SU‐FF‐T‐162: Quality Assurance for a Tomotherapy Machine: New Procedures and Comparison to TG‐40 Recommendations for Conventional Linear Accelerators

Purpose: To compare daily and monthly quality assurance, QA, procedures on a TomoTherapy machine with TG‐40 recommendations for a conventional linear accelerator. Method and Materials: A commercially available diode check‐unit is retrofitted for use in daily and monthly TomoTherapy QA. Output consistency is checked with this unit. The housing of the check unit is equipped with 1‐mm diameter lead markers that provide for checks of megavoltage CT imaging, laser alignment, and couch motion. On a monthly basis more quantitative checks of laser alignment, beam energy, beam profiles, and couch motion are done with a diode array. A very efficient test procedure has been developed that uses this array. Results: On a daily schedule, output consistency, laser alignment, and couch motion are check with a single measurement that takes approximately 10 minutes. Daily output checks over 118 days of operation show the output to vary with a 1‐standard deviation of 2%. Based on megavoltage imaging of the morning check unit, the vertical and superior‐inferior green‐laser alignment and couch motion are found to vary by less than 1 mm. In this time period, the left‐to‐right green‐laser drifted past a 2 mm limit and adjustments were made to correct the alignment. On a monthly schedule absolute beam output was measured for 1, 2.5 and 5.0‐cm jaw widths and found to vary by ±1%. Beam energy shifts were found to be ±1.2%. Monthly comparisons to commissioning profiles indicate small changes in the left‐to‐right direction, which are indicative of small changes in beam energy. Conclusion: New procedures and tolerances had to be developed for daily and monthly checks of the TomoTherapy unit. Over a five month period the machine stability has been acceptable. Many TG‐40 checks for conventional linear accelerators are not applicable to a TomoTherapy machine.