Use of Two-dimensional Ionization Chamber Array on Brachytherapy Dose Verification

  Objective  To investigate the feasibility of two-dimensional ionization chamber array MatriXX for brachytherapy plan verification.

  Methods  The well-type ion chamber was used to verify the source activity of ¹⁹²Ir though the result of 60-second readout by calibration factor. The radioactive source was used to complete the calibration of EBT3 film through the Valencia skin applicator and water equivalent solid phantom. After the dose gray curve was obtained, the percentage depth dose(PDD) curves under Valencia skin applicator was drawn and then applied to calibrate the two-dimensional ionization chamber array MatriXX and get a calibration factor Kuser. The Freiburg Flap applicator with the needle was inserted, and then combined with solid water to build a phantom. After CT scanning, the catheter reconstruction and dose distribution calibration were completed in the treatment planning system (TPS). The planned dose distribution and measurement dose distribution were analyzed by software.

  Results  The ¹⁹²Ir dose distribution under Valencia skin applicator was similar to accelerator beam. The transverse and longitudinal symmetries were 0.17% and 0.16%, respectively. The reference dose was at 3 mm under the applicator surface, and the PDD value at 11.6 mm under the applicator surface was 46.5%. The measured dose distribution and TPS calculated dose plane was compared using gamma analysis method (3 mm/3%). The ratio of pixels within γ ≤ 1 was 97.67%, and the absolute dose measurement and calculation of dose deviation remained in 5%.

  Conclusions  MatriXX has good repeatability when used as a brachytherapy dose distribution verification tool. It avoids using film and is more rapid and convenient for brachytherapy dosimetry quality assurance.