## 4 Discussion  
Radioembolization with \({}^{90}\)Y-loaded microspheres is increasingly used in radiation therapy for the treatment of HCC and metastatic liver lesions. The treatment requires accurate activity planning and dose determination to ensure a good therapeutic response with as few side effects as possible. At present, \({}^{99\text{m}}\)Tc-MAA imaging is largely used as a surrogate of microsphere distribution to assess potential extra-hepatic shunts as well as to estimate the absorbed doses and to personalize the activity to be administered. This procedure enables predictive patient-specific dosimetry to be performed prior to RE treatment. However, the gamma camera used for activity quantification has several inherent limitations, which restrict the spatial resolution of the images and affect the accuracy in the determination of the time-integrated activity.  
Although SIRT is a well-established molecular radiotherapy approach, there are ongoing efforts to further improve treatment planning using personalised dosimetry. In fact, the general consensus is that compared with standard dosimetry, personalised dosimetry may significantly improve the response rate in patients with locally advanced hepatocellular carcinoma [24]. Personalized dosimetry can be developed using MAA-based dosimetry, which is the only evaluation available prior to \({}^{90}\)Y-loaded microspheres injection. At present, the Recent research from [PERSON] et al. [25] demonstrated that response of hepatic malignancies to SIRT with resin microspheres strongly depends on tumour dose, highlighting the importance of personalized dosing regimens in SIRT therapy for improved outcome. In a recent study by [PERSON] [26], \({}^{90}\)Y PET/CT with pre- and post-treatment FDG PET/CT images were used to derive individual clinically relevant radiobiological parameters with the aim to evaluate the dosimetry and the treatment outcome specific to each patient. In another study by [PERSON] and colleagues [27] DVHs from SIRT procedures were investigated and compared to metabolic lesion response as measured with [\({}^{18}\)F]FDG PET/CT to investigate the dose-response relationship for liver lesions treated with \({}^{90}\)Y resin microspheres.  
Today, most literature studies addressing the issue of activity quantification in reference conditions have focused on validating the scanner geometry using simple phantoms. However, it is generally acknowledged that the validation of the quantification process on imaging phantoms based on real anatomic data has the potential to improve the quan  
Figure 11: AVHs of ROIs delineated on Abdo-Man phantom corrected using the \(RC_{j}\) reported in Figure 9.  
titative accuracy in vivo, if optimized acquisition protocols are used. At present, few researchers have addressed the problem of quantitative accuracy using real anatomy and specific to the patient cohort of interest.  
With this in mind, we tried to set-up an accurate workflow to assess the quantitative accuracy of SPECT imaging using \({}^{99\text{m}}\)Tc-MAA for target liver lesions combining cutting edge imaging technology with a 3D-printed multi-compartmental anthropomorphic phantom.  
Preliminary investigations on Abdo-Man anthropomorphic phantom have been conducted using a SPECT/CT Symbia T2 without the xSPECT tool by [PERSON] et al. [15]. The authors reported that SPECT overestimates normal liver activity (+11%, approximately) and underestimates the activity in the lesions (\(-11\%\) for the 40-mm lesion, \(-12\%\) for the 40-mm shell lesion and the 20-mm lesion, approximately). In their study, the authors put forward the idea that Abdo-Man should be validated using alternative reconstruction algorithms, such as the Siemens X-SPECT and Hermes SUV-SPECT software [15].  
Against this backdrop in this work the SPECT/CT Symbia Intevo system provided with xSPECT quantitative software was used to assess the accuracy in quantification procedure. A number of conclusions can be drawn from the present study and are summarized in the following subsections.