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SU‐E‐T‐121: Determination of MLC QA Criteria for Non‐Split IMRT Fields Based On Clinical Quantification
Author(s) -
Schinkel C,
Mutaf Y,
Prado K,
Yi B
Publication year - 2013
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 - 10.1118/1.4814556
Subject(s) - nuclear medicine , head and neck , monte carlo method , dosimetry , radiation treatment planning , mathematics , medicine , radiation therapy , radiology , statistics , surgery
Purpose: To identify clinically‐driven MLC performance criteria for non‐split step‐and‐shoot IMRT fields by quantifying the dosimetric impact of Varian MLC leaf abutment miscalibration on the delivered dose. The maximum tolerable leaf gap that can be used as a criterion for MLC leaf gap QA will be determined. Methods: Ten IMRT plans with large (X > 15 cm), non‐split fields were selected, 5 whole pelvis and 5 head and neck. All were planned using Pinnacle v9.0. The delivered dose for each plan was calculated using Monte Carlo simulations assuming leaf abutment gaps from 0 to 3 mm, in 0.5 mm increments. All calculations were done with 1 mm resolution. Three dimensional dose difference maps (ΔD) were calculated by subtracting the 0 mm dose matrix from each non‐zero one. Cumulative ΔD histograms were constructed to determine the maximum tolerable gap where 5% volume received ΔD >= 3%. Results: The gap where % volume received ΔD >= 3% ranged from 0.7 to 2.2 mm (average 1.0 mm). For 8/10 plans this gap was >= 1.0 mm. The head and neck plans showed more sensitivity to gaps than the whole pelvis plans (p = 0.08). Increasing gap size did not adversely affect target coverage. Maximum OAR dose increased with increasing gap; a 3 mm gap led to a 4% dose increase. Conclusion: Non‐split fields should be used for IMRT with caution. This method of treatment is acceptable as long as MLC leaves abut within 0.5 mm. Standard IMRT QA using a 4 mm calculation grid and gamma of 3%/3mm is not sensitive enough to detect dose differences from leaf abutment miscalibration. If treating this way, leaf abutment should be checked as part of weekly QA with a 0.5 mm criterion in addition to the TG‐142 recommended MLC QA.