres is a locoregional therapy that is used in efforts to improve hepatic disease control and survival. This study aims to describe the survival outcomes and toxicities associated with radioembolization for hepatic-predominant metastatic NET in a large single-institution cohort. A total of 59 patients underwent radioembolization for metastatic NET with hepatic predominant disease at a single academic center. Patient outcomes were analyzed by Kaplan-Meier survival analysis and toxicities were detailed and described. Ten patients within the cohort underwent post-treatment dosimetric analysis using PET-MRI and normal liver dosimetry was correlated with hepatic fibrosis and toxicity. Median overall survival from time of radioembolization in the patient cohort was 31 months, and the 1- and 2-year overall survival was 80.4% and 65.6% respectively. Median hepatic progression-free survival and overall progression-free survival were 18 and 13 months, respectively. Three patients died of hepatic failure that was possibly therapy-related. Ten patients underwent evaluation of post-treatment dosimetry following radioembolization. In patients who did not develop hepatotoxicity or hepatic fibrosis, mean dose to normal liver was 25.4 Gy, while the mean liver dose in patients who experienced toxicity (hepatic fibrosis in n=2 and death from hepatic failure in n=1) was 59.1 Gy. Overall survival following radioembolization for hepatic metastases from NET is excellent; however, deaths that are potentially treatment-related have been observed. Preliminary data regarding dose to normal liver is suggestive of a relation between dosimetry and toxicity, however further work is required to further elucidate the mechanism, correlation with dosimetry, as well as additional patient and tumor factors that may predispose these patients to toxicity.

37. 3D Personalized dosimetry for Yttrium-90 microsphere radioembolization of liver tumors: feedback and clinical cases

Introduction Today, dose computation for Selective internal radiation therapy (SIRT) is based on methods easy to apply clinically. Several studies highlight dose-effect relationship for SIRT. As a result, interest enhances for advanced calculations and dose analysis tools as the ones used in external beam radiation therapy (EBRT). The Nuclear Medicine department of Montpellier University Hospital (MUH) implemented a new system to meet this need. This study describes the methodology applied, its interest and the questions raised illustrated by some clinical cases. Methods For every patient treated by SIRT, the dosimetry referred as “standard” applied at MUH follows the method described by Garin et al. [1] . This is based on the partition model for which the volumes of interest are defined using Syngo® software (Siemens, Erlangen, Germany) on SPECT images acquired after the injection of macro-aggregates of albumin (MAA) labelled with 99Tcm. Recently, the implementation of a treatment planning system (TPS), PLANET® Dose (DOSIsoft, Cachan, France), allows to perform dosimetry at the voxel level. This is done through two major steps: predictive dosimetry based on MAA-SPECT images and in vivo control dosimetry using PET images acquired after the injection of yttrium-90 (90Y) microspheres ( Fig. 1 ). Dose distribution is especially evaluated for tumoral and non-tumoral liver delineated on the injected CT or MR images by the radiology team. Dose computation rests on a convolution method based on voxel S factors. Pre and post treatment dosimetries are analyzed by the mean dose to the volume, the dose-volume histograms (DVH) and the isodose curves. Results The comparison between both dosimetric approaches (standard and at the voxel level) pointed out some differences. This particularly allowed to highlight the TPS contribution according to various aspects that were observed through clinical cases. A first aspect is related to the volume definition (functional only vs. anatomical and/or functional) for which the TPS contribution is especially interesting for complex clinical cases (multiple tumor sites, heterogeneous or partially targeted lesions, liver failure). This kind of system then allows dose evaluation in any compartment (volume at risk, portal vein thrombosis, multiple lesions, viable tumor, etc.). Moreover, voxelized dosimetry allows to analyze dose distribution uniformity with advanced tools (isodoses, DVH, etc.) in order to better assess areas of over/under dosing. Finally, being able to perform post-treatment dosimetry in order to know the real delivered dose and to sum several treatment sessions can play an important role in the patient therapeutic care. Conclusion Integrating a TPS like the ones used in EBRT in the SIRT process allowed to increase the medical team confidence and accuracy in the prescription of the activity to administer and to better control the dose delivered during the treatment. Setting up these new tools open the way to a more advanced personalization of this therapy.

Individualized dosimetry for patients with graves’ disease

Introduction Following Directive 2013/59 Euratom, all therapeutic exposures of target volumes shall be individually planned and their delivery appropriately verified. We have developed a home-made software for I-131 dosimetry in hyperthyroid patients using gammacamera and capture probe for prior and post treatment. Purpose This study describes the dosimetry method, the differences between pre and post treatment purposes, and compare its results. Materials and methods We base our procedure on the EANM Dosimetry Committee Guidelines using an INa Probe for uptake measurements at 2, 24 and 96 h after administration of 25 μCi tracer activity (also measured before its administration) and two Tc-99m planar images to estimate the thyroid mass. A two compartimental model is used to calculate the required activity to achieve 160 Gy to the target. Two uptake measurements are acquired at 96 and 168 h post-treatment to calculate the real absorbed dose to the thyroid, using a monocompartimental model and dead time correction for saturation effect through a previous probe calibration curve. 78 patients treated during 2015 have been analyzed. Results 163,1 Gy and 37,7 Gy are the average and standard deviation of administered absorbed doses, ranging from 87,3 Gy to 240,1 Gy. The mean activity to be administered according the used model is 9,1 mCi which differs from previously used (Marinelli algorithm, 7,3 mCi) and administered (8,9 mCi). Differences of means between activities by Marinelli and by our method using t-Student are significant (p Conclusion Individualized pre and post treatment dosimetry for hyperthyroidism I-131 therapy is mandatory.

SU-F-BRA-15: Physical Aspects and Clinical Applications of Post-Treatment Yttrium-90 PET-Based Dosimetry

Purpose: The accuracy of PET-based post-treatment dosimetry of yttrium-90 microspheres has been improving over the past decade and is now at a stage, permitting volumetric dose-outcome studies. We outline the recent advances and identify the physical limitations to the accuracy of the dose calculations. Methods: Convolution of the measured PET activity density distribution with a pre-calculated voxel-dose-kernel (VDK) is the most widely used method for dose reconstruction. Therefore, accurate knowledge of the beta+ branching ratio as well as the micro-dosimetric characteristics of electron interactions within the microsphere is essential for the computation of the dose kernel. We implement a model of the microspheres used in our clinic to calculate the modified electron energy spectrum at the microsphere’s surface and to determine the impact of self-shielding on the dose reconstruction. The three-dimensional dose distributions obtained for 10 patients treated with radio-embolization with yttrium-90 microspheres are evaluated and various DVH markers are investigated for correlation with outcome. Results: The methods for reducing the overall computation uncertainty are systematically outlined in this presentation. Since the latest experimental data on yttrium-90 beta+ branching ratio has a relative uncertainty of 1.5%, all contributing factors derived from Monte Carlo simulations must be brought to a sub 1% level. The self-shielding within the microspheres is found to be responsible for up to 6% reduction of the reconstructed dose in low-gradient regions and must be taken into account. The contribution of trace amounts of other beta+ emitters introduced during the manufacturing process is also discussed. Conclusion: The accelerating pace of clinical adoption of PET-based post-treatment dosimetry is mainly due to advances in both quantitative PET imaging and physical models of dose deposition. We show that the overall physical dose uncertainty in the convolution step can be further reduced, thus making ongoing multi-institutional dose-outcome studies even more reliable.

Sector analysis of dosimetry of prostate cancer patients treated with low-dose-rate brachytherapy

Brachytherapy is an effective single treatment modality for low- and intermediate-risk prostate cancer. Here, we compare the radiation doses in different prostate sectors between the preimplant planning images and the postimplant dosimetry. Two hundred fifteen consecutive patients treated for prostate cancer by (125)I seed brachytherapy were assessed. Pretreatment plans using transrectal ultrasound images of the prostate were compared with the dose calculated on posttreatment MRI and CT scans obtained 1month after seed implantation. Twelve sectors were generated by dividing the prostate base, midgland, and apex into four quadrants each. Pretreatment and posttreatment dosimetry were compared between the 12 different sectors of the prostate. Average V100 (percentage of prostate volume that receives 100% of the prescribed dose) in the preimplant planning images of the prostate was 99.9±0.25% compared with postimplant V100 of 94.8±3.77% (p < 0.0001). Prostate V100 in the postimplant dosimetry was >91% in all sectors, except the anterior base sector, in which it was 64.87±20.96%. Average 1-month D90 (the dose to 90% of the prostate volume) was 114.5±10.55%. D90 at 1month compared with preimplant planning was lower in the prostate base and higher in the prostate apex (p < 0.001). Our results show that in (125)I seed brachytherapy, prostate base receives a lower dose and apex receives a higher dose compared with preimplant planned dose coverage.

The role of SPECT/CT in radioembolization of liver tumours

Radioembolization (RE) with (90)Y microspheres is a promising catheter-based therapeutic option for patients with unresectable primary and metastatic liver tumours. Its rationale arises from the dual blood supply of liver tissue through the hepatic artery and the portal vein. Metastatic hepatic tumours measuring >3 mm derive 80 - 100 % of their blood supply from the arterial rather than the portal hepatic circulation. Typically, an angiographic evaluation combined with (99m)Tc-macroaggregated albumin ((99m)Tc-MAA) scan precedes therapy to map the tumour feeding vessels as well as to avoid the inadvertent deposition of microspheres in organs other than the liver. Prior to administration of (99m)Tc-MAA, prophylactic coil embolization of the gastroduodenal artery is recommended to avoid extrahepatic deposition of the microspheres. SPECT/CT allows direct correlation of anatomic and functional information in patients with unresectable liver disease. SPECT/CT is recommended to assess intrahepatic distribution as well as extrahepatic gastrointestinal uptake in these patients. Pretherapeutic SPECT/CT is an important component of treatment planning including catheter positioning and dose finding. A post-therapy bremsstrahlung (BS) scan should follow RE to verify the distribution of the administered tracer. BS SPECT/CT imaging enables better localization and definition of intrahepatic and possible extrahepatic sphere distribution and to a certain degree allows posttreatment dosimetry. In this paper we address the usefulness and significance of SPECT/CT in therapy planning and therapy monitoring of RE.

Reply to Letter re: General Theory of Predictive Dosimetry for Yttrium-90 Radioembolization to Sites Other Than the Liver

We fully approve the content of this Letter, as it provides a comprehensive roadmap on predictive dosimetry and aims at standardisation and good clinical practice for Yradioembolization to sites other than the liver. The field of application is well defined in the dosimetric assumptions and general points. The formulas are clear and applicable at any site. As stated in the introduction of this Letter, the safety and efficacy of selective internal radiotherapy (SIRT) is a combination of ‘‘technical’’ and ‘‘clinical’’ success. In this Reply, we would like to highlight some technical aspects that need special attention in order to achieve a therapeutic success. Before simulation with digital subtraction angiography and Tc-macroaggregated albumin (MAA), presimulation imaging should be performed by CT(or MRI-) angiography to detect vascular variants and abnormalities, to direct additional embolization of vessels, and to plan the most appropriate positioning of the catheter [1]. It is obvious that the position of the catheter and the flow rate should be similar for simulation and treatment. Therefore, it is important to choose a stable position of the catheter, as far as possible from any arterial bifurcation and to store the position of the tip of the catheter (e.g., by means of a screen capture) during simulation to assure a similar distribution of the Tc-MAA and Ymicrospheres. Posttreatment imaging can assess the distribution of the Y-microspheres and should be used to verify and validate the predictive dosimetry, especially in the setting of radioembolization to sites other than the liver. 3D distribution of the Y-microspheres can be assessed by Y-Bremsstrahlung Emission Computed Tomography (BECT) [2, 3] or Y-Time of Flight Positron emission tomography (TOF-PET) [4, 5]. Although the latter technique provides high-resolution images enabling quantification and real posttreatment dosimetry, its application is restricted to a research setting due to the very long acquisition times (at least the double compared to BECT) to obtain sufficient counts. As tissue mass is an essential component of the equations used for dosimetry, accurate delineation of volumesof-interest (VOI) is another technical challenge. For organs, the most obvious choice for estimation of the tissue mass is the volumes and radiographic densities derived from CT. However, the use of CT for delineation of viable tumour tissue, especially in small or necrotic target lesions often is less obvious. In this setting, functional imaging might be more appropriate for accurate VOI-delineation. Hybrid imaging by SPECT/CT is indeed a minimal technical requirement for optimal tissue delineation in SIRTdosimetry, but also requires standardization of the methodology used for VOI-delineation in order to enhance reproducibility and comparability of published data.

Integration of geometric data recorded during irradiation sessions for post-treatment dosimetry calculation in external radiotherapy

Introduction Accelerators geometric inaccuracies and anatomical variations are two elements that can affect treatment quality. It can be interesting to investigate the dosimetric impact induced by the uncertainty on these parameters. Material and methods In Poitiers hospital, the record and verify Mosaiq (Elekta, v2.41) ensures that the parameters defined with the TPS (ISOgray, v4.1) are included in a certain range of tolerance. These tolerances, usually determined from the uncertainties measured during the accelerator commissioning, can potentially cause significant dosimetric errors. A in-house tool compatible with the ICOM protocol (Elekta) has been developed in order to incorporate some mechanical data recorded during the treatment (Elekta Synergy Beam Modulator, v7.01SP2) to the original RTPlan. These RTPlan, modified with respect to each treatment sessions, were then imported into the TPS to recalculate the dosimetry. Used daily, the CBCT (Cone Beam Computed Tomography) allows a more precise positioning of the treatment isocenter by shifting images performed before each session on the initial dosimetric imaging. However, this adaptation does not take into account every anatomical changes. To quantify the influence of these variations, each CBCT has been incorporated into the initial dosimetry. This influence could be quantitatively compared to that of the treatment machine. Results Dose differences were analyzed within a series of patients treated for a prostate disease, taking into account the specificities of the TPS algorithm and the accuracy of the measurement techniques. Conclusion This study established if the methodology can detect relevant dosimetric effect, and whether it may need to be taken into account.

Evaluation of the visibility of a new thinner 125I radioactive source for permanent prostate brachytherapy

PurposeThe 125I source currently used for prostate brachytherapy at St. James’s Institute of Oncology is a standard size seed (≈4.5mm in length and 0.8mm in diameter). A new, thinner seed is under evaluation. This is designed to be implanted using narrower needles, potentially reducing edema and improving the dose distribution. This study investigated the visibility of the thinner source on multimodality images and compared it with that of standard size seeds. Methods and MaterialsImages of dummy seeds of both thinner and standard size models were taken using ultrasound, fluoroscopy, computed tomography (CT), and magnetic resonance (MR) imaging. The ultrasound, fluoroscopy, and CT images were acquired with the seeds inserted into phantoms positioned in a water tank. The MR images were acquired using phantoms containing single seeds. The images were analyzed visually and quantitatively. The resolution of closely spaced seeds on CT images was investigated. ResultsThe visibility of both seeds was similar on ultrasound, fluoroscopy, and MR images. On CT images, the thinner seeds give reduced artifacts and better resolution. ConclusionsThe use of the thinner seed would have minimal effect on ultrasound and fluoroscopy imaging during treatment. However on CT images, the use of the thinner seeds may improve seed identification for post-treatment dosimetry. Further study is required into the suitability of MR images alone for post-treatment dosimetry.

Quantitative Evaluation of Scintillation Camera Imaging Characteristics of Isotopes Used in Liver Radioembolization

BackgroundScintillation camera imaging is used for treatment planning and post-treatment dosimetry in liver radioembolization (RE). In yttrium-90 (90Y) RE, scintigraphic images of technetium-99m (99mTc) are used for treatment planning, while 90Y Bremsstrahlung images are used for post-treatment dosimetry. In holmium-166 (166Ho) RE, scintigraphic images of 166Ho can be used for both treatment planning and post-treatment dosimetry. The aim of this study is to quantitatively evaluate and compare the imaging characteristics of these three isotopes, in order that imaging protocols can be optimized and RE studies with varying isotopes can be compared.Methodology/Principal FindingsPhantom experiments were performed in line with NEMA guidelines to assess the spatial resolution, sensitivity, count rate linearity, and contrast recovery of 99mTc, 90Y and 166Ho. In addition, Monte Carlo simulations were performed to obtain detailed information about the history of detected photons. The results showed that the use of a broad energy window and the high-energy collimator gave optimal combination of sensitivity, spatial resolution, and primary photon fraction for 90Y Bremsstrahlung imaging, although differences with the medium-energy collimator were small. For 166Ho, the high-energy collimator also slightly outperformed the medium-energy collimator. In comparison with 99mTc, the image quality of both 90Y and 166Ho is degraded by a lower spatial resolution, a lower sensitivity, and larger scatter and collimator penetration fractions.Conclusions/SignificanceThe quantitative evaluation of the scintillation camera characteristics presented in this study helps to optimize acquisition parameters and supports future analysis of clinical comparisons between RE studies.