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A Novel Multielectrode Linac Beam Monitoring Chamber: Real-Time Detection of Linac Output as Well as MLC Segment Sizes and Locations

Z Han*, D Brivio, J Kukluk, Y Lyatskaya, P Zygmanski, Brigham & Women's Hospital, Harvard Medical School, Boston Boston, MA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To develop a beam monitoring detector for real-time verification of linac output as well as other clinically relevant parameters such as MLC segment size and location. The latter are important to ensure targeting accuracy (mm), while the former can be used for real-time dose rate control (cGy/s).

Methods: The beam monitoring device resembles a large area parallel plate ion chamber, which is further diagonally segmented into 2 complementary chambers. Each chamber can be read out independently. The underlying conceptual design of the detector was described in our earlier work. In the current study, we have developed a custom linac beam-detector response model and extended its application to radiation therapy. This new device makes use of the 2-signal inputs to derive dosimetric and geometric quantities of the MLC fields. These quantities include linac output, effective sizes, and center locations of MLC segments. Through a simulation study, we have applied this model to both static and dynamic MLC fields and compared the simulated parameters to TPS plans.

Results: We designed a novel segmented ion chamber and developed a beam-response model for the system. We determined model parameters including MLC transmissions, gap offset, cGy-mm conversion factor, and head scatter. The simulated MLC parameters for a variety of static and dynamic MLC fields were found to be in relatively good agreement with TPS considering its simplicity.

Conclusion: The early results reveal that this novel beam monitoring device is capable of providing not only output but also relevant MLC parameters for real time quality control. Further work includes improvement in the model based on extensive measurements.

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