Retrospective log-based quality assurance of real-time MLC adaptation on a helical tomotherapy platform

Real-time multileaf collimator (MLC) adaptation is available on some helical tomotherapy treatment units. However, these systems are not supplied with software that facilitates the retrospective review of adapted delivery data.

This study used in-house software to complete a retrospective review of 1,566 treatment fractions with the aim of verifying the suitability and accuracy of MLC adaptations provided by a real-time target tracking system.

A data extractor tool was used to access files containing planned MLC sinogram, the telemetry treatment MLC sinogram, the telemetry delivery timing data, and the 3-dimensional target offset data from each treatment fraction. These data were used to calculate target offsets in the beam's eye view so that expected adaptations could be evaluated and compared to the adaptations recorded by the MLC telemetry system. Further investigations were undertaken using dosimeter array measurements with a motion platform.

While MLC telemetry matched expected adaptations for the vast majority of treatment fractions, erroneous MLC leaf motions lasting >1 s were identified in four treatment fractions distributed across one lung and two prostate patients (amounting to 0.3% of the total number of fractions examined), producing dose discrepancies of up to 0.3 Gy which equated to 0.6% of the total prescription dose from the affected lung treatment. Phantom measurements demonstrated that a patch provided by the vendor successfully reduced the dosimetric impact of these errors to a negligible level.

A retrospective review of 1,566 treatment fractions led to the identification of a rare error which was communicated to and subsequently patched worldwide by the vendor, exemplifying the importance of post-treatment quality assurance of adaptive radiotherapy.