AbstractID: 7301 Title: Challenges in low energy photon dosimetery of a point source: Differences in international chamber calibrations and dose calculation protocols.
Challenges in low energy photon dosimetery of a point source: Differences in international chamber calibrations and dose conversion protocols.
An ongoing effort to improve the accuracy of the dose measured in water around a 50 kV interstitial x-ray source has illuminated several weaknesses in our understanding in the calibration and use of parallel plate ionization chambers for dose measurement at this beam quality. We compared the PTW 23342 parallel plate chamber to a newly commissioned design, the model 34013 chamber which has the same basic design, but is smaller (0.0017cc volume). Differences in the depth-dose curves were observed for the two chambers. We observed a depth-dependant difference at short distances due to the different size of the detectors. We also observed a persistent 8-10% difference at larger distances (>25mm) where the doses should have been equal. Research into this systematic difference turned up two areas that contribute to the effect. Firstly, the exposure calibrations showed a 4% systematic difference between NIST and PTW for the 34013 chamber. Secondly, the German standard for calculating dose uses a factor, K aÍw
to correct the in-air calibration factor for use in water. This factor is 1.08 for 23342 and 1.02 for 34013 chambers respectively.
We had four 23342 and six 34013 chambers calibrated at four calibration laboratories. The 4% difference between PTW and NIST for the 34013 chambers is presently unexplained. These calibrations show good agreement in some cases and larger than expected differences in others.