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On the Use of Transmission Detectors to Detect MLC Errors During Treatment Delivery

L Vazquez Quino1*, C Huerta Hernandez2 , V Gutti3 , A Morrow4 , O Calvo5 , H Chung6 , (1) Baylor Scott & White Health, Temple, TX, (2) ,Little Rock, AR, (3) Baylor Scott & White Health, Temple, TX, (4) Baylor Scott and White Health, Temple, TX, (5) Baylor Scott & White Health, Temple, TX, (6) Baylor Scott & White Health, Temple, TX


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To explore the use of transmission detector in MLC positioning verification for VMAT delivery.

Methods: The Discover detector from Scandidos was tested in the detection of MLC errors introduced in VMAT plans, delivered by a Varian TrueBeam equipped with Millenium-120 MLCs. The Discover detector consist of 4040 detectors with a 1.5mm spacing along MLC trajectories and can be used for in-vivo dosimetry. MLC shift were introduced in RapidArc plans for a variety of treatment sites. The shitfs ranged from 0.1 to 1mm to explore the ability of the detector in catching such error introduced. The original plan was delivered and used as the baseline. Next the plans with the shifts were delivered and analyzed to using a criteria of a maximum allowed leaf tip deviation of 1mm. We set an internal criteria of passing for which the plan pass if 95% of the MLC positions have a gamma index of <=1 with such criteria. One more parameter used to evaluate was the dose delivered versus the gantry angle.

Results: Original VMAT fields were measured and produced gamma pass rates ranging from 99.5-100% passing rate. As the shift is increased in the MLC positioning, the gamma pass rates is reduced. The fields with a 1mm shift resulted in a 70% gamma pass rate, on average. The dose delivered versus gantry angle graph results in deviations from the original arc delivery.

Conclusion: The preliminary results are very promising in the detection of MLC errors for in vivo dosimetry using the Discover detector. This detector was capable of detecting any shift error introduced and quantifying the deviations based in the criteria used. Furthermore the dose delivered versus gantry angle is a useful feature to evaluate the performance of the LINAC and in finding any errors during delivery.


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