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A clinically observed discrepancy between image‐based and log‐based MLC positions
Author(s) -
Neal Brian,
Ahmed Mahmoud,
Kathuria Kunal,
Watkins Tyler,
Wijesooriya Krishni,
Siebers Jeffrey
Publication year - 2016
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4949002
Subject(s) - multileaf collimator , nuclear medicine , dosimetry , offset (computer science) , image guided radiation therapy , quality assurance , medical imaging , position (finance) , computer science , mathematics , computer vision , artificial intelligence , radiation treatment planning , medicine , radiation therapy , radiology , external quality assessment , finance , pathology , economics , programming language
Purpose: 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.