Coincidence of radiation and mechanical isocentre using computed radiography

Background: Verification of the coincidence between the radiation and mechanical isocentres is an essential quality assurance (QA) procedure for linear accelerators (linacs) in radiotherapy. Ensuring this alignment is critical for accurate beam targeting, especially in conformal and intensity-modulated radiotherapy techniques. Objective: This study aimed to evaluate the feasibility and effectiveness of using computed radiography (CR) with a bare imaging plate (IP) to assess the coincidence between radiation and mechanical isocentres, as a practical QA method, especially applicable in resource-limited settings. Materials and Methods: The study was conducted on a Precise linac installed at Bac Ninh General Hospital No2. A Fujifilm IP and an FCR Prima T2 reader were used. Mechanical isocentre marking was performed via 1 monitor unit (MU) open-field exposure. Radiation isocentre verification involved multiple exposures with narrow rectangular fields at various gantry, collimator and couch angles. The latent images were processed using FCR View software to analyse displacement diameters between mechanical and radiation centres. Results: The CR system demonstrated sufficient sensitivity to detect exposures as low as 1 MU. Deviations (diameters) were 0·44–1·04 mm with expanded uncertainty U up to 0·44 mm ( k = 2). All measurements were within the ±2 mm tolerance recommended by American Association of Physicists in Medicine (AAPM) for non-stereotactic systems. Conclusion: The use of a bare IP in CR offers a reliable and low-cost alternative for verifying the coincidence of radiation and mechanical isocentres. This method is suitable for routine QA procedures in clinical environments lacking access to radiochromic film or electronic portal imaging devices.