## 1 Introduction  
Transarterial radioembolization (TARE) is one the most effective treatments for unresectable primary and secondary hepatic lesions. In TARE, \({}^{90}\)Y-loaded resin (SIR-Spheres(r), Sirtex Medical Ltd., Woburn, MA, USA) or glass (TheraSphere(tm), Boston Scientific, Marlborough, MA, USA) microspheres or \({}^{166}\)Ho-loaded PLLA microspheres (QuiremSpheres(r), Quirem Medical B.V. subsidiary of Terumo Corporation, Tokyo, Japan) are selectively injected in the hepatic arteries. The injection leads to a preferential deposition in the tumoral tissues, hence sparing the healthy liver lobes. The use of \({}^{99m}\)Tc-macro aggregated albumin (\({}^{99m}\)Tc-MAA) SPECT/CT for treatment planning to assess the target coverage is largely established and recommended in the operating instructions of these devices as well as in the international recommendations [1; 2; 3]. Moreover, the \({}^{99\text{m}}\)Tc-MAA SPECT/CT images are mandatory to evaluate the possible extrahepatic shunts [4] or to identify patients with a Lung Shunt Fraction (LSF) > 20%, both representing exclusion criteria for TARE patients. More recently, exclusively for \({}^{166}\)Ho-based TARE treatment, \({}^{99\text{m}}\)Tc-MAA images have been replaced by \({}^{166}\)Ho-Scout dose (using an activity up to 250 MBq) to accurately predict the activity distribution of the therapeutic injection of \({}^{166}\)Ho-loaded microspheres [5]. Nevertheless, \({}^{99\text{m}}\)Tc-MAA imaging remains the state-of-the-art approach for predicting the therapeutic \({}^{90}\)Y-loaded TARE activity distribution. Specifically, pre-treatment \({}^{99\text{m}}\)Tc-MAA SPECT/CT 3D voxel-based dosimetry allows the calculation of the absorbed dose distribution expected in patients after the TARE procedure, which can be used to identify the \({}^{90}\)Y activity to be administered in order to increase the tumor control while reducing the possible risk of toxicity. In addition, the post-treatment \({}^{90}\)Y PET/CT-image-based dosimetry allows the verification of the planned absorbed dose distribution. Pre-treatment voxel-based dosimetry requires the importing of DICOM images, delineation of volumes of interest (VOIs), calculation of the absorbed dose distribution and computing and evaluation of the dose volume histograms (DVHs). These operations can be performed using CE or FDA marked treatment planning systems (TPSs) [6]. In particular, regarding the operation of VOI contouring, nuclear medicine or interventional radiologist physicians can manually or semi-automatically delineate the tumor, lungs, whole liver and non-tumoral liver (NTL) on SPECT/CT images. Moreover, the available images for TARE generally also include contrast-enhanced computed tomography (CECT) and \({}^{90}\)Y PET/CT images. In each of these modalities, all the VOIs might be manually delineated to obtain target volumes or to perform post-treatment dosimetry. Alternatively, deformable or rigid image registration algorithms are implemented in commercial TPSs to transfer VOIs from any reference modality (e.g., \({}^{99\text{m}}\)Tc-MAA SPECT/CT) to another (e.g., \({}^{90}\)Y PET/CT) [6]. Unfortunately, the manual delineation of VOIs is time-consuming and might affect the accuracy of the volume, shape and position of the target and organs at risk (OARs), due to the inter-operator delineation variability on a given image [7]. This has been previously shown in TARE, where the variability between different readers led to tumor-absorbed doses with a low intra-class correlation coefficient (ICC), while showing higher ICC values for liver-absorbed doses [8]. In addition, other sources of VOI delineation discrepancies among image modalities are due to respiratory motion and liver deformation [9]. Several authors investigated the impact of the dose calculation method (e.g., dose kernel convolution vs. local deposition) on absorbed dose distributions [10; 11; 12; 13] and the agreement between pre- and post-treatment image-based dosimetry [14; 15; 16; 17; 18].