SU‐FF‐T‐385: Commissioning of a PTW Starcheck 2‐D Ion Chamber Array

Abstract

Purpose: To investigate and commission a PTW Starcheck 2‐D array in megavoltage photon beams and determine to what extent it can be used to replace in‐water ion chamber scans. Method and Materials: The Starcheck was introduced in 2008 and consists of 4 main arrays (0°, 90° and two diagonals) and is designed for both live diagnostics of linac beams and 2‐D QA measurements. Three parameters were investigated — dose linearity, relative precision, energy dependence (kQ) — and the results from the Starcheck mounted in a Virtual Water phantom were compared with scans obtained in a water tank (using both an ion chamber and diode) and with a 1‐D ion chamber array (Victoreen THEBES). Results: The dose linearity was found to be better than ± 0.1 % over the range 0.25 Gy to 8 Gy and short‐term repeatability was at the same level. The measured relative calibration was found to be significantly better than the manufacturer's specification, with a relative precision of 0.3 % (1 sd). The absolute calibration results for 6, 10 and 25 MV photon beams were surprising as the kQ factors for the central chamber did not follow the expected water‐air stopping power ratio, indicating a significant perturbation (perhaps due to the construction of the electrodes). The comparison of scans for a 10×10 cm field showed that the Starcheck in solid phantom agreed with the in‐water ion chamber scan within 0.5% at all points. Conclusion: The Starcheck is an easy‐to‐use device that meets, and generally exceeds, the manufacturer's specifications for relative dosimetry and can be used as a substitute for ion chamber scans where a precision of 0.5% is acceptable.