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MRIdian Linac Daily QA

P Yadav*, K Mittauer , K Vredevoogd , J Bayouth , School of Medicine and Public Health UW-Madison, Madison, WI


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: MRgRT is an emerging technology and requires establishment of QA procedures that support the novel types of therapy being provided with the MRIdian Linac. The purpose of this study is to develop and validate a series of daily radiation dose and imaging QA tests to be performed on the MRIdian Linac system to establish and maintain an effective and safe MRgRT. These highly sensitive tests evaluate the precision of MR and RT isocenter alignment, spatial fidelity imaging and patient handling systems, and the performance of the linear accelerator.

Methods: On daily basis safety interlocks, laser alignment with known offset from isocenter, imaging and treatment isocenter coincidence, radiation dose output, couch motion and small-field MLC positioning as commonly required for IMRT are tested. The ViewRay daily QA phantom is used for all tests except safety interlock tests. At MR and RT isoceneter coincidence, the ViewRay daily QA phantom is irradiated with an extremely small sensitive field of 0.4x0.83 cm2 . This test requires submillimeter agreement MLC positioning, beam steering, sub-degree gantry angle accuracy, to provide a radiation dose within ±10% of the expected dose. Radiation dose and dose rate constancy of the linear accelerator are measured using standard field size of 9.96x9.96 cm2, with a specification of ±3%.

Results: Over 04 months period the radiation output was within ±3%. Our monthly output results were compared with daily output and were within 0.5-2%. Ratio of daily dose and reference dose for small field size shows that MLC position is within set spec of 2 mm with random variation which might be due to setup errors.

Conclusion: Tests performed on daily basis help us to keep track of key component such as linac, couch, MLC, MR imaging system which are critical for accuracy of MRgRT.

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