Characterization and use of a 2D-array of ion chambers for brachytherapy dosimetric quality assurance.

The two-dimensional (2D) ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed by IBA Dosimetry (IBA Dosimetry, Germany) for megavoltage real-time absolute 2D dosimetry and verification of intensity-modulated radiation therapy (IMRT). The purpose of this study was to (1) evaluate the performance of ion chamber array for submegavoltage range brachytherapy beam dose verification and quality assurance (QA) and (2) use the end-to-end dosimetric evaluation that mimics a patient treatment procedure and confirm the primary source strength calibration agrees in both the treatment planning system (TPS) and treatment delivery console computers. The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X-ray beams (100, 180 and 300 kVp). The detector calibration factor was determined using 300 kVp X-ray beams so that we can use the same calibration factor for dosimetric verification of high-dose-rate (HDR) brachytherapy. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector, and water-equivalent slab of RW3 to provide full scattering conditions. The treatment planning system (TPS) (Oncentra brachy version 3.3, Nucletron BV, Veenendaal, the Netherlands) dose distribution was calculated on the computed tomography (CT) scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro-I'mRT software. The quality of agreement was quantified by the gamma (γ) index (with 3% delta dose and distance criterion of 2 mm) for 9 sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end-to-end dosimetric evaluation for routine QA checks. The 2D ion chamber array dose dependence was found to be linear for 100-300 kVp and the detector response (k(user)) showed strong energy dependence for 100-300 kVp energy range. For the Ir-192 brachytherapy HDR source, dosimetric evaluation k(user) factor determined by photon beam of energy of 300 kVp was used. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test plans have shown agreement with >94.5% for γ ≤1. Dosimetric QA can be performed with the 2D ion chamber array to confirm primary source strength calibration is properly updated in both the TPS and treatment delivery console computers. The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir-192 brachytherapy HDR source.