Assessment of Dosimetric Accuracy and Plan Quality in Prostate SBRT: Retrospective Evaluation of 36.25 Gy in 5 Fractions Using RapidArc on Varian Millennium MLC with MLC Parameter Analysis

Introduction Prostate cancer is one of the most commonly diagnosed cancers among men worldwide. Advances in radiotherapy, particularly Stereotactic Body Radiotherapy (SBRT), have enabled ultra-hypofractionated treatment schedules that enhance tumor control while reducing treatment time. This study focuses on evaluating the dosimetric accuracy and plan quality of prostate SBRT using RapidArc technology on a Varian Millennium Multileaf Collimator (MLC) system. Objectives To retrospectively evaluate the dosimetric parameters, plan quality metrics, and multileaf collimator performance in prostate SBRT delivered via RapidArc, and to assess target coverage, organ-at-risk (OAR) constraints, and delivery efficiency. Materials and Methods A total of 24 patients with localized prostate adenocarcinoma received SBRT with a prescribed dose of 36.25 Gy in five fractions. Treatment planning was performed using Eclipse v15.6 with Acuros XB algorithm, utilizing three 6 MV flattening filter-free (FFF) arcs. Planning Target Volume (PTV) coverage, OAR doses, Paddick Conformity Index (PCI), Gradient Index (GI), and Monitor Units (MU) were analyzed. Multileaf collimator motion was evaluated through log files, including leaf speed and position dynamics. Quality assurance was performed using electronic portal imaging devices (EPID) and gamma pass rate evaluation. Results The mean PTV D 95 was 35.80 ± 0.46 Gy, with mean Dmax and Dmean being 39.80 ± 1.05 Gy and 37.10 ± 0.40 Gy, respectively. V 95% averaged 99.13 ± 1.14%, confirming adequate coverage. Slight violations of rectum and bladder Dmax constraints were observed but remained clinically acceptable. The mean PCI and GI were 0.905 ± 0.18 and 3.08 ± 0.16, respectively. Gamma pass rates exceeded 99.6% for 2%/2 mm criteria. MLC leaf speed remained below the 3 cm/s threshold, ensuring mechanical safety and dose delivery accuracy. Conclusion Prostate SBRT delivered via RapidArc on a Varian Millennium MLC system demonstrated high plan conformity, efficient delivery, and acceptable OAR sparing. The integration of intra-fraction CBCT improved setup accuracy, while MLC analysis confirmed mechanical precision. This approach supports the clinical adoption of RapidArc-based SBRT as a feasible and effective option for localized prostate cancer treatment.