The Study on the Use of a Cylindrical Ionization Chamber for the Calibration of a 6 MeV Electron Beam / 의학물리

The standard dosimetry systems based on an absorbed dose to water recommend to use a planeparallel chamber for the calibration of such a low-megavoltage electron beam as a nominal energy of 6 MeV. For this energy ranges of an electron beam a cylindrical chamber should not be used for the routinely regular beam calibration, but the feasibility of the temporary use of a cylindrical chamber was studied to give temporary solutions for special situations users meet. The PTW30013 chambers and the electron beam quality of R(50)=2.25 g/cm2 were selected for this study. 10 PTW30013 chambers, a cylindrical type of chamber, were calibrated in KFDA, the secondary standards dosimetry laboratories, and given the absorbed dose-to-water calibration factors, respectively. A "temporary" kappa(Q,Q0) for each chamber were calculated using the absorbed dose determined by a cross-calibrated planeparallel chamber, with the result of an average 0.9352 for 10 chambers. This value for PTW30013 chamber was used to determine an absorbed dose to water at the reference depth. The absorbed doses determined by PTW30013 chambers were in an agreement within 2% with that by ROOS chamber. In a certain situation where a cylindrical chamber be used instead of a planeparellel chamber, the value of 0.9352 might be useful to determine an absorbed dose to water in the same beam quality of electron beam as this study.

Evaluation of the Long-Term Stability for the Cylindrical Ionization Chambers / 대한방사선종양학회지

PURPOSE: To analyze the long-term stability of Farmer-type cylindrical ionization chambers by calibration factor provided from the KFDA (Korea Food Drug Administration) MATERIALS AND METHODS: The cylindrical ionization chambers used in this study were the PTW 30001 (30006), 30013, 30002, 30004, 23333, the Capintec PR06C, the NE 2571, the Exradin A12 and the Wellhofer FC65G (IC70). We were analyzed that the N(k) and N(D,W) calibration factor for the cylindrical chambers and compared between the measured N(D,W) and calculated N(D,W) calibration factor. RESULTS: We have observed that the long-term stability of the PTW 30013 (30006), the Wellhofer FC65G (IC70) and the NE 2571 has varied within 0.2%. The measured N(D,W) calibration factor was about 1.0% higher than the calculated N(D,W) that determined by the N(k) calibration factor. CONCLUSION: The study has evaluated that the long-term stability of the cylindrical chambers through analysis for the Nk and N(D,W) calibration factor. It has contributed to the improvement of clinical electron dosimetry in radiotherapy centers.