Purpose: For Pencil Beam Scanning systems, AAPM TG-224 recommends measuring the depth of the distal 90% depth dose (DR90) to within 1 mm of baseline daily. To meet this criteria, often expensive QA devices specifically designed for energy verification must be purchased. We present a novel technique to add this capability to an inexpensive daily QA device and evaluate its accuracy.
Methods: 1 cm of plastic water was placed on the surface of a standard daily QA device and held in position by a custom 3-D printed mount to minimize daily variation. The beams were delivered parallel to the surface of the QA device into the plastic water. The scatter was detected on the ion chambers below to obtain dose versus depth. The beams were delivered from gantry 90 and 270 to mitigate the effect of positioning errors. Custom software was developed to obtain the raw readouts from each chamber. A cubic spline was fit to each distribution and the DR90 for the two measurements were extracted, averaged, and compared to baseline. To test sensitivity, energies expected to have DR90 just outside the 1 mm threshold were delivered
Results: The DR90 spot positions measured using this method have a standard deviation under 0.2 mm from baseline using this method, meeting the measurement accuracy recommended by TG-224. The DR90s from the sensitivity test had an average discrepancy of 0.3 mm from expectation. The software reliably flagged spots with an expected 1.5 mm difference as failing.
Conclusion: Compared to dedicated spot range measuring devices such as multi-layer ion chambers, use of the commercial daily QA device has the advantage of being relatively inexpensive and simpler for operators to use. The same device is used for daily machine output and spot position checks with the same setup, speeding up the daily QA measurement.
Funding Support, Disclosures, and Conflict of Interest: Dr. Cardan receives honorarium and consulting fees from Varian Medical Systems