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Determination of MLC QA Criteria for Non-Split IMRT Fields Based On Clinical Quantification

C Schinkel

C Schinkel*, Y Mutaf, K Prado, B Yi, University of Maryland School Of Medicine, Baltimore, MD

SU-E-T-121 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

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.

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