Purpose: To evaluate the accuracy of virtual simulation, which is less time-consuming than physical simulation, with the new laser system Dorado CT4 in 96 prostate cancer patients. Patients and Methods: Virtual simulation was based on a spiral scan with 8 mm reconstruction index and 8 mm slice thickness in 64 patients (group A), and 3 mm reconstruction index and 3 mm slice thickness in 32 patients (group B). Both groups were evaluated for impact on maximum difference (Δmax) regarding the isocenters obtained from virtual simulation versus those obtained from physical simulation. Results: In the entire cohort, mean differences were as follows: Δmax 5.7 ± 3.5 mm, Δx (left/right) 2.8 ± 2.9 mm, Δy (anterior/posterior) 4.5 ± 3.8 mm, and Δz (cranial/caudal) 2.1 ± 2.2 mm. In group A, mean values were Δmax 6.2 ± 3.8 mm, Δx 2.9 ± 3.1 mm, Δy 4.9 ± 4.2 mm, and Δz 2.3 ± 2.3 mm. In group B, mean values were Δmax 4.8 ± 2.8 mm, Δx 2.7 ± 2.7 mm, Δy 3.7 ± 2.7 mm, and Δz 1.7 ± 2.0 mm. Time of radiotherapy (primary vs. salvage RT) and radiation regimen (external-beam radiotherapy [EBRT] vs. high-dose-rate brachytherapy [HDR-BT] plus EBRT) had no significant impact on Δmax. Conclusion: Virtual simulation with the new laser system Dorado CT4 was very precise for both primary and salvage RT in the treatment of prostate cancer patients. High precision was achieved for both EBRT and HDR-BT plus EBRT. Virtual simulation should be performed with a planning CT with 3 mm reconstruction index and 3 mm slice thickness for high accuracy.