SU-E-T-144: Daily and Monthly Quality Assurance with TomoDose

Examine the use of TomoDose for Tomotherapy quality assurance.

Baseline TomoDose profiles are acquired annually in accordance with TG-148. During monthly quality assurance, TomoDose is irradiated with a static beam and referenced to baseline. Differences between superimposed profiles are evaluated monthly. TomoDose is further utilized in verifying transverse laser alignment. The TomoDose is aligned to virtual isocenter lasers and statically irradiated with a 40cm × 5.02cm treatment beam. Next, the TomoDose's internal diodes are imaged using the MVCT. Transverse laser, imaging beam, and treatment beam field center are verified to less than 1mm coincidence difference. For daily TomoDose irradiation the couch is set to a fixed height and irradiated. The dose to the central axis and dose rate is recorded. To check energy, the measurement is repeated with an aluminum block placed centrally on the TomoDose. The output reading relative to the dose rate is compared to the baseline determined at the time of monthly verification. A ratio of the output and energy readings serves as a daily energy constancy check.

Over 28 months, for nominal field sizes of 1.05, 2.50, and 5.02cm, TomoDose measured an average field size of 1.10, 2.51, and 5.02cm with maximum deviations of 0.09, 0.09, and 0.10cm, respectively. For transverse laser alignment, TomoDose software calculates beam center location. We have found the alignment to be sensitive to sub-millimeter changes. Our nominal daily energy ratio is 0.689. We have observed a decline from an estimated 0.693 to 0.680 with target wear, providing a good indicator of degradation.

TomoDose serves as a reliable constancy device with added benefits of verifying transverse laser alignment and predicting target health. I own common stock in Accuray (Tomotherapy).