Background and purpose: IMRT QA is commonly performed in a phantom geometry but the clinical interpretation of the results in a 2D phantom plane is difficult. The main objective of our work is to move from film measurement based QA to 3D dose reconstruction in a patient CT scan. In principle, this could be achieved using a dose reconstruction method from 2D detector array measurements as available in the COMPASS system (IBA Dosimetry). The first step in the clinical introduction of this system instead of the currently used film QA procedures is to test the reliability of the dose reconstruction. In this paper we investigated the validation of the method in a homogeneous phantom with the film QA procedure as a reference. We tested whether COMPASS QA results correctly identified treatment plans that did or did not fulfil QA requirements in head and neck (H&N) IMRT.
Materials and methods: A total number of 24 treatments were selected from an existing database with more than 100 film based H&N IMRT QA results. The QA results were classified as either good, just acceptable or clinically rejected (mean gamma index 0.5, respectively with 3%/3 mm criteria). Film QA was repeated and compared to COMPASS QA with a MatriXX detector measurement performed on the same day.
Results: Good agreement was found between COMPASS reconstructed dose and film measured dose in a phantom (mean gamma 0.83 +/- 0.09, 1SD with 1%/1 mm criteria, 0.33 +/- 0.04 with 3%/3 mm criteria). COMPASS QA results correlated well with film QA, identifying the same patients with less good QA results. Repeated measurements with film and COMPASS showed changes in delivery after a modified MLC calibration, also visible in a standard MLC check in COMPASS. The time required for QA reduced by half by using COMPASS instead of film.
Conclusions: Agreement of COMPASS QA results with film based QA supports its clinical introduction for a phantom geometry. A standard MLC calibration check is sensitive to
- IMRT QA
- 2D ionization chamber array
- Dose reconstruction
- Film dosimetry
- Gamma index analysis
- INTENSITY-MODULATED RADIOTHERAPY
- PIXEL IONIZATION-CHAMBER
- MONITOR UNIT SETTINGS
- DOSE VERIFICATION
- MULTILEAF COLLIMATION