TU-FG-201-08: Investigation of Systematic Low RPC Calibration Result for Tomotherapy

Authors


Abstract

Purpose:

Independent IROC (formally RPC) TLD-based output checks have found a ∼−3% systematic calibration difference for our (and general) Tomotherapy treatment machines. The purpose of this study is to identify source of this bias.

Methods:

The entire Tomotherapy calibration process is analyzed. Unlike conventional linacs, Tomotherapy unit absolute dose calibrations are based on calibration plans (tomoPhant plans on a tomocheese phantom) created at the time of commissioning. Two main areas in this process are investigated. First, the accuracy of the dose computation for the original calibration plan. Second, whether the historic commissioning plan can be relied upon for current calibrations. Dose calculations were performed with an updated local HU-density table and an updated MLC latency curve. Machine output adjusted based on our findings was verified via a helical plan delivery to an ion chamber in real water.

Results:

The vendor provided phantom image and HU-density table over-estimated the phantom electron density by 2%, which results in a −1% systematic dose error. In addition, the dose calibration is affected by the MLC latency curve. For the two machines in our site, the change in MLC latency affected the dose calibration by −1% and −2% respectively. After correcting the phantom density and re-compute the dose, we found that the reference dose increased by 2.5–3.0% indicating our machines were previously calibrated low by the same amount. After increasing the output by 3.0%, the measured dose in real water agrees with the TPS prediction.

Conclusion:

We identified the problem with the absolution calibration for our Tomotherapy units. The same calibration issues likely exist for other tomotherapy units. Tomotherapy users should ensure that their MLC latency curve is current and should re-compute reference dose with the correct phantom density before they perform absolute dose calibration.

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