From Films to Logs: A History of MLC QA at a Large Cancer Center
Abstract
Purpose
A robust and routine quality assurance (QA) program for multileaf collimators (MLCs) is essential for accurate external beam radiotherapy delivery. Since the initial implementation of MLC-equipped linear accelerators (LINACs) at our cancer centre, QA procedures have evolved alongside technological advances. This study reviews the development of the MLC QA program: From film-based methods to portal dosimetry and log file–based techniques.
Methods
Different approaches to performing and evaluating Picket Fence tests across a 13-LINAC, multi-vendor fleet were assessed. Four MLC QA techniques were examined: film-based measurements, electronic portal imaging device (EPID) megavoltage acquisitions with analysis performed using Pylinac or through an inhouse software, and a hybrid method combining log file analysis with EPID imaging. Accelerator time requirements, achievable accuracy, and technical limitations were evaluated for each implementation. Results interpretability and ease of integration into the QA database were also compared.
Results
Film analysis of MLC positions was time-consuming, operator dependent, and required precise visual alignment. Errors were often identified late in the workflow, requiring full repetition of the procedure and additional machine/operator time. EPID-based tests on Elekta Agility systems introduced complexity due to treatment fields exceeding the EPID sensitive area and varying trigger sensitivity across the LINAC fleet. While these methods reduced accelerator usage time and improved relative measurement precision, the absolute position information was either lost or required further acquisitions. Transitioning to a hybrid EPID and log file–based approach on Varian TrueBeam systems enabled a simplified, semi-automated workflow, allowing higher testing frequency with minimal additional effort.
Conclusion
The MLC QA program at our cancer centre has evolved in parallel with advances in LINAC technology. Progression from film-based analysis to modern semi-automated EPID and log file based techniques has reduced operator time while enabling more frequent, precise, and reproducible QA measurements.