SU-E-T-382: An Analysis of VMAT Monthly Quality Assurance

The purpose of this study is to quantify the change in monthly VMAT quality assurance (QA) and determine the tests to maintain consistent delivery with a baseline.

VMAT monthly QA has been performed for over 14 months on two Elekta Synergy LINACs. A baseline was established at acceptance and the monthly QA results were compared to those initial values. Films were used to test the dependence on varying dose rate, gantry speed, rotational direction, and MLC speed. These parameters were tested independently and then together in a test called the synchronicity test. Ion chamber readings test a DMLC field with a varying dose rate and MLC speed. Introducing intentional errors into the tested fields allowed the detectable limits of the QA to be determined.

The monthly QA has consistently matched the baseline within a 3% dose limit on film. Analyzing the synchronicity film with a gamma test using a 5%/0.5mm tolerance showed a monthly pass rate of over 99%. The DMLC test has been identical for the entire course of VMAT QA. Furthermore, intentional changes in the MLC speed were noticed on the synchronicity test in the form of a smaller gamma pass rate as the MLC error was increased.

There is a monthly agreement with all films testing individual parameters, ion chamber DMLC readings, as well as agreement with the synchronicity test. This collection of data has lead to the conclusion that only the synchronicity test and DMLC readings need to be performed on a monthly basis. If those tests fail, then individual parameters need to be tested to determine the singular cause of the error. Having a single test used as a red flag increases the efficiency of the monthly QA and is being implemented with an EPID to eliminate the use of film.