Assessing the suitability of a two-dimensional array for routine quality assurance checks of flatness and symmetry.

Quality assurance of radiotherapy linear accelerator beams demands the use of equipment with high resolution and reliability of reproducible results over time. This study examines the suitability of a commercially available two-dimensional ionization chamber array-the StarCheck array (PTW, Frieburg, Germany) to measure symmetry and flatness for both photon and electron beams. The study was conducted over a period of 4 years whereby the reliability of the array could be established. The reproducibility, uniformity of chamber response, and comparison of both photon and electron profiles acquired with the StarCheck array to that of the water-tank were examined. The most significant result was that across all profiles acquired using the StarCheck array, a defective chamber was detected, manifested by a 'dip' in the cross-plane profile at the same position. Assuming all detectors are functional, StarCheck was shown to be within 1% of the water-tank results for both flatness and symmetry. The detector array exhibited many calibration and detector issues over the period in which it has been used in the department. Furthermore, PTW recommends that recalibration of the array should to be performed every 2 years at either PTW, Freiburg, or at a qualified calibration laboratory. Therefore, the department requires a backup device for the recalibration time, approximately 4 months as worst case.