Three dimensional radiation dosimetry using polymer gels

The use of polymer gels to record radiation dose distributions in three dimensions was first proposed in 1992 as a proof-of-principle [1]. The accuracy of these early polymer gel dosimeters and scanning methods was not sufficient for most clinical applications [2]. Since then, several efforts were made to increase the accuracy and precision of both the physical gel dosimeter and the scanning technique [3,4]. Different read out techniques have been investigated. Magnetic resonance imaging was soon followed by optical scan techniques [5-7] and X-ray CT [8]. The potential applicability of polymer gel dosimeters has been shown for a variety of applications such as for dose verification in conformal radiotherapy [9], brachytherapy [10], proton therapy [11], boron neutron capture therapy [12] and diagnostic X-ray CT [13]. The polymer gel composition can be optimized for each application. The complete dosimetric performance with respect to dose rate insensitivity, temperature insensitivity, temporal stability, dose integrity and energy sensitivity has only been determined for a few polymer gel compositions [14,15]. The large variety of polymer gel compositions and scanning methods may explain why so little polymer gel dosimeters are currently used in everyday radiation quality assurance. Current efforts are directed towards less-toxic polymer gel dosimeters and alternative scanning techniques that could be easily implemented by the radiation physicist. Polymer gel dosimetry Gel dosimeters are radiation sensitive chemical dosimeters that aim at displaying the integrated absorbed dose in three dimensions (3D). Basically, gel dosimetry involves three steps: First, the radiation sensitive gel is fabricated in a chemical laboratory. Polymer gel dosimeters consist of a hydrogel in which monomers are dissolved. Upon fabrication, the gel solution can be poured in a plastic cast with an anthropomorphic shape (figure 1a) or in a recipient that can be suspended in an anthropo-morphic shaped carrier phantom. Second, the 3D gel phantom is irradiated. Upon irradiation, a radiation induced polymerization reaction occurs in which the amount of created polymer is related to the absorbed dose. The polymer structures are fixed by the gelatin matrix. Thirdly, the gel dosimeter is read out by use of a dedicated quantitative imaging technique. A frequently used imaging technique is magnetic resonance imaging (MRI). This is based on the fact that the spin-spin (T2) NMR relaxation time in a polymer gel decreases with increasing amounts of polymer. After calibration a set of dose images is obtained (figure 1b). Several methods to compensate for MR imaging artifacts that compromise the accuracy in the dose images have been developed [3]. A small change in mass density in polymer gels upon irradiation is responsible for a measurable change in Hounsfield units which enables the use of X-ray CT to read out polymer gel dosimeters [13]. In order to gain sufficient signal-to-noise, several image averages are required. In this context, attempts have also been made to increase the signal-to-noise ratio by adequate image filtering [16]. The change in optical opacity has also lead to the development of optical scan techniques [7]. Basically, optical scan techniques use a methodology similar to X-ray CT but with an optical laser beam instead of an X-ray tube and by (a) (b) Figure 1. Gel phantom irradiated according to a conformal radiotherapy treatment (a). The white region is due to irradiation induced polymerization. Maps of absorbed radiation dose are obtained by use of high-accuracy quantitative R2 imaging (b). Dose [Gy]