Periodic QC of linear accelerators with an electronic diode matrix

The MapCHECK (Sun Nuclear Corporation) is an instrument developed as a tool for quality control of IMRT treatments in radiotherapy. It is a 2D instrument with a grid of 445 diode detectors placed at 2 cm water equivalent depth. It comes with a software where IMRT treatment plans can be imported, field by field, to be compared with measurements made with the MapCHECK. The instrument can easily be calibrated for absolute dose. The aim of this work is to evaluate whether the instrument can replace radiographic film for relative measurements of beam symmetry, beam flatness and electron energy. To obtain measurements, the detector is placed on the treatment couch with the detector plane at isocenter (SAD =100 cm). Best resolution over the largest area is obtained by positioning the detector with its diagonals aligned with the major axis. The relative response of the detector is calibrated separately for photons and electrons. Raw data are imported into an excel worksheet. For photons and electrons, a macro extract profiles along the major axis, calculates flatness, symmetry and field size. Initially during commissioning, the electronic detector and radiographic film (Kodak EDR) were used in parallel to compare the results of the new method with the well verified film method. We have used the device for all our photon (4, 6, 10, 18 MV) and electron (4, 6, 8, 10, 12, 15, 18, 20 MeV) beams at seven linear accelerators. To measure electron energy we have modified the system we used with film. It consists of two brass wedges placed with their thinnest parts 2.1 cm apart at the central axis of the beam. From the profile beneath the wedge, the distance from centre to 50% penetration is determined. This distance is correlated to the mean electron energy at the phantom surface, E0, which in turn is directly related to R50 in water. Results and discussion. The profiles for symmetry and flatness were comparable with film, with a few exceptions. The photon energies of 4, 6 and 10 MV do all compare with film, no extra build up material was needed to obtain acceptable profiles. An example of the resemblance is given. To get the same results for 18 MV, an additional slab of 1 cm build-up was required. For electrons the results did not show the same level of compliance. A problem was that 4 MeV hardly reaches the diodes at 2 cm depth and therefore results in rather noisy profiles. This problem was slightly less for 6 MeV. For the higher energies, the results compare well with film. Since January 2006, the detector has been used as the only device for routine QC of beam symmetry, beam flatness and electron energy. Until March 2006, two occasions of measurements outside tolerance have been detected. These deviations were verified by a diode array (BMS from Schuster) and the accelerators were adjusted accordingly. The overall stability of the MapCheck detector system has been very good. The system has detected deviations on occasions which also have been verified with a second system. The results shows that the electronic detector system can, in most cases, replace radiographic film in periodic QC procedures for linear accelerators. The only exception is the lower electron energies where the diode detectors are positioned to deep in the device. The advantage of this system compared with film is that the time spent at the accelerator is essentially shorter (QC is performed during normal working hours). The time for processing, evaluation and analysis has also been decreased mainly due to elimination of handling of radiographic film, e.g. developing and scanning. The latter is also an environmental advantage