MR Based Polymer Gel Dosimetry (MRPD) versus Film Dosimetry using Dose Modulation Transfer Function (DMTF)

Precise methods of modern radiation therapy such as intensity modulated radiotherapy (IMRT), brachytherapy (BT) and high LET irradiation allow for high dose localization in volumes of a few mm3. However, most dosimetry methods—ionization chambers, TLD arrangements or silicon detectors, for example—are not capable of detecting sub-mm dose variations or do not allow for simple dose imaging. Magnetic resonance based polymer dosimetry (MRPD) appears to be well suited to three-dimensional high resolution relative dosimetry but the spatial resolution based on a systematic modulation transfer function (MTF) approach has not yet been investigated. In this study a theoretical construct for addressing the spatial resolution in different dose imaging systems, i.e. the dose modulation transfer function (DMTF) approach, have been used. The absorption grid is made of brass, which offers a good compromise between effective absorption necessary to obtain sufficient modulation and easy mechanical processing to obtain the fine grid structures. Quasi-periodic slits at lateral width of about 280, 525 and 1125 μm were sawed by means of fine saw blades in the 8 mm thick brass absorber material. The modulated dose is detected by the polymer gel and then, for film dosimetry the polymer gel is replaced by a film layer which was located under the absorption grid. We concluded that the DMTF in high frequencies is related to the imaging system, and with the current imaging system the spatial resolution in film densitometry and MRPD for 280μm resolution is nearly the same.