AIM: To assess the dosimetric results of an online adaptive stereotactic body radiotherapy (SBRT) program to treat patients with prostate cancer using a conventional linear accelerator. METHODS AND MATERIALS: Prostate SBRT with five fractions is performed at the XXXX using an online adaptive method previously described (XXXX). A CBCT based adaptive "plan-of-the-day" (POD) is generated for each treatment fraction, which consists of a single volumetric modulated arc therapy. A dosimetric evaluation of the PODs was performed for the first twenty patients and included analysis of the target (prostate), organs-at-risks (OARs), and patient-specific quality assurance (PSQA). Each of the PODs was also compared with the corresponding conventional image-guided radiotherapy (IGRT) method. Finally, the adaptive treatment timing is analyzed for the one hundred PODs in this study RESULTS: The online adaptive treatment method ensured optimal target coverage in all treatment fractions for all patients. However, the simulated IGRT plans did not result in adequate prostate coverage (V40 Gy ≥ 95%) in 70% of fractions. Small average differences between PODs and IGRT plans were found in the OARs. The dose sparing in the rectum and bladder provided by some simulated IGRT plans, was associated with a compromised prostate coverage. The PSQA resulted in an excellent agreement between the online-calculated plans and the independent dose checks performed for all one hundred PODs. The average duration of the plan adaptation was 20.1 ± 6.1 minutes and the average overall session time including adaptation and treatment delivery was 26.4 ± 33.2 minutes. CONCLUSIONS: This study demonstrates The online adaptive program using a conventional linac to treat prostate cancer described in this study is clinically feasible and in adherence with the acceptance criteria set by the PACE B.