Over-Response Correction of SNC ۳۵۰p ROOS and PTW Markus Chambers in the Surface Dose and Build-Up Region Dosimetry of ۶MV Photon Beam

Introduction: The design characteristics of ionization chambers are critical for the accurate measurement of Percentage Surface Dose (PSD) and Percentage Depth Dose (PDD) in the build-up region. This study assesses the over-response correction in PSD and PDD for different field sizes, using Markus and ROOS fixed separation parallel-plate ionization chambers. Material and Methods: The Sun Nuclear Corporation SNC ۳۵۰P ROOS and PTW Markus parallel plate ionization chambers were used to measure ۶ MV photon beam PDD along the central axis in the build-up region.The experiment was performed on a Solid Water® HE phantom with a density of ۱.۰۳۲ g/cm۳.The Measurements were taken at a Source Surface distance (SSD) of ۱۰۰ cm with Field sizes of ۱۰x۱۰cm۲,۲۰x۲۰ cm۲, and ۳۰x۳۰cm۲ and at variable depths. Results: For field sizes of ۱۰x۱۰ cm۲, ۲۰x۲۰ cm۲, and ۳۰x۳۰ cm۲ at a source-to-axis distance (SAD) of ۱۰۰ cm, the PSD values obtained with the Markus chamber, before and after applying Rawlinson's equation for over-response correction, were as follows: ۳۵.۱۱%, ۴۲.۹۲%, ۵۰.۷۲% (before correction) and ۲۸.۳۷%, ۳۶.۱۹%, ۴۴.۰۵% (after correction). Similarly, the PSD values measured using the ROOS chamber, before and after applying over-response correction, were as follows: ۴۱.۶۱%, ۵۰.۵۰%, ۵۸.۰۴% (before correction), and ۳۳.۹۶%, ۴۲.۸۷%, ۵۰.۳۸% (after correction). Conclusion: The application of over-response correction using Rawlinson's equation improved the accuracy of measurements for both chambers, emphasizing the significance of employing appropriate correction techniques to ensure precise dose measurements in the build-up region using ionization chambers.