TH-AB-202-04: Auto-Adaptive Margin Generation for MLC-Tracked Radiotherapy

Purpose: To develop an auto-adaptive margin generator for MLC tracking. The generator is able to estimate errors arising in image guided radiotherapy, particularly on an MR-Linac, which depend on the latencies of machine and image processing, as well as on patient motion characteristics. From the es...

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Bibliographic Details
Published in:Medical physics (Lancaster) 2016-06, Vol.43 (6), p.3857-3857
Main Authors: Glitzner, M, Fast, M, Denis de Senneville, B, Nill, S, Oelfke, U, Lagendijk, J, Raaymakers, B, Crijns, S
Format: Article
Language:English
Subjects:
60 APPLIED LIFE SCIENCES
BIOMEDICAL RADIOGRAPHY
Cumulative distribution functions
Dosimetry
Error correction
ERRORS
IMAGE PROCESSING
Impact testing
Liver
Medical radiation safety
Multileaf collimators
PARTICLE TRACKS
RADIATION PROTECTION AND DOSIMETRY
RADIOTHERAPY
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Summary:Purpose: To develop an auto-adaptive margin generator for MLC tracking. The generator is able to estimate errors arising in image guided radiotherapy, particularly on an MR-Linac, which depend on the latencies of machine and image processing, as well as on patient motion characteristics. From the estimated error distribution, a segment margin is generated, able to compensate errors up to a user-defined confidence. Method: In every tracking control cycle (TCC, 40ms), the desired aperture D(t) is compared to the actual aperture A(t), a delayed and imperfect representation of D(t). Thus an error e(t)=A(T)-D(T) is measured every TCC. Applying kernel-density-estimation (KDE), the cumulative distribution (CDF) of e(t) is estimated. With CDF-confidence limits, upper and lower error limits are extracted for motion axes along and perpendicular leaf-travel direction and applied as margins. To test the dosimetric impact, two representative motion traces were extracted from fast liver-MRI (10Hz). The traces were applied onto a 4D-motion platform and continuously tracked by an Elekta Agility 160 MLC using an artificially imposed tracking delay. Gafchromic film was used to detect dose exposition for static, tracked, and error-compensated tracking cases. The margin generator was parameterized to cover 90% of all tracking errors. Dosimetric impact was rated by calculating the ratio between underexposed points (>5% underdosage) to the total number of points inside FWHM of static exposure. Results: Without imposing adaptive margins, tracking experiments showed a ratio of underexposed points of 17.5% and 14.3% for two motion cases with imaging delays of 200ms and 300ms, respectively. Activating the margin generated yielded total suppression (
ISSN:0094-2405
2473-4209
DOI:10.1118/1.4958068