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Long Term Dosimetric Stability of 6 TomoTherapy Systems

J Smilowitz

J Smilowitz*, D Dunkerley , M Geurts , P Hill , P Yadav , University of Wisconsin, Madison, WI


SU-F-P-11 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall


The dosimetric stability of six TomoTherapy units was analyzed to investigate changes in performance over time and with system upgrades.


Energy and output were tracked using monitor chamber signal, onboard MVCT detector signal and external ion chamber measurements. The systems (and monitoring periods) include 3 Hi-Art (67, 61 and 65 mos.), 2 HDA (29 and 25 mos.) and one research unit (7 mo.). Dose Control Stability system (DCS) was installed on 4 systems. Output stability is reported as deviation from reference monitor chamber signal for all systems, and from an external chamber for 4 systems. Energy stability was monitored using the relative (center versus off-axis) MVCT detector signal and/or the ratio of chamber measurements at 2 depths. The results from the clinical systems were used to benchmark the stability of the research unit, which has the same linear accelerator but runs at a higher dose rate.


The output based on monitor chamber data of all six systems is very stable. Non-DCS had a standard deviation of 1.7% and 1.8%. As expected, DCS systems had improved standard deviation: 0.003-0.05%. The energy was also very stable for all units. The standard deviation in exit detector flatness was 0.02-0.3%. Ion chamber output and 20/10 cm ratios supported these results. The stability for the research system, as monitored with a variety of metrics, is on par with the existing systems.


The output and energy of six TomoTherapy units over a total of almost 10 years is quite stable. For each system, the results are consistent between the different measurement tools and techniques, proving not only the dosimetric stability, but that these quality parameters can be confirmed with various metrics. A research unit operating at a higher dose rate performed as well as the clinical treatment units.

Funding Support, Disclosures, and Conflict of Interest: University of Wisconsin and Accuray Inc. (vendor of TomoTherapy systems) have a research agreement which supplies funds for research to the University. This project was partially supporting with these funds.

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