The superior predictive value of 166Ho-scout compared with 99mTc-macroaggregated albumin prior to 166Ho-microspheres radioembolization in patients with liver metastases

Purpose As an alternative to technetium-99m-macroaggregated albumin (99mTc-MAA), a scout dose of holmium-166 (166Ho) microspheres can be used prior to 166Ho-radioembolization. The use of identical particles for pre-treatment and treatment procedures may improve the predictive value of pre-treatment analysis of distribution. The aim of this study was to analyze the agreement between 166Ho-scout and 166Ho-therapeutic dose in comparison with the agreement between 99mTc-MAA and 166Ho-therapeutic dose. Methods Two separate scout dose procedures were performed (99mTc-MAA and 166Ho-scout) before treatment in 53 patients. First, qualitative assessment was performed by two blinded nuclear medicine physicians who visually rated the agreement between the 99mTc-MAA, 166Ho-scout, and 166Ho-therapeutic dose SPECT-scans (i.e., all performed in the same patient) on a 5-point scale. Second, agreement was measured quantitatively by delineating lesions and normal liver on FDG-PET/CT. These volumes of interest (VOIs) were co-registered to the SPECT/CT images. The predicted absorbed doses (based on 99mTc-MAA and 166Ho-scout) were compared with the actual absorbed dose on post-treatment SPECT. Results A total of 23 procedures (71 lesions, 22 patients) were included for analysis. In the qualitative analysis, 166Ho-scout was superior with a median score of 4 vs. 2.5 for 99mTc-MAA (p < 0.001). The quantitative analysis showed significantly narrower 95%-limits of agreement for 166Ho-scout in comparison with 99mTc-MAA when evaluating lesion absorbed dose (− 90.3 and 105.3 Gy vs. − 164.1 and 197.0 Gy, respectively). Evaluation of normal liver absorbed dose did not show difference in agreement between both scout doses and 166Ho-therapeutic dose (− 2.9 and 5.5 Gy vs − 3.6 and 4.1 Gy for 99mTc-MAA and 166Ho-scout, respectively). Conclusions In this study, 166Ho-scout was shown to have a superior predictive value for intrahepatic distribution in comparison with 99mTc-MAA.


Introduction
Treatment planning for radioembolization (known as Selective Internal Radiation Therapy or SIRT) of liver tumors is generally performed with technetium-99m macroaggregated albumin particles ( 99m Tc-MAA). Aside from predicting the lung shunt and (other) non-target embolization, 99m Tc-MAA is also used to predict the activity distribution in the liver. Several studies have shown that the value of 99m Tc-MAA to predict the distribution of 90 Y in the liver is limited [1][2][3][4]. Shape, size, density, and number of injected particles of 99m Tc-MAA differ greatly from 90 Y-microspheres. This may explain the difference in biodistributions.
As an alternative to the yttrium-90 ( 90 Y)-microspheres, microspheres containing the radioactive element holmium-166 ( 166 Ho) have recently become available (Quiremspheres®, Quirem Medical, Deventer, the Netherlands). The radionuclide 166 Ho provides several advantages over 90 Y with regard to imaging possibilities. The element holmium itself is paramagnetic and can therefore be visualized-and quantifiedon MRI [5,6]. In addition, 166 Ho emits low-energy gamma radiation, which can be used for quantitative SPECT/CT imaging [7]. A scout dose of 166 Ho-microspheres (consisting of approximately 3 million microspheres with an activity of 250 MBq) can be used for treatment planning instead of 99m Tc-MAA [8]. This has the theoretical benefit of using the exact same 166 Ho-microspheres for both procedures. The aim of the current study was to analyze whether the intrahepatic distribution of 166 Ho-scout has a better agreement with the 166 Ho-therapeutic dose distribution in comparison with 99m Tc-MAA.

Patients and procedures
Patients treated with 166 Ho-radioembolization in the phase 1 and phase 2 Holmium Embolization Particles for Arterial Radiotherapy (HEPAR) studies were analyzed (Clinicaltrials. gov numbers NCT01031784 and NCT01612325). All patients had unresectable liver metastases from various primaries. The institutional review board approved these studies and all patients provided written informed consent. All patients received treatment planning with the conventional 99m Tc-MAA and subsequently with a scout dose of 250 MBq of 166 Ho-microspheres (60 mg; approximately 3 million microspheres). Patients were included for analysis if they had received both pre-treatment administrations of 99m Tc-MAA and 166 Ho-scout before treatment with 166 Ho-microspheres. All radioembolization procedures were performed according to the HEPAR study protocol [9]. Non-target vessels were only coil-embolized in case no safe injection position could be found distal to these non-target vessels. Approximately 150 MBq 99m Tc-MAA (0.8 mg, approximately 1.8 million particles, TechneScan LyoMAA; Mallinckrodt Medical B.V., Petten, the Netherlands) was injected. 99m Tc-MAA was injected slowly at approximately 5 ml/min. All injections (both 99m Tc-MAA and 166 Ho-microspheres) were performed with a standard 2.4F or 2.7F microcatheter (Progreat®, Terumo, Japan). At the end of the procedure, the access site in the groin was compressed for hemostasis. 99m Tc-MAA injection was followed by planar-and SPECT/CT imaging to check for extrahepatic deposition and excessive lung shunting (> 30 Gy lung absorbed dose). If no contra-indications were found, the patient was scheduled for treatment. In case the 99m Tc-MAA procedure had to be repeated because of extrahepatic activity, only the most recent 99m Tc-MAA data were used for analysis. No vessels were coil-embolized after the final 99m Tc-MAA procedures. On the day of treatment, patients underwent a second treatment-planning procedure in the morning. During this procedure, the injection position(s) from the first treatment planning procedure was mimicked and patients received a scout dose of 250 MBq of 166 Ho-microspheres (approximately 3 million microspheres). At the end of the procedure, the catheter and microcatheter were removed. The vascular sheath was left in the groin in order to secure vascular access for the treatment procedure in the afternoon. The sheath was connected to a pressurized bag of saline (for continuous flushing) and secured with sterile tape. This second treatment-planning procedure was again followed by planar imaging and SPECT/CT. If there were no contraindications for SIRT, patients returned to the angio-suite in the afternoon to receive treatment with 166 Ho-microspheres. Catheters were introduced via the vascular sheath that had remained in situ. Again, injection position(s) of the treatment-planning procedures were mimicked. Total treatment activity was planned based on an aimed whole liver absorbed dose of 20, 40, 60, and 80 Gy for patients who were treated in the phase 1 HEPAR study and 60 Gy for patients in the phase 2 HEPAR study [10,11]. The total number of microspheres injected was the same for all patients (i.e., 600 mg; approximately 30 million microspheres). After injection, the catheters and sheath were removed and a vascular closure device was used for hemostasis.
In order to exclude cases in which differences in injection positions might have influenced distribution, agreement of catheter-tip positions between the three procedures (i.e., 99m Tc-MAA, 166 Ho-scout, and 166 Ho-therapeutic dose) was retrospectively analyzed by three observers (M.S, A.B., and M.L.). These observers independently reviewed the agreement of all injection positions per procedure on digital subtraction angiography images. Agreement was rated on a 4point scale (1, very poor agreement, difference in catheter tip position > 10 mm; 2, poor agreement, difference 5-10 mm; 3, good agreement, difference 3-5 mm; 4, very good agreement, difference < 3 mm). Only the patients with good or very good agreement (point scale 3 and 4) between all the injection positions were included in this study.
In case of procedures with multiple injection positions, the ratio between the injected activities at each injection position had to be the same for the three procedures. This ratio was based on CT-volumetry (e.g., a 2:1 activity ratio for a right liver lobe of 1000 mL and a left liver lobe of 500 mL). Agreement between the procedures was checked based on the administered activity per injection position, which was corrected for any residual activity. A maximum deviation of 10% between the two pre-treatment and post-treatment injections was accepted.

Imaging
Pre-and posttreatment SPECT/CT imaging was performed on a dual-headed gamma camera (Forte, Philips Medical Systems, 6 procedures) and a SPECT/CT camera (Symbia 16T, Siemens Health Care, 17 procedures). In all cases, the same scanner was used for pre-and posttreatment SPECT/CT. Pre-treatment 99m Tc-MAA SPECT images were acquired on a 512 × 512 matrix (15 procedures) and a 128 × 128 matrix (8 procedures). An energy window of 129.1-to 150.5-keV and a low-energy general-purpose collimator were used. Pre-and post-treatment 166 Ho SPECT images (interval; mean 4 days, range 3-6 days) were acquired on a 128 × 128 matrix (22 procedures) and a 512 × 512 matrix (1 procedure) in combination with a 74.9-to 87.1-keV energy window and a medium-energy general-purpose collimator. Imaging was performed with 120 projections over a noncircular orbit of 360°( 99m Tc-MAA: 30s/projection (Philips) or 20s/projection (Siemens) and 166 Ho; 30s/projection). An in-house developed and validated Monte-Carlo-based reconstruction algorithm (Utrecht Monte-Carlo System (UMCS)) intrinsically correcting for attenuation and scatter was used for the reconstruction of the 99m Tc-MAA and 166 Ho HEPAR I data [12]. The reconstruction of all HEPAR II data was done by using the Siemens Flash3D ordered-subsets expectation maximization ( 99m Tc-MAA; 6 iterations, 8 subsets, 166 Ho; 5 iterations 8 subsets). The scatter correction for the HEPAR II 99m Tc-MAA data was applied by using the dual energy window method with two adjacent energy windows (both 15% width) and a lower window weight of 0.5. The HEPAR II 166 Ho data was reconstructed without scatter correction. For 99m Tc-MAA data, a Gaussian post-reconstruction filter of 5 mm in full width at half maximum was also included. Reconstructed

Qualitative analysis
Two nuclear medicine physicians independently compared the intrahepatic distribution of activity of 99m Tc-MAA and 166 Hoscout with the post-treatment 166 Ho-therapeutic dose. Image sets of all three SPECT scans were coupled, blinded, and simultaneously presented to the reviewers in our Picture Archive and Communication System (Sectra PACS, Almere, the Netherlands). The post-treatment SPECT was marked "post-treatment" to allow the reviewers to compare it with the other two pre-treatment SPECT scans (which were marked "pre-treatment"). No information was provided regarding the type of pre-treatment scan. To prevent bias, both pre-treatment scans were placed randomly on the screen for each patient.
Visual agreement was rated on a 5-point scale (1-5, very poor agreement-very good agreement).

Quantitative analysis
Agreement of intrahepatic distribution was also measured quantitatively. FDG-PET/CT pre-treatment scans were used for segmentation of lesion and liver volumes (volumes of interest, or VOIs). In-house-developed software was used for this purpose (VolumeTool, version 1.6.5.) [13]. Delineation of VOIs was performed manually on FDG-PET images. The normal liver was defined as the whole liver minus tumors. All VOIs were manually registered (non-deformable) to the 99m Tc-MAA SPECT, 166 Ho-scout SPECT, and 166 Ho-therapeutic dose SPECT images (Fig. 1). In order to compensate for co-registration errors and blurring effects, the VOIs were enlarged by a 1-cm margin when measuring the activity. Also, small lesion VOIs with a volume smaller than 10 mL were excluded, because of their higher sensitivity to these errors. The pre-treatment 99m Tc-MAA SPECT and 166 Ho-scout SPECT and the post-treatment 166 Ho-therapeutic dose SPECT were converted into units of 166 Ho-activity concentration by normalization of the total number of reconstructed counts to the total administered activity of 166 Ho during therapy. This resulted in two predictive amounts of 166 Ho (MBq/ cm 3 ) for each VOI, based on both pre-treatment SPECT images, in comparison with an actual amount of 166 Ho based on post-treatment SPECT. The lesion-absorbed dose was calculated by dividing the activity measured in the lesion VOI plus the 1-cm margin by the volume of that same lesion VOI without the 1-cm margin. Normal liver activity was calculated by subtraction of the measured activity of all lesion VOIs within one patient (including the VOI's with a volume smaller than 10 mL) without 1-cm margin from the activity of the liver VOI with 1-cm margin. The volume of the normal liver compartment was calculated by subtraction of the total volume of the lesion VOIs without margin from the volume of the liver without margin. Finally, the calculated activity was divided by this volume to calculate the normal liver absorbed dose. No extrahepatic activity distribution and no interval change in liver morphology were assumed. Contribution of gamma radiation was neglected in calculating the absorbed dose. To illustrate clinical implications, the calculated activity concentrations were converted into units of Gray for both the lesion and normal liver segmentations, using a conversion factor of 15.78 • 10 −3 Gy/(MBq/kg) assuming a liver density of 1.06 kg/L [10].

Statistical analysis
Statistical data analysis was performed using a commercial statistical software package (SPSS for Windows, version 21.0; SPSS Inc.). For the qualitative analysis, medians and interquartile ranges were calculated and the Wilcoxon signed rank test was used to analyze differences in overall agreement scores between both pre-treatment scans and the 166 Ho treatment scan. The Weighted Kappa (squared) was calculated to classify the interrater agreement [14,15].
Bland-Altman plots were used for evaluating agreement between pre-and post-treatment activity biodistributions [16,17]. The confidence interval of the 95% limits of agreement was used to test the significance of the difference in agreement for both plots. The precision of the 95% limits of agreement was estimated by the number of analyzed procedures or lesions (n), the standard deviation of the differences (s), and the degrees of freedom (n − 1). In this estimation, no corrections were made for any correlation between analyzed lesions within a patient. The 95% confidence intervals were calculated by the multiplication of the standard error (standard error = ffiffiffiffiffiffiffiffiffiffiffi 3s 2 =n p ) of the upper and lower 95% limit of agreement and the value of the t distribution with n − 1 degrees of freedom according to the method described by Bland and Altmann [16]. Agreement was considered statistically significantly different if the bandwidth of the 95% limits of agreement, including the confidence intervals, of either scout method was smaller than the bandwidth of the other scout method.

Results
Fifty-three patients underwent 55 treatment procedures between December 2009 and March 2015 (two patients underwent separate lobar procedures) (Fig. 2). Before treatment, every patient received 99m Tc-MAA and 166 Hoscout in a separate procedure ( 99m Tc-MAA interval; median 7 days, range 2-21 days and 166 Ho-scout; same day). Thirtytwo procedures were excluded from analysis: in 13 procedures, a mismatch in catheter tip positioning was found; in 10 procedures, data was not completely available (one or more missing angiographic images (n = 4); no pre-treatment FDG-PET/CT (n = 2); no post-treatment SPECT/CT (n = 2); 166 Hoscout was not administered (n = 2)), and in nine procedures, the ratio of activity between the injection positions was not similar. A total of 23 procedures in 22 patients were included for analysis (Table 1). In these 22 patients, 71 lesions were analyzed. The mean administered treatment activity of 166 Homicrospheres per procedure was 5,470 MBq (range 1,957-12,897). Figure 3 shows the overall agreement scores of both pretreatment scans with the 166 Ho post-treatment scan, based on the visual assessment of the two nuclear medicine physicians. The median score for 99m Tc-MAA was 2.5 compared to 4 for 166 Ho-scout (p < 0.001). In 15 of the 23 procedures, overall agreement between 166 Ho-scout and 166 Ho-therapeutic dose was rated higher than 99m Tc-MAA (Fig. 4). 99m Tc-MAA showed better overall agreement in five procedures and in three procedures equal agreement was observed. The interrater agreement was moderate with a weighted kappa of 0.52.
Quantitative analysis showed that the overall mean differences in pre-minus post-treatment calculated absorbed dose for all lesions were 16.5 Gy for 99m Tc-MAA and 7.5 Gy for 166 Ho-scout. For 99m Tc-MAA the 95% limits of agreement of the differences were − 164.1 and 197.0 Gy (Fig. 5). 166 Ho- Quantitative analysis of the normal liver absorbed dose showed that the mean difference in the pre-and posttreatment calculated absorbed dose was 1.3 Gy for 99m Tc-MAA and 0.2 Gy for 166 Ho-scout (Fig. 6). The 95% limits of agreement of 99m Tc-MAA and 166 Ho-scout were comparable, − 2.9 and 5.5 Gy for 99m Tc-MAA and − 3.

Discussion
This study showed that treatment planning prior to radioembolization can be improved by using a scout dose of 166 Ho-microspheres instead of 99m Tc-MAA. The qualitative and quantitative analysis showed that the agreement between 166 Ho-scout and 166 Ho-therapeutic dose was significantly superior to 99m Tc-MAA.
The difference in agreement between scout dose distribution and treatment distribution is subject to several influencing factors. First, as stated before, catheter positioning is known to be a key factor [2]. In order to accurately measure agreement, the catheter tip should be positioned identically for all administrations in each patient to prevent differences in flow. Attempts were made to eliminate this factor by always paying close attention to the exact positioning of the catheter and by retrospectively excluding patients for whom the catheter positions showed suboptimal agreement. However, even with seemingly identical catheter positions on 2D images, the catheter position in 3D may still be different.
Secondly, the assumption of a homogeneous distribution in the normal liver and lesion segmentations is not representative, leading to an overestimation of agreement. From a clinical perspective, normal liver-absorbed dose and tumorabsorbed dose are the most important parameters in terms of safety and efficacy. However, the VOIs of the normal liver compartment cover a much larger volume than the lesion VOI's and therefore the level of overestimation of the agreement will be higher for this part of the analysis. In the qualitative analysis, agreement was visually analyzed on a more detailed, sub-segmental level. Interestingly, this analysis showed that in 65% of the 23 procedures, overall agreement between 166 Ho-scout and 166 Ho-therapeutic dose was better than with 99m Tc-MAA. In addition, the mean of the overall agreement scores of all procedures of 166 Ho-scout was significantly higher compared to 99m Tc-MAA.
Thirdly, the width of the standard error of the 95% limits of agreement is strongly influenced by the number of analyzed procedures. Due to strict exclusion criteria, only 23 procedures were eligible for analysis. A larger study would require more patients to undergo two types of treatment-planning administrations, which is a burden to the patients, costly and arguably unethical.
Lastly, technical aspects may have influenced the outcome of this study. Imaging of 166 Ho and 99m Tc-MAA is inherently different in terms of background noise and resolution, which could have influenced both the qualitative and quantitative analyses. These inherent differences are unavoidable for conducting this study. In addition, the type of gamma camera, reconstruction software and matrix size was not uniform across the study. We think the influence of these factors on Ho-scout and 166 Ho-therapeutic dose were performed on the same day. It is possible that the scout dose injection induced embolic effects that influenced the distribution of 166 Ho-microspheres during the treatment injection later that day. The majority of the analyzed patients were treated with radioembolization for colorectal liver metastases that are known to be relatively hypovascular tumors. Especially in these patients, embolic effects may lead to alterations in microsphere distribution [18]. This may have led to an enhanced disagreement between 166 Ho-scout and 166 Ho-therapeutic dose. Regardless of the type of scout-dose, embolic effects can be experienced during 166 Ho-radioembolization, even though the number of 166 Ho-microspheres injected for therapy (approximately 30 million) is lower than the typical number of resin 90 Y-microspheres injected (estimated 50 million) [9,11]. On the other hand, however, the larger time interval between 99m Tc-MAA (i.e., approximately 1 week versus same day for 166 Ho-scout) and 166 Ho-therapeutic dose may have led to interval changes in flow dynamics and thus altered distribution. Any type of scout dose performed on the same day as treatment may be more predictive than scout doses injected earlier. The difference in time interval may also result in 99m Tc-MAA  being administered by another operator than the operator injecting the 166 Ho-therapeutic dose. Administration speed and technique may differ between operators. Also, the administration box used for 166 Ho-scout and therapy administration with the possibility to intermittently change between contrast and microsphere injections was not available for 99m Tc-MAA injections.
A downside of 166 Ho-scout is that it is not as readily available as 99m Tc-MAA, which can be easily extracted from a generator. 166 Ho-scout needs to be ordered, activated in a nuclear reactor and delivered to the treatment site.
This study focused on intrahepatic distribution, but lung shunt calculation is another important aspect of 166 Ho-scout. Elschot et al. showed that a scout dose of 166 Ho-microspheres Fig. 4 Example of discrepancy between 99m Tc-MAA and 166 Hotherapeutic dose. Despite identical catheter positions (upper row), there is a remarkable difference in activity distribution between the three procedures. SPECT-CT (middle and lower row) shows that the activity distribution in the liver of 166 Hoscout is more similar to the therapy distribution than 99m Tc-MAA. Overall agreement of 99m Tc-MAA was rated 2.5 compared to 4.5 for 166 Ho scout for this patient is more accurate for lung shunt calculation on SPECT/CT than 99m Tc-MAA [19]. This reduces the chance that a patient is unnecessarily excluded from therapy or receives an unnecessary dose reduction. In addition, Braat et al. studied safety concerns of an unintended extrahepatic deposition of 166 Hoscout. They showed that extrahepatic depositions did not lead to any adverse events in their study. This supported the previously published findings by Prince et al. [8,20].
Individualized treatment planning becomes increasingly important for patients who are selected for radioembolization. However, pre-treatment activity planning based on 99m Tc-MAA lacks dosimetric accuracy and needs improvement in order to increase the clinical benefit of radioembolization treatment. A recent study by Kafrouni et al. showed that body surface area (BSA) based activity planning (i.e., the "BSAmethod") often leads to under-dosing, most likely because it insufficiently corrects for the tumor-to-normal (T/N) uptake ratios [21]. Despite this limitation, the BSA-method currently is the most commonly used treatment planning method [22]. As an alternative to the BSA-method, the so-called partition model can be used. Analysis of SPECT-based biodistribution of 99m Tc-MAA is used to account for distribution differences between tumor and non-tumor compartments. This method is known to be a more personalized and accurate treatment planning method. However, the predictive value of 99m Tc-MAA seems limited in many cases and the partition model is rarely used in clinical practice. The therapeutic activity choice in our treatment is based on one-compartment modeling or the socalled MIRD model, using an average absorbed dose to the target volume without differentiation between tumor and normal tissue absorbed doses. Analysis of 166 Ho-scout and 166 Ho-therapeutic dose imaging, together with treatment outcomes in terms of efficacy and safety, will lead to threshold values for personalized dosimetry-based treatment planning, which ultimately needs to be validated in prospective clinical studies. This study showed that 166 Ho-scout may serve as a predictive "biomarker" for safe and effective treatment.
In conclusion, both the qualitative and quantitative analyses showed that the intrahepatic distribution of the 166 Hoscout agreed better with the distribution of the therapy dose Fig. 6 Bland-Altman plots for healthy-liver segmentation. (1) Difference between 99m Tc-MAA and 166 Ho-therapeutic dose activity is plotted against mean activity for each procedure. (2) Difference between 166 Hoscout and 166 Ho-therapeutic dose activity is plotted against mean activity for each procedure. The 95% limits of agreement (LoA) are indicated and surrounded by two dotted lines indicating the standard error of the 95% LoA than 99m Tc-MAA, although the comparison has some inherent limitations. These results support the use of a scout dose of 166 Ho-microspheres for radioembolization treatment planning.
Funding This study was funded by the Dutch Cancer Society. Ethical approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Compliance with ethical standards
Informed consent Informed consent was obtained from all individual participants included in the study.
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