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Evening Output Check Improves Daily QA Reliability

I Veltchev

I Veltchev*, R Price, C Ma, Fox Chase Cancer Center, Philadelphia, PA

SU-E-T-141 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the benefits of performing a routine evening output check during IMRT QA. Having a secondary dosimetrically accurate measurement at the end of the treatment day proves extremely useful when deterioration of machine output is inferred from the morning QA.

Methods: A two-dimensional ionization chamber array (MatriXX) is used for IMRT QA on six linear accelerators. Out of the 12 beams studied six are 6MV, while the rest are 10MV. As part of our IMRT QA protocol, a 10x10 100MU field is routinely recorded as a way to correct for machine output variations. The central axis dose is stored in a database and compared with the morning QA database over a period of 18 months. The distributions of central axis readings are analyzed in terms of range and relative standard deviation.

Results: The standard deviation of the readings (averaged over all 12 beams) in the morning QA measurements is 0.88%, while the evening variability is reduced to 0.68%. The range of the evening data (the difference between the lowest and highest reading) is 40% less than the spread in the morning readings signifying stabilization of the machine output during the course of the treatment day. In the case of one treatment machine the morning QA readings of both 6MV and 10MV showed a downward trend not observed in the evening data - a warning sign predicting morning QA equipment failure.

Conclusion: The additional beam output information routinely collected in the evening during IMRT QA shows reduced variability compared to the data obtained by the morning QA device. Additionally, the trend of evening output can be compared with the daily QA results to predict equipment failure, and can be used to guide the decision making when the morning readings exceed the predetermined warning limits.

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