Master of Science (MS)


Physics and Astronomy

Document Type



Purpose: To determine the sensitivity of a topographic stepwedge procedure to energy variations in a helical tomotherapy system. Method and Materials: Topographic procedures were followed using an aluminum stepwedge suspended in air on the end of the couch for a TomoTherapy HI-Art II system. Exit detector data from these procedures, collected and processed using TomoTherapy quality assurance (TQA) software were used to determine the effective beam energy. Energy sensitivity tests of the technique were made by varying the injector current. Topographic procedures were run daily over the lifetime of an x-ray target. Additionally, water phantom scanning data (lateral profiles and percent depth dose) were measured monthly over the same time period. Results: The attenuation data from the stepwedge demonstrated a smooth decrease in effective energy with time over the life of the target. The energy difference showed dramatic spikes in trending due to target changes and injector current adjustments. Monthly lateral and depth-dose profiles showed variations due to an energy change but not while a target was simultaneously degrading. Analysis of the data show that the stepwedge procedure is capable of detecting a change in the effective energy of greater than 2%. Conclusion: The stepwedge topographic procedure provides a good method to monitor the energy difference over time and may be used to help diagnose impending target failures.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Gibbons, John