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Estimation of Dynamic Leaf Gap for IMRT Dose Calculation

W Yao

W Yao*, D Goodyear, M Gillard, M Corsten, Eastern Health, St. John's, NL

SU-E-T-30 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To obtain the proper dynamic leaf gap (DLG) of a Millennium 120 MLC such that the calculated dose matches the measured.

Methods: The DLG for round leaves is often measured via extrapolating the size of fields formed by sweeping synchronized MLC leaves to the size under which the measured dose equals the MLC transmission. In this situation, the tongue and groove effect would be minimal. However, practical IMRT fields are formed by non-synchronized MLC leaves. Therefore, we measured doses from both synchronized and non-synchronized dynamic fields. Each non-synchronized field had fixed but different neighbouring leaf extensions (tongue-and-grove, T&G). The DLG was obtained for each set. Doses calculated with various values of DLG and transmission rate (TR) were compared with the measured dose. The relative discrepancies were put into a lookup table as function of gap and T&G. The table was used to estimate the pass rate of patient IMRT plans, in which each field was decomposed into sub-fields. Each sub-field was formed by a pair of opposite leaves. The T&G was calculated from the neighbouring leaves formed the sub-field.

Results: The extrapolated DLG was 1.8, 1.6, 1.7, 2.5 and 1.6mm for the T&G 0, 5, 10, 15 and 20mm, respectively. The MLC TR was 1.6%. For typical patient IMRT plans, the T&G values were broadly distributed from 0 to 20mm but with median around 15mm. From investigating many combinations of TR and T&G, we see the pass rate was extremely sensitive to the combination, and the highest pass rate went to the combination of transmission 1.6% and T&G 2.3mm. This result excellently agreed with IMRT QA.

Conclusion: The extrapolated DLG depends on the T&G. The proper value of DLG can be estimated from the study of non-synchronized sweeping gaps and T&G distributions in typical patient IMRT plans.

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