###### Abstract  
This study aimed to compare the commercial dosimetric software Planet(r) Dose (version 3.1.1) from DOSIsoft and the open-source toolkit GATE. Dosimetry was performed for six patients receiving 200 mCi of Lutzhera(r) every 8 weeks for four treatment cycles. For the dose calculation with Planet(r), SPECT/CT images were acquired at 4, 24, 72 and 192 h post-injection. After the registration of all the time points to T0, the organs of interest (OOIs) were segmented. Time-activity curves were produced and the absorbed dose was calculated using the bi- and tri-exponential fitting methods. Regarding GATE simulations, the SPECT images of the 24 h time point were utilized for the radiopharmaceutical biodistribution in the OOs and the attenuation maps were produced using the CT images. For liver and spleen, the average relative difference between GATE and Planet(r) was 9.6% and 11.1% for biexponential and 12.4% and 30.5% for triexponential fitting, respectively. The right and left kidneys showed differences up to 10.7% and 10.4% for the biexponential and up to 60.6% and 11.9% for the triexponential model, respectively. The absorbed dose calculated with GATE, Planet(r)(bi-exp) and Planet(r)(tri-exp) was in agreement with the literature. The results of the bi-exponential fitting were similar to the GATE-resulted calculations, while the tri-exponential fitting had a higher relative difference. Monte Carlo simulation; GATE; patient-specific dosimetry; peptide receptor radionuclide therapy; [177 Lu-[DOTA0, Tyr3]-octreotate ] 20233 subtype 2, which is found in high levels on the surface of NETs. The most recent isotope employed for PRRT is \({}^{177}\)Lu, which uses DOTA linker to form \({}^{177}\)Lu-DOTA-Tyr\({}^{3}\)-octrotate (\({}^{177}\)Lu-DOTATATE) and \({}^{177}\)Lu-DOTA\({}^{0}\)-Tyr\({}^{3}\)-octreotide (\({}^{177}\)Lu-DOTATOC) [6; 7]. In recent years, there has been a significant increase in the use of computer science within the field of medical physics, particularly in the realm of personalized medicine. One of the most popular methods is the use of Monte Carlo (MC) simulations for dosimetry in both diagnostic and therapeutic applications [8; 9]. Numerous studies have utilized MC simulations to calculate the absorbed doses for patients undergoing therapy with \({}^{177}\)Lu [10; 11; 12; 13; 14; 15; 16]. MC simulations offer high-accuracy patient-specific dose calculations and are considered as the gold standard [17; 18; 19]. [PERSON] et al. aimed to validate absorbed dose calculations with the GATE MC simulation toolkit at the clinical scale for nuclear medicine dosimetry [20]. Their findings indicate that GATE provides a reliable approach for radiopharmaceutical voxel-based dosimetry [20]. However, there are practical limitations currently preventing its widespread use in daily clinical routine [21]. The main problem is that direct MC simulations are very time-consuming. Even with the technological advancements, they still need a significant amount of time, making analytical and faster alternatives more appealing [22]. Dosimetry in nuclear medicine departments has historically been a daunting task, as no commercial software was available to carry out integrated dosimetry. Advancements in dosimetry led to the development of commercial software packages, which may be used in clinical routine [23]. [PERSON] et al. conducted a comparative analysis of commercial dosimetric software platforms and highlighted the necessity for the advancement of tools and standards to facilitate the comparison among such platforms. To calculate the absorbed dose, it is necessary to choose the proper fitting model for the time-activity curves [24]. Various studies suggest different fitting methods for estimating absorbed doses [25; 26; 27; 28]. In their research, [PERSON] et al. conducted a comparison between Planet\({}^{\circledR}\) Dose, Dosimetry Toolkit\({}^{\circledR}\), and OLINDA/EXM\({}^{\circledR}\) V1.0, utilizing mono-exponential fitting for their analyses [29]. Similarly, [PERSON] et al.