Linac beam data measurement using a detector array

Introduction As new treatment advances are implemented in a radiotherapy department, the QA workload increases requiring more resources. Unplanned maintenance work in the middle of the daily linac schedule also demands quick methods to check beam parameters without sacrifices in the quality of measurements. New measurement modalities that can simplify procedures are thus of great value. Purpose To investigate the reliability of a detector array when beam adjustments are carried out by the linac engineer or QA work is being undertaken. Detector arrays provide an attractive alternative to water phantoms due to their compactness and ease of setup. Materials and methods A multi-axis ion chamber array (IC PROFILER ™ , SUN NUCLEAR) was used to collect reference data for clinical linac beams. Energies measured were photons of 6, 15 and 18 MV and electrons in the range 6 to 18 MeV. The data were compared to corresponding scans and point measurements obtained using the PTW MP3 (Freiburg, Germany) water phantom for the respective beams. Parameters checked were output constancy, dose rate dependence, energy constancy, beam profiles in cardinal and diagonal directions, beam flatness and symmetry and electron applicator output constancy. The array allowed also a quick check of beam startup characteristics and the verification of beam parameter stability at various gantry angles. Results All parameters (constancy, output factors, energy dependence, dose rate, beam symmetry and flatness), measured with the multi-axis chamber array and the water phantom for the various beams, were in accordance within 1% at most. The detector array provides comparable results and combines the reliability of a water phantom with the advantage of reduced linac time taken up by solid phantom setups. Conclusion The detector array has shown to have the reliability needed to substitute the water phantom for acquiring linac beam data. This is particularly useful for QA measurements and checking linac parameters during and after unplanned technical interventions.