Purpose: The Xstrahl RADiant radiotherapy machine (70 to 80 kV) presents a challenge in dosimetry calibration due to its short SSD (5.0 cm). This study investigated the potential of utilizing 3D-printing technologies for the fabrication of an ion chamber holder to significantly reduce setup variations in output measurements.
Methods: A block-like, solid ion chamber holder was designed, then 3D-printed from PLA. A circular opening through the block has a diameter slightly less than the reference conical applicator (4.0cm). The rim of the opening locks the cone, preventing in-plane and cross-plane setup variation. A cylindrical portal through the side of the block holds an ion chamber in a fixed vertical position. We have utilized this holder to measure machine output weekly for the last three months. Geometric setup variation was determined by two alternating physicists repeating 5 independent setup and measurements on the same day.
Results: Without using the chamber holder, the measurement was highly dependent on the placement of the reference chamber. Based on our initial experience with the RADiant unit, the setup variations (~1.0 mm) lead to a 4.1% dosimetric uncertainty. By using our custom-designed holder, the weekly dose variation was reduced to 1.34% based upon our 15 weekly data points. The geometric setup uncertainty is estimated to be 0.5%.
Conclusion: The AAPM Task Group 61 report suggests that the setup variation for the in-air measurement needs to be 2.0% or less in order to achieve within 5% cumulative dosimetric uncertainty in clinical practice. Without an alignment tool, the 5% goal was determined to be unrealistic. By holding the cone and the ion chamber in a fixed geometry, our 3D-printed chamber holder can reduce setup variation and consequently reduce the final uncertainty to 5% or less.
Not Applicable / None Entered.