Dosimetric Quality Assurance for Intensity–Modulated Radiotherapy

Abstract

Test and comparison of various 2-D real-time detectors for dosimetric quality assurance (QA) of intensity-modulated radiotherapy (IMRT) with the vision to replace radiographic films for 2-D dosimetry. All IMRT treatment plans were created with the Konrad software (Siemens OCS). The final dose calculation was also carried out in Konrad. A Mevatron Primus (Siemens OCS) linear accelerator which provides 6-MV and 15-MV highenergy photon beams was used for the delivery of segmented multileaf-modulated IMRT. Three different 2-D detectors, each based on a different physical (interaction) principle, were tested for the field-related IMRT verification: (1) the MapCheck diode system (Sun Nuclear), (2) the I'mRT QA scintillation detector (Scanditronix/Wellhöfer), and the Seven29 ionization chamber array (PTW). The performance of these detector arrays was evaluated against IMRT dose distributions created and calculated with Konrad and the results obtained were compared with film measurements performed with radiographic films (EDR2, Kodak). Additionally, measurements were performed with point detectors, such as diamond, diodes (PTW) and ionization chambers (PTW, Scanditronics/ Wellhöfer) and radiochromic films (GafChromic film MD55, ISP). The results obtained with all three 2-D detector systems were in good agreement with calculations performed with the treatment-planning system and with the standard dosimetric tools, i.e., films or various point dose detectors. It could be shown that all three systems offer dosimetric characteristics required for performing field-related IMRT QA with relative dose measurements. The accuracy of the 2-D detectors was mostly +/- 3% normalized to dose maximum for a wide dynamic range. The maximum deviations did not exceed +/- 5% even in regions with a steep dose gradient. The main differences between the detector systems were the spatial resolution, the maximal field size, and the ability to perform absolute dosimetric measurements. Commercial 2-D detectors have the potential to replace films as an "area detector" for field-related verification of IMRT. The on-line information provided by the respective systems can even improve the efficiency of the QA procedures.