Assessment of Dosimetric Accuracy and Plan Quality in Prostate SBRT Using RapidArc with Analysis of Varian MLC Parameters

Introduction: Prostate cancer is a highly prevalent malignancy worldwide. Advances in radiotherapy, particularly SBRT, have enabled ultra-hypofractionated treatments that improve tumor control while reducing treatment time. This study evaluates the dosimetric accuracy and plan quality of prostate SBRT delivered using RapidArc technology on a Varian Millennium Multi Leaf Collimator (MLC) system.Material and Methods: Twenty-four patients with localized prostate adenocarcinoma received SBRT with a prescribed dose of ۳۶.۲۵ Gy in five fractions. Treatment planning was performed using Eclipse v۱۵.۶ with the Acuros XB algorithm, employing three ۶ MV Flattening Filter Free (FFF) arcs. Planning Target Volume (PTV) coverage, Organs At Risk (OAR) doses, Paddick Conformity Index (PCI), Gradient Index (GI), and Monitor Units (MU) were analyzed. MLC performance was assessed using trajectory log files to evaluate leaf speed and positional accuracy. Pre-treatment Quality Assurance (QA) was conducted using Electronic Portal Imaging Device (EPID)-based gamma analysis.Results: The mean PTV D۹۵ was ۳۵.۸۰ ± ۰.۴۶ Gy, with Dmax and Dmean of ۳۹.۸۰ ± ۱.۰۵ Gy and ۳۷.۱۰ ± ۰.۴۰ Gy, respectively. V۹۵% averaged ۹۹.۱۳ ± ۱.۱۴%, confirming adequate coverage. The bladder and rectum Dmax slightly exceeded constraints, while all other OAR objectives were met. Gamma pass rates exceeded ۹۹.۶% for the ۲%/۲mm criterion. MLC leaf speed remained below ۲.۵ cm/s, ensuring delivery accuracy.Conclusion: RapidArc-based prostate SBRT demonstrated high conformity, efficient delivery, and acceptable OAR sparing. Intra-fraction CBCT improved setup accuracy, and MLC log-file analysis confirmed mechanical precision, supporting the feasibility of this approach for localized prostate cancer.