A clinically observed discrepancy between image-based and log-based MLC positions.
OBJECTIVE
To present a clinical case in which real-time intratreatment imaging identified an multileaf collimator (MLC) leaf to be consistently deviating from its programmed and logged position by >1 mm.


METHODS
An EPID-based exit-fluence dosimetry system designed to prevent gross delivery errors was used to capture cine during treatment images. The author serendipitously visually identified a suspected MLC leaf displacement that was not otherwise detected. The leaf position as recorded on the EPID images was measured and log-files were analyzed for the treatment in question, the prior day's treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position.


RESULTS
Whereas the log-file reported no difference between planned and recorded positions, image-based measurements showed the leaf to be 1.3 ± 0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations.


CONCLUSIONS
It has been clinically observed that log-file derived leaf positions can differ from their actual position by >1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log-based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trust log-file records. Intratreatment EPID imaging provides a method to capture departures from MLC planned positions.
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Brian
B
Neal
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Mahmoud
M
Ahmed
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Kunal
K
Kathuria
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Tyler
T
Watkins
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Krishni
K
Wijesooriya
Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia 22908.
Jeffrey
J
Siebers