Patient Specific Quality Assurance for Helical Tomotherapy: An Evolving Process in Reducing Discrepancy between Measured and Calculated Dose

Claus Chunli Yang, Chuan Wu, Fanqing Guo, Tianxiao Liu, James Purdy

Department of Radiation Oncology, University of California, Davis, Sacramento, CA, USA

Purpose: The Tomotherapy Hi-Art system’s (Tomotherapy Inc., Madison, WI USA) acceptance test/commissioning (data gathering) is performed in a relatively short time period mainly by the company’s physicists with the institutions’ physicists merely observing. This new approach, while very efficient in terms of starting patient treatments, leaves the institution’s physicists not nearly as familiar or experienced as in the case of the conventional linac. This report is to document the evolutionary process by which our group has reached an acceptable level in patient specific quality assurance (QA) measurement in a relatively short period of time.

Method and Materials: 61 delivery quality assurance (DQA) tests were performed in the previous four months on the Tomo machine. For each DQA plan, two point doses were verified simultaneously using two Exradin A1SL ion chambers in high dose but possibly low dose gradient area. EDR2 film is used only for qualitative evaluation of the 2-D dose distribution.

Results: The four monthly ion chamber measurement results (excluding signs) are: Feb. 2006 average: 3.3% (42 measurements) ±2.0% (SD), range: from -3.3% to 8.5%; March average: 1.4% (28) ±0.9%, from -2.5% to 3.7%; April average: 1.3% (21) ±1.0%, from -2.7% to 2.9%; May average: 1.5% (30) ±1.0%, from -3.6% to 2.7%.

Conclusion: We have implemented a step by step procedure to achieve a high standard in patient specific QA measurements (97% of the ion chamber measurements are within ±3% from the calculation). A more quantitative (and efficient) QA tool is needed for evaluating the 2-D dose distribution.