• Dosimetry Check
• Multi Leaf Collimator
• Quality Assurance
• Radiotherapy

Delivery of dynamic intensity modulated beams requires extensive knowledge of Multi Leaf Collimator (MLC) position accuracy and reproducibility, since when accurate leaf positioning is lost, significant dose delivery errors can occur. The leaf speed of motion and the transmission are also parameters that highly influence the dose delivered to the patient. The 2009 report of the AAPM Task group 142 [1] provides an excellent review of MLC Quality Assurance (QA) issues. During the past years radiographic films have been widely used for checking the accuracy of the MLC positioning. However, the use of an Electronic Portal Imaging Device (EPID) can be very helpful in saving time for daily routine tests, providing images with high spatial resolution, quickly obtained, directly digitalized and stored in a workstation. Dosimetry Check (Math Resolutions, LLC, Columbia, MD, U.S.), is a commercial software for pre-treatment dosimetric verification, in vivo-dosimetry and also MLC QA. The aim of this work is to redact a QA protocol for the Elekta Agility MLC, based on EPID images analyzed with the commercial software Dosimetry Check (DC).

First of alls a test to validate EPID measurements of leaf position was performed with a Diode detector: the same field was acquired with Diode and EPID and the leaf position measurements were compared for 10 leaf pairs by using a Matlab code. The difference resulted to be less than 0.8mm for all the analyzed leaves.
Leaf position accuracy was checked with Garden Fence test acquisitions at the four cardinal gantry angles, while short term and long term reproducibility were respectively tested with 5 Garden Fence test acquisitions repeated in one day and 6 acquisitions repeated in a time interval of 70 days. All the EPID images were analyzed with DC. For what concern the reproducibility, each leaf position resulted to differ from its average value less than 0.3mm for the short term repeated measurements and less than 0.4mm for the long-term ones. For the Garden Fence test at different gantry angles, the maximum difference between the measured and the prescribed position was 0.8 mm, except for 270.
The radiation field dimension was checked by acquiring 6 different square fields, and comparing the measured and the prescribed dimensions. EPID acquisitions were analyzed with DC, and measurements resulted to be always within 1 mm on one side compared to the prescription.
The average MLC transmission was measured, for all the beam energies (6MV, 10MV and 15MV), by acquiring a rectangular field and dividing the signal measured under the MLC with that measured at the center of the field. The measured transmission was <0.5% for all the energies, according to the Elekta specifications.
A swipe acquisition, with a rectangular field translating along the X direction, was used as leaf speed test. Six swipe were acquired with the same dose rate but with different total erogated Monitor Units (MU), in this way a curve of the relation between dose and leaf speed was assessed. This test could be used as reference for the comparison of the swipe acquired in the same conditions during future MLC QA in order to assess leaf speed loss from baseline.

Different types of test were implemented, by using the EPID detector and DC for the image analysis, in order to check all the parameters suggested in [1] for the MLC QA, and the results were all within the tolerance. The combined use of EPID and DC allowed to assess an exhaustive MLC QA protocol that could be proposed as routine test in radiotherapy departments.

[1] E E Kleinb, J Hanley, J Bayouth, F-F Yin, WSimon, S Dresser, C Serago, F Aguirre, L Ma, B Arjomandy, C Liu, Quality assurance of medical accelerators, Report of Task Group No. 142, Med. Phys. 36(9), September 2009.