## 1 Introduction  
In the treatment of primary and secondary liver cancer, radioembolization with holmium-166 microspheres (\({}^{166}\)Ho-RE), also known as SIRT (Selective Internal RadiationTherapy), appears as a promising therapy. This procedure, as in other liver radioembolization techniques (\({}^{90}\)Y-based), involves injecting radioactive microspheres into the hepatic artery through a microcatheter placed in the femoral artery [1]. A small activity of \({}^{166}\)Ho microspheres, typically 200-250 MBq, serves as a \"scout dose\" to assess the microspheres' distribution within the liver and detect any extra-hepatic shunting. This is achieved through a post-scout SPECT/CT scan, which guides the planning and administration of the main treatment involving a higher activity of \({}^{166}\)Ho microspheres. After this verification and planning first step, a therapeutic activity of typically 2-6 GBq is injected, mimicking the \"scout dose\" distribution. After administering the therapeutic activity, a post-therapy SPECT/CT scan is performed after a suitable interval of time has elapsed. This delay ensures that the SPECT detectors are not saturated by the high count rate immediately following the treatment due mainly to Bremsstrahlung radiation emission [2]. The second SPECT scan enables the assessment of the doses delivered to both the liver and the tumor. \({}^{166}\)Ho is a high-energy beta-emitting radioactive isotope (maximum energy: 1.85 MeV, abundance 49%). It also emits low-energy gamma rays at 80.6 keV (abundance 6.6%), which enables the visualization of microspheres' distribution through SPECT (Single-Photon Emission Computed Tomography) [1; 3; 4]. Additionally, there are gamma emissions at higher energies, including 1.34 MeV (0.9%), 1.58 MeV (0.2%), and 1.66 MeV (0.1%) [5]. When working with \({}^{166}\)Ho, it is crucial to account for the presence of Bremsstrahlung radiation and Compton scattering. The high-energy photons in the Bremsstrahlung spectrum can pass through the collimator septa, leading to a reduced image quality and causing inaccurate results [6]. MC algorithms that simulate scattering within the object have been effective in recovering image information. The Hybrid Recon(tm) software by Hermes Medical Solutions AB (Stockholm, Sweden) incorporates a scatter modeling method named \"convolution-based forced detection\", applying pre-calculated probability density functions of the \({}^{166}\)Ho spectrum generated using the PENELOPE MC code [7; 8]. This approach by Hybrid Recon(tm) includes a full MC collimator simulation, which comprehensively addresses all photon interactions at the collimator, including both primary and scattered photons, while also considering scatter within the object [8]. As the interest in post-therapy dosimetry grows [9], accurate quantification of \({}^{166}\)Ho becomes necessary. Thus, properly determining the CF, which is required to convert SPECT image counts into radioactivity concentration units (MBq/mL), plays a critical role in activity quantification [10; 11]. It is important to note that radioembolization enables direct activity conversion from SPECT scans, as nearly all activity stays within the liver, removing the need for a calibration factor. It is also essential to address the PVE when discussing \({}^{166}\)Ho activity quantification. PVE is an important factor that affects the measurement of activity concentration in emission tomographic imaging and can significantly impact the accuracy of radionuclide uptake determination in quantitative SPECT and PET imaging. To address this issue and potentially correct it, one approach is to use ARC along with the corresponding activity concentration recovery curves [12]. The primary goal was to enhance the OSEM reconstruction parameters (number of iterations and subsets) for \({}^{166}\)Ho SPECT and assess the impact of advanced MC collimator modeling through Hybrid Recon(tm) on both image quality and quantification.