Beam Characterization and QA Recommendations for 2.5 MV Imaging Beams In a Large Institution Network
Abstract
Purpose
Current AAPM guidelines (TG-104 and MPPG 2.b) primarily address kV or standard MV portal beams for imaging and are not well suited for low-dose-rate 2.5 MV imaging beams, particularly in establishing dose and image quality baselines. This study developed and implemented a commissioning and routine quality assurance (QA) workflow for the 2.5 MV imaging beam on C-arm linear accelerators (LINACs) across a clinical network comprising 27 LINACs.
Methods
Commissioning measured beam characteristics including PDD10, Dmax, and absolute dose calibration using a 1D water tank and A12 ion chamber following TG-51 protocols. Flatness and symmetry were assessed using a 5-point ion chamber method at 1.0 and 1.5 cm depths at 100 cm SAD. Image quality (spatial resolution, contrast-to-noise ratio (CNR), contrast, and uniformity) was evaluated using an MC2 phantom and compared against 6 MV baselines. A routine QA schedule with items specific to the 2.5 MV imaging beam was established: Daily (safety/interlocks and radiation isocenter alignment), Monthly (image scaling), and Annual (beam quality, dosimetry, and image quality). Tolerance levels were proposed.
Results
Commissioning data indicated a Dmax of 5.5 mm and PDD10 of 52.1%. Off-axis profiles for a 30×30 cm² field with a 5 cm shift showed stable symmetry near 0%. Imaging tests demonstrated that 2.5 MV (High Quality) provided superior CNR (40.8 vs. 30.6) and contrast (37.3% vs. 22.0%) compared to 6 MV, with comparable spatial resolution (0.4 lp/mm). Monthly output checks showed stability within 0.3% ± 0.4% for five consecutive months. Commissioning results were adopted as baseline values, and standardized QA templates with tolerances were developed for network-wide implementation.
Conclusion
The 2.5 MV imaging beam improves portal imaging quality compared to standard treatment beams. A comprehensive commissioning and routine QA program were established, expanding TG-104 and MPPG 2.b recommendations, to ensure safety and consistency across a large network.